Computer Tools for Bifurcation Analysis

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Tutorials and Reviews

International Journal of Bifurcation and Chaos, Vol. 9, No. 6 (1999) 9831008c World Scientific Publishing Company

COMPUTER TOOLS FOR BIFURCATION ANALYSIS:

GENERAL APPROACH WITH APPLICATION TO

DYNAMICAL AND DISTRIBUTED SYSTEMS

LEONID M. PISMEN and BORIS Y. RUBINSTEINDepartment of Chemical Engineering,

Technion I.I.T., Technion City,Haifa 32 000, Israel

Received February 2, 1996; Revised December 15, 1998

A general approach of the automated algorithms for derivation of amplitude equations for dy-namical and distributed systems is presented. A brief description of the Mathematica functionsdesigned using this approach is given. Application of the functions is illustrated by severalexamples dealing with dynamical and distributed systems.

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9842. Bifurcations of Dynamical Systems . . . . . . . . . . . . . . . . . . . . . . . 986

2.1. Multiscale Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . 9862.2. Monotonic Bifurcation . . . . . . . . . . . . . . . . . . . . . . . . . . 987

2.2.1 Fold Bifurcation . . . . . . . . . . . . . . . . . . . . . . . . . 9872.2.2 Transcritical Bifurcation . . . . . . . . . . . . . . . . . . . . . . 9872.2.3 Higher-Order Bifurcations . . . . . . . . . . . . . . . . . . . . . 988

2.3. Hopf Bifurcation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9882.4. Automated Generation of Normal Forms . . . . . . . . . . . . . . . . . . 9902.5. Computation of Coefficients . . . . . . . . . . . . . . . . . . . . . . . 992

2.5.1 Function CalculateCoefficient . . . . . . . . . . . . . . . . . . . 9922.5.2 Lorenz Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 9922.5.3 Brusselator Model . . . . . . . . . . . . . . . . . . . . . . . . . 9922.5.4 Exothermic Reaction in a Stirred Tank Reactor . . . . . . . . . . . . 993

3. General Algorithm for Distributed Systems . . . . . . . . . . . . . . . . . . . 9953.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9953.2. Linear Analysis and Dispersion Relation . . . . . . . . . . . . . . . . . . 9963.3. Multiscale Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . 9963.4. Function BifurcationTheory . . . . . . . . . . . . . . . . . . . . . . . 997

4. Amplitude Equations for ReactionDiffusion Problems . . . . . . . . . . . . . . 9984.1. Long-Scale Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . 9984.2. Turing Bifurcation . . . . . . . . . . . . . . . . . . . . . . . . . . . 999

4.2.1 Hopf Bifurcation . . . . . . . . . . . . . . . . . . . . . . . . 1000

983


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984 L. M. Pismen& B. Y. Rubinstein

4.3. Brusselator Model . . . . . . . . . . . . . . . . . . . . . . . . . . 10004.3.1 Monotonic Bifurcation . . . . . . . . . . . . . . . . . . . . . . 10004.3.2 Oscillatory Bifurcation . . . . . . . . . . . . . . . . . . . . . . 1001

4.4. Two-Level Laser Model . . . . . . . . . . . . . . . . . . . . . . . . 10014.4.1 Oscillatory Bifurcation . . . . . . . . . . . . . . . . . . . . . . 1001

5. Amplitude Equations for Convective Problems . . . . . . . . . . . . . . . . . 10025.1. Convective Instabilities . . . . . . . . . . . . . . . . . . . . . . . . 1002

6. Resonant and Degenerate Cases . . . . . . . . . . . . . . . . . . . . . . . 10036.1. Three-Wave Resonance in Turing Bifurcation . . . . . . . . . . . . . . . 1003

6.1.1 General Results . . . . . . . . . . . . . . . . . . . . . . . . . 10036.1.2 Calculations for Brusselator Model . . . . . . . . . . . . . . . . 1004

6.2. Four-Wave Resonance in Oscillatory Instability . . . . . . . . . . . . . . 10056.3. Algebraic Degeneracy HopfTuring Bifurcation . . . . . . . . . . . . . 1005

A Linear Analysis and Amplitude Equations for Lorenz Model . . . . . . . . . . . 1006B Linear Analysis for Brusselator Model . . . . . . . . . . . . . . . . . . . . 1007

1. Introduction

Amplitude equations describe behavior of evolu-tionary systems in the vicinity of threshold, or crit-ical values of parameters that mark a point of qual-itative change in behavior, like a transition froma quiescent state to convection, or from rigid me-chanical equilibrium to vibration, or from a homo-geneous to patterned state of a material medium.Near the criticality, the behavior of systems of dif-ferent physical origin is described by representa-

tive equations belonging to one of the universalclasses that are determined by the character of thetransition.

A common procedure of derivation of amplitudeequations starts from a certain system of evolutionequations symmetric to spatial translations and ro-tations and time translations; a necessary conditionis that it does not include explicit dependence onspatial coordinates and time. This underlying sys-tem may be, in the order of ascending complexity,a dynamical system (autonomous ODEs), an au-tonomous distributedsystem described by PDEs but

not restricted by boundary conditions, and a dis-tributed system with vertical structure constrainedby boundary conditions along some spatial coordi-nates but retaining translational symmetry in other(unconstrained) directions. A solution that retainsall symmetries of the underlying system is calleda basic state. This state may b ecome unstable toinfinitesimal pertubations in a certain parametricdomain. The instability may be connected with the

emergence of new solutions with a more complicatedspatiotemporal structure.

In dynamical systems, possible transitions arebifurcation of multiple stationary states (monotonicbifurcation) or transition to oscillatory behavior(Hopf bifurcation). If the phase trajectory of thedynamical system remains in the vicinity of the ba-sic state even after the latter becomes unstable,it can be replaced there by certain reduced equa-tions called normal forms[Guckenheimer & Holmes,1983; Hale & Kocak, 1991]. In distributed systemsthere are two more possibilities: transition to a sta-tionary pattern (Turing bifurcation) or to a wavepattern (Hopf bifurcation at nonzero wavenumber).Like normal forms of dynamical systems, amplitudeequations describe behavior in the vicinity of thebasic state; they may include, however, also differ-ential terms describing spatial inhomogeneities onan extended scale or spatial modulation of a pri-mary pattern.

There is a number of celebrated universalequations which have b een extensively studied in

physical literature. The general structure of theseequations can be often predicted by symmetry con-siderations. Some of them were first suggested asmodels, others were derived as rational approxima-tions in a certain physical context, and then discov-ered again in apparently unrelated problems. Al-though these equations, strictly speaking, may bevalid only in a very limited parametric domain, theyretain a variety of dynamic behavior due to a high


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Computer Tools for Bifurcation Analysis 985

parametric sensitivity in the vicinity of a transi-tion point. Parametric boundaries between regionsof qualitatively different dynamic behavior can bedrawn as relations between coefficients of amplitudeequations or normal forms. If these coefficients, intheir turn, are expressed through measurable physi-cal parameters, the boundaries can be mapped uponthe actual parameter space, and then further con-

tinued into a region where universal equations areno longer valid quantitatively but still faithfully de-scribe qualitative features of behavior of a complexnonlinear system.

It is important therefore not only to be ableto predict a general form of amplitude equations ornormal forms appropriate to different kinds of bi-furcations but to have at hand reliable tools for ac-tual computation of their coefficients, starting withspecific underlying evolution equations. The appli-cable algorithms, known as center manifold meth-ods in mathematical literature [Guckenheimer &Holmes, 1983; Hale & Kocak, 1991], or eliminationof slaved variables in physical literature [Haken,1987], are well established, at least in standardcases. Actual computations are, however, repetitiveand time-consuming, and have been carried out inscientific literature on a case-to-case basis. Onlyvery rarely can they be done manually by anybodylacking Leverriers patience. Application of mod-ern computer algebra systems first, REDUCEand MACSYMA, and later, MAPLE and Mathe-matica is an obvious remedy. It is difficult to

estimate, to what extent they are used nowadaysfor the study of specific bifurcation problems, as itis not always explicitly acknowledged in resultingpublications. A case-to-case application is, how-ever, still arduous and inefficient, compared to therealization of a general algorithm.

A MACSYMA-based (not fully automated) al-gorithm for derivation of normal forms of dynamicalsystems has been written by Rand and Armbruster[1987]. Another well-known bifurcation analysispackage AUTO by Doedel, as well as the algorithmby Guckenheimer et al. [1997] for the analysis of

Hopf bifurcation, are restricted to numerical track-ing of solutions of dynamical systems. No generalcomputer-aided algorithms have been suggested sofar, to the best of the authors knowledge, for theanalysis of bifurcations in distributed systems.

This tutorial review describes a Mathematica-based automated algorithm for the derivation ofamplitude equations for both dynamical and dis-tributed systems in either symbolic or numerical

form. The algorithm makes use of Mathematicacapabilities as a programming language [Wolfram,1991, 1996] that allows to define special functionscarrying out complicated tasks in response to sim-ple and well-defined inputs.

In the course of our work on this algorithm,we arrived at the conclusion that computations aremost efficient, in terms of CPU time and computerresourses, when any specific bifurcation problem isapproached as a part of a broader class of prob-lems leading to an amplitude equation of the samestructure. Accordingly, all computations are car-ried out in two stages. The general algorithm basedon multiscale bifurcation expansion is implementedby the function BifurcationTheory . The input ofthis function must be given in a generalized form,without specifying particular nonlinear functions,eigenvectors, etc. The inderlying system should bedefined as a single operator equation where vari-

ous vectors or arrays are denoted by symbols ratherthan being specified explicitly. The output dependstherefore only on the type of the bifurcation, appli-cable symmetries and the desired order of expan-sion. It includes the amplitude equation and unre-solved expressions for its coefficients, written in ageneralized symbolic form and containing unimple-mented inverse operators.

The next stage is the actual computation im-plemented by the function CalculateCoefficient .It takes the necessary data for a specific system anduses them in the output of BifurcationTheory to

produce either explicit symbolic expressions for co-efficients or their numerical values. Thus, the pro-gram works much in the way as an intelligent math-ematician would: first preparing general formulaeand then working out a specific example. This givesan order of magnitude acceleration compared to anearlier version of our algorithm [Pismen et al., 1996]where amplitude equations containing explicit ex-pressions for coefficients were computed directly forspecific problems.

The paper is organized as follows. Althoughone and the same internal algorithm works for

dynamic and distributed systems, we start, forpedagogical reasons, with a detailed treatmentof dynamical systems in Sec. 2. First, thegeneral algorithm based on multiscale bifurca-tion expansion is outlined, and then the functionBifurcationTheory designed to implement this al-gorithm is introduced and applied to both mono-tonic and Hopf bifurcations. This is followed byspecific examples.


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Starting from Sec. 3, we repeat the same schemefor distributed systems. The general algorithm isoutlined in Sec. 3 for a problem written in the mostgeneral nonlinear operator form. In Sec. 4, the func-tion BifurcationTheory is applied to reactiondiffusion problems, followed by specific examples ofcomputation of coefficients.

In Sec. 5, we consider along the same lines

some problems involving convection that cannotbe reduced to a reactiondiffusion form. Resonantand algebraically degenerate cases are discussed inSec. 6. The primary cause of degeneracy in dis-tributed systems with two unrestrained directionis rotational symmetry that makes excitation ofstationary inhomogeneities or waves with differentdirections of the wavenumber vector. Strong res-onances may involve in such systems three wave-forms in the case of a monotonic (Turing), and fourwaveforms in the case of a wave (Hopf) bifurca-tion. We shall also give an example of a paramet-ric (accidental) degeneracy involving mixed Hopfand Turing modes. The algorithm is applicable onlywhen the degeneracy is algebraic but not geometric.The latter case requires essential modification of themultiscale expansion procedure, and is outside thescope of this communication.

2. Bifurcations of Dynamical Systems

2.1. Multiscale expansion

The standard general algorithm for derivation ofamplitude equations (normal forms) is based onmultiscale expansion of an underlying dynamicalsystem in the vicinity of a bifurcation point. Con-sider a set of first-order ordinary differential equa-tions in Rn:

du/dt = f(u, R) , (1)

where f(u; R) is a real-valued n-dimensionalvector-function of an n-dimensional array of dy-namic variables u, that is also dependent on anm-dimensional array of parameters R. We ex-

pand both variables and parameters in powers ofa dummy small parameter :

u = u0 + u1 + 2u2 + ,

R = R0 + R1 + 2R2 +

(2)

Next, we introduce a hierarchy of time scales tkrescaled by the factor k, thereby replacing the func-tion u(t) by a function of an array of rescaled time

variables. Accordingly, the time derivative is ex-panded in a series ofpartial derivatives k /tk:

d

dt= 0 + 1 +

22 + . (3)

Let u = u0(R0) be an equilibrium (a fixedpoint) of the dynamical system (1), i.e. a zero ofthe vector-function f(u; R) at a point R = R0 in

the parametric space. The function f(u; R) can beexpanded in the vicinity of u0, R0 in Taylor seriesin both variables and parameters:

f(u; R) = fu(u u0) + 12

fuu(u u0)2

+1

6fuuu(u u0)3 + + fR(R R0)

+ fuR(u u0)(R R0)

+1

2fuuR(u u0)2(R R0) + .

(4)

The derivatives with respect to both variables andparameters fu = f/u, fuu =

2f/u2, fR =f/R, fuR =

2f/uR, etc. are evaluated atu = u0, R = R0.

Using Eqs. (2)(4) in Eq. (1) yields in the first-order

0u1 = fuu1 + fRR1 . (5)

The homogeneous linear equation

Lu1

(fu

/t0)u1 = 0 (6)

obtained by setting in Eq. (5) R1 = 0 governsthe stability of the stationary state u = u0 to in-finitesimal perturbations. The state is stable if alleigenvalues of the Jacobi matrix fu have negativereal parts. This can b e checked with the help ofthe standard Mathematica function Eigenvalues.Computation of all eigenvalues is, however, super-fluous, since stability of a fixed point is determinedby the location in the complex plane of a leadingeigenvalue, i.e. with the largest real part. Generi-cally, the real part of the leading eigenvalue vanishes

on a codimension one subspace of the parametricspace called a bifurcation manifold. The two typesofcodimension onebifurcations that can be locatedby local (linear) analysis are a monotonic bifurca-tion where a real leading eigenvalue vanishes, anda Hopf bifurcation where a leading pair of complexconjugate eigenvalues is purely imaginary. Addi-tional conditions may define bifurcation manifoldsof higher codimension.


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2.2. Monotonic bifurcation

2.2.1. Fold bifurcation

A monotonic bifurcation is also a bifurcation ofequilibria of the dynamical system (1). Setting u1= const, i.e. 0u1 = 0, allows to find a shift ofthe stationary solution due to small variations ofparameters

u1 = f1u fRR1 . (7)The n m array f1u fR is recognized as the para-metric sensitivity matrix. Continuous dependenceon parameters can be used to construct a branch ofequilibria which terminates at a bifurcation pointR0.

On a monotonic bifurcation manifold, the ma-trix fu has no inverse. This means that one canneither construct a stationary solution at values ofparameters close to this point, nor characterize thestability of the equilibrium in the linear approxi-

mation. The dynamics in the vicinity of the bifur-cation manifold is governed by a nonlinear ampli-tude equation to be obtained in higher orders of theexpansion.

Generically, the zero eigenvalue is nondegener-ate. Let U be the corresponding eigenvector satis-fying fuU = 0. Then

u1 = a(t1, t2, . . .)U (8)

is the solution of Eq. (6) that remains stationaryon the fast time scale t0. The amplitude a is so

far indeterminate, and can depend on slower timescales.

The inhomogeneous equation (5) has solutionsconstant on the rapid time scale, provided its inho-mogeneity does not project on the eigenvector U.This condition is

1 UfRR1 = 0 , (9)

where U is the eigenvector of the transposed matrixfu = Transpose[fu] satisfying fu

U = 0; we as-sume that the eigenvector is normalized: UU = 1.

Equation (9) defines the tangent hyperplane to thebifurcation manifold at the point R = R0.

Before writing up the second-order equation,we require that the second-order deviation u2 re-main constant on the rapid time scale (otherwise itmight outgrow u1 at long times). The dependenceupon slower time scales must be expressed exclu-sively through the time dependence of the ampli-tude a. Using Eq. (8), we write the second-order

equation as

fuu2 = 1aU fRR2 afuRUR1 1

2a2fuuUU .

(10)The solvability condition of this equation is

1a = 2 + 1a + 0a2 . (11)

The parameters of Eq. (11) are

2 = UfRR2 +

1

2UfRRR1R1 ,

1 = UfuRUR1 ,

0 =1

2UfuuUU .

(12)

The indices correspond to the scaling of respectiveparametric deviations from the bifurcation point.In a generic case, one can consider only parametricdeviations transverse to the bifurcation manifold,

and set R1 = 0 to satisfy Eq. (9); then 1 = 0, andgeneric equation for slow dynamics near the bifur-cation manifold becomes

1a = 2 + 0a2 . (13)

This equation can be also obtained by shifting theamplitude in Eq. (11). On the one side of the bi-furcation manifold, where 2 has the sign oppo-site to that of 0, there are two stationary statesa = 2/0. When viewed as a solution ofEq. (13), one of them is stable, and the other, un-stable. The stable solution corresponds to a stableequilibrium of the original system Eq. (1), providedthe rest of eigenvalues of the matrix fu have nega-tive real parts. On the other side of the bifurcationmanifold, where the signs 2 and 0 are the same,there are no stationary states. The trajectory ofthe dynamical system is deflected then to anotherattractor, far removed from u0. Thus, the systemundergoes a first-order phase transition when thebifurcation manifold is crossed. If the value of somedynamic variable or other characteristic of the solu-tion is drawn as a function of parameters, the bifu-

ration locus can be seen as the projection of a fold ofthe solution manifold on the parametric plane; ac-cordingly, the generic monotonic bifurcation is alsocalled a fold bifurcation.

2.2.2. Transcritical bifurcation

It may happen that the matrix fR as well the tensorfRR vanish identically. This would be the case whenu0 is a trivial solution that remains constant at


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all values of parameters. Then Eq. (9) is satisfiedidentically, and Eq. (11) reduces to

1a = 1a + 0a2 . (14)

This equation has two solutions, a = 0 and a =1/0, on both sides of the bifurcation manifold,but the two solutions interchange stability when

this manifold is crossed. This bifurcation is calledtranscritical.

2.2.3. Higher-order bifurcations

If 0 = 0, the expansion should be continued tothe next order. The coefficient 0 may vanish iden-tically due to the symmetry of the original prob-lem to inversion of u. Otherwise, it can be equalto zero at certain values of the parameters of theproblem. Generally, the two conditions, Det[fu] = 0and 0 = 0 are satisfied simultaneously on a codi-mension two manifold in the parametric space thatcorresponds to a cusp singularity.

In order to continue the expansion, we restrictparametric deviations in such a way that the depen-dence on t1 be suppressed. Deviations transverseto the bifurcation manifold have to be restrictedby the condition 2 = 0, which is stronger thanEq. (9). First-order parametric deviations R1 par-allel to the bifurcation manifold, which are still al-lowed by Eq. (9), should be now restricted by thecondition 1 = 0. If the array R contains two pa-rameters only, the conditions 1 = 0 and 1 = 0 im-

ply, in a nondegenerate case, that first-order para-metric deviations should vanish identically. Whenmore parameters are available, parametric devia-tions satisfying both these conditions are superflu-ous, since they correspond just to gliding into acloser vicinity of another point on the cusp bifur-cation manifold in a higher-dimensional parametricspace. Further on, we shall set therefore R1 to zeroidentically.

The dynamics unfolding on a still slower timescale t2 should be determined from the third-orderequation

fuu3 = 2aU fRR3 afuRUR2

afuuUu2 1

6a3fuuuUUU . (15)

The second-order function u2 has to be found bysolving Eq. (10), now reduced to the form

fuu2 = fRR2 1

2a2fuuUU . (16)

Only the solution of the inhomogeneous equation,which does not project on the eigenvector U, is rel-evant. It can be expressed as

u2 = U(2)2 + a

2U(0)2 . (17)

The solvability condition of Eq. (15) is obtainedthen in the form

2a = 3 + 2a + 0a3 , (18)

where

3 = UfRR3 ,

2 = UfuRUR2 + U

fuuUU(2)2 ,

0 =1

6UfuuuUUU + U

fuuUU(0)2 . (19)

Equation (18) presents a parametric unfoldingof dynamics in the vicinity of a cusp bifurcation.Three equilibria two stable and one unstable exist in the cusped region

2 > 0 , |3| < 23/2

33/21/2. (20)

Outside this region, there is a unique stable equilib-rium. A second-order phase transition occurs whenthe parameters change in such a way that 2 crosseszero. Other transitions occuring in the vicinity ofthe cusp bifurcation are weakly first-order.

The condition 0 = 0 defines a singular bifur-

cation manifold of codimension three. Again, it ispossible to fix parametric deviations to suppress thedynamics on the scale t2, and obtain in the nextorder a quartic equation that represents the unfold-ing of the butterfly singularity. The procedure canbe continued further if a sufficient number of freeparameters is available. The unfolding of a codi-mension q singularity is presented by a polynomialof order q+ 1:

qa =q1

p=0qp+1a

p + 0aq+1 , (21)

where the parameters k depend on parametric de-viations proportional to k.

2.3. Hopf bifurcation

At the Hopf bifurcation point the parametric de-pendence of the stationary solution u = u0(R) re-mains smooth; a linear correction can be obtained


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from the stationary equation (7), and higher cor-rections from higher orders of the regular expansion(4). In order to simplify derivations, we shall elim-inate this trivial parametric dependence by trans-forming to a new variable u = u u0(R). Theresulting dynamic system, du/dt = f(u, R) has thesame form as (1) but contains a modified vector-

function f(u, R) = f(u + u0, R). Since, by defini-tion, u0 satisfies f(u0, R) = 0, u = 0 is a zero of

f(u, R). Now we can drop the hats over the symbolsand revert to the original form (1) while keeping inmind that u = 0 is a stationary solution for all Rand, consequently, all derivatives fR, fRR, etc. com-puted at u = 0 vanish.

At a Hopf bifurcation point the Jacobi matrixfu has a pair of imaginary eigenvalues = i0.The first-order Eq. (6) has a nontrivial oscillatorysolution

u1

= a(t1

, t2

, . . .)(t0

) + c.c. ; (t0

) = ei0t0U(22)

with an arbitrary complex amplitude a(t1, t2, . . .)dependent on slower time scales tk, k > 0; U is theeigenvector of fu with the eigenvalue i0:

fuU = i0U . (23)

The function (t0) and its complex conjugate(t0) are two eigenfunctions of the linear opera-tor L, Eq. (6), with zero eigenvalue. The operatorL acts here in the space of 2/0-periodic complex-valued vector-functions with the scalar product de-fined as

u, v = 02

2/00

u(t) v(t)dt . (24)

The eigenfunctions of the adjoint operator L =fu + /t0 are

(t0) = ei0t0U

and its complex conjugate; U is the eigenvector of

fu with the eigenvalue i0.The second-order equation can be written in

the form

Lu2 = 1u1 12

fuuu1u1 fuRu1R1 . (25)

The inhomogeinity of this equation contains boththe principal harmonic, ei0t0, contributed by thelinear terms 1u1 and fuRu1R1, and terms with

zero and double frequency stemming from thequadratic term 1/2fuuu1u1. The scalar prod-ucts of the latter terms with the eigenfunctions(t0),

(t0) vanish, and the solvability conditionof Eq. (25) is obtained in the form:

1a = 1a , (26)

where 1 is given by Eq. (12) (note that this pa-rameter is now complex).

Equation (26) has a nontrivial stationary solu-tion only if the real part of 1 vanishes. This con-dition defines a hyperplane in the parametric spacetangential to the Hopf bifurcation manifold. Theimaginary part of 1 gives a frequency shift alongthe bifurcation manifold. In order to eliminate thetangential shift, we set as before R1 = 0. ThenEq. (26) reduces to 1a = 0, so that the ampli-tude may evolve only on a still slower scale t2. Thesecond-order function u2 has to be found by solvingEq. (25), now reduced to the form

Lu2 = 12

(a2e2i0t0fuuUU + |a|2fuuUU + c.c) .(27)

The solution of this equation is

u2 = 12

[|a|2fu1fuuUU

+ a2e2i0t0(fu 2i0)1fuuUU + c.c.] . (28)

In the third-order,

Lu3 = 2u1 fuRu1R2

fuuu1u2 16

fuuuu1u1u1 . (29)

The amplitude equation is obtained as the solvabil-ity condition of this equation. Only the part of theinhomogeinity containing the principal harmoniccontributes to the solvability condition, which takesthe form

2a = 2a + 0|a|2a , (30)

where 2, 0 are defined as

2 = UfuRUR2 ,

0 =1

2UfuuuUUU

UfuuU(fu1fuuUU)

12

UfuuU((fu 2i0)1fuuUU) .

(31)


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The expansion has to be continued to higher orders,after readjusting the scaling of parametric devia-tions, if the real part of 0 vanishes.

2.4. Automated generationof normal forms

The procedure of derivation of a sequence of ampli-tude equations described in the preceding section isimplemented by the function BifurcationTheory .

BifurcationTheory[operatoreqn , u, R, t, {U,U}, amp, coef, eps, order, freq, options] derivesa set of amplitude equations for a dynamical systemof ordinary differential equations written in an op-erator form.

BifurcationTheory has the following argu-ments:

operatoreqn denotes an operator equation de-scribing the problem; u denotes a array of variables;

R denotes a array of bifurcation parameters; t denotes a time variable; U and U stand for the normal mode eigenvectors; amp is an amplitude of the normal mode appear-

ing in the linear solution; coef denotes the name of coefficients of the re-

sulting amplitude equations; eps denotes the small parameter of the problem

();

order is the highest order of the expansion; if it

is higher than the codimension of the bifurcationmanifold, corrections to the principal (lowest or-der) amplitude equation are produced;

freq denotes the frequency of the arising limitcycle (for monotonic bifurcations it can bedropped).

The differentiation operator /t used in theoperator equation operatoreqn is represented bythe symbols Nabla[t]. Action of this operator onfunctional expressions is denoted by the symbol alias to NonCommutativeMultiply .

The function BifurcationTheory admits theoption ScalarProduct specifying a pure functionfor scalar product used in solvability condition. Thedefault value is ScalarProduct -> Dot.

As it is discussed in the preceeding section thefunction automatically shifts the basic solution inthe process of derivation of the normal forms forHopf bifurcation. As for monotonic bifurcationsthere is no shift with only one exception. In case of

transcritical monotonic bifurcation, where the ba-sic state solution remains constant at all values ofparameter, the shift of the solution is admissible.This case needs an application of another optionTranscritical -> True which forces the functionto make a shift in monotonic case. The default valueof this option is Transcritical -> False.

As an example of usage of the function

BifurcationTheory, we consider the Hopf bifur-cation. Unfortunately, the version 2.2.2 of Mathe-matica used in development of the package has noperfect typesetting abilities of the version 3.0. As aresult the output of the function is very lengthy andis difficult to understand for inexperienced users.We present the Mathematica output only once andfurther we will use the standard textbook notation.

The original dynamical problem in the standardnotation has a form:

ut = f(u, R) . (32)

Hence the first argument operatoreqn will take aform: f[u, R] Nabla[t]**u == 0 and the func-tion BifurcationTheory is used as follows:

dynoscbif = BifurcationTheory[f[u, R] Nabla[t]**u == 0, u, R, t, {U,Ut}, a, c, eps, 2, w].The result produced by the function can be pre-sented as a set of two amplitude equations describ-ing the slow dynamics of the amplitude a at thetime scales ti =

it0, (i = 1, 2) [cf. Eqs. (26) and

(30)]:

{a(1,0)[t[1], t[2]] == a[t[1], t[2]] c[2,2][R[1]] / c[2, 1],a(0,1)[t[1], t[2]] ==a[t[1], t[2]]2 Conjugate[a[t[1], t[2]]]c[3, 2] / c[3, 1] +a[t[1], t[2]] c[3, 3][R[1], R[2]] / c[3,1]},or in the usual notation:

1a =c2, 2(R1)

c2, 1

a ,

2a =c3, 2c3, 1

|a|2a + c3, 3(R1, R2)c3, 1

a . (33)

The coefficients cij of the amplitude equations aregenerated separately and, hence, can be calculatedand simplified separately. The coefficients c2, 1 =

c3, 1 = UU determine the normalization of the

eigenvectors.


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The coefficient in Eq. (26) 1 = c2, 2(R1)/c2, 1.Formulae for calculation of coefficients c are givenby the set of the replacement rules:

{c[2, 1] -> Ut . U,c[2, 2][R[1]] -> Ut . f(1,1)[u, R] . U .R[1]}.The second equation is a well-known Landau equa-

tion, and the coefficient at the nonlinear term 0 =c3, 2/c3, 1 is called the Landau coefficient. The signof this coefficient determines the type of the bifurca-tion the subcritical one with unstable limit cyclecoefficient arises if the real part of the coefficient ispositive, and the supercritical otherwise. The un-normalized Landau coefficient c3,2 is produced bythe function in the following form:

c[3, 2] -> -Ut . f(2,0)[u, R] . U .LinearSolve[f(1,0)[u, R], f(2,0)[u, R] .DiracConjugate[U] . U] -

Ut . f(2,0)[u, R] . DiracConjugate[U] .LinearSolve[-2 I w IdentityTensor[2] +f(1,0)[u, R],f(2,0)[ u , R ] . U . U ] / 2 +Ut . f(3,0)[u, R] . DiracConjugate[U] . U .U / 2 ,

and can be written in the standard notation as:

c3, 2 = UfuuUR(fu)fuuUU UfuuUR(2iwI + fu)fuuUU/2

UfuuuUUU

/2 , (34)where I denotes the identity matrix, whichcorresponds to the IdentityTensor[2] (ten-sor of the rank 2) generated by the functionBifurcationTheory . The R(m) denotes the re-solvent matrix of the matrix m if the matrix isregular, resolvent coincides with the inverse matrix,otherwise, resovent is constructed using the spec-trum of the original matrix m. A special functionResolvent[matrix] is designed for the construc-tion of the resolvent of a singular matrix.

Finally the linear term coefficient in the Lan-dau equation depends on parametric deviations ofthe first- and second-order:

c3, 3(R1, R2)

= (UR(iwI+fu)fuRUR1)(UfuRUR1)/(UU)+ UfuRUR2UfuRR(iwI+fu)fuRUR1R1+ UfuRRUR1R1/2 . (35)

It can be easily seen that the condition R1 = 0reduces the above general expression to linear coef-ficient 2 in (31) depending on R2 only.

The case of a monotonic bifurcation is morecomplicated, the resulting formulae are more cum-bersome, and we present the results of the Math-ematica calculations in the standard notation. Itmust be noted that it has been indicated in theprevious section that the first-order parametric de-viation R1 is automatically set to zero.

The set of two amplitude equations describingthe slow dynamics of the amplitude a now takes aform:

1a =c2, 2c2, 1

a2 +c2, 3(R2)

c2, 1,

2a =c3, 2c3,, 1

a3 +c3, 3(R2)

c3, 1a +

c3, 4(R3)

c3, 1.

(36)

Now the nontrivial amplitude equation appearsalready at the first time scale t1. It containsquadratic in amplitude term with unnormalized co-efficient given by:

c2, 2 = UfuuUU/2 , (37)

and corresponds to 0 in (12); the coefficient c2, 3coincides with 2 in (12).

The equation at the slower time scale gives cor-rections to the principal equation. Its Landau coef-ficient can be cast as follows:

c3, 2 = (UR(fu)fuuUU)(UfuuUU)/(2UU)

UfuuUR(fu)fuuUU/2+ UfuuuUUU/6 . (38)

The linear term coefficient is given as:

c3, 3(R2)

= UfuRUR2

+ (UR(fu)fuuUU)(UfuRUR2)/(UU) UfuuUR(fu)fRR2 .

(39)

Finally the free term in the equation coincides with3 in (19).


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2.5. Computation of coefficients

2.5.1. Function CalculateCoefficient

In order to perform the calculation of the coeffi-cient of the amplitude equations describing slowdynamics in the vicinity of a bifurcation point ofa particular problem it is needed to specify val-ues of all elements in the output of the functionBifurcationTheory , substitute them into the for-mulae and carry out required simplifications. Thelast two steps of this procedure is performed auto-matically by the function CalculateCoefficient .

CalculateCoefficient[coefrule , urule,Rrule, freqrule, funcrules, eigenrules,addrules] performs calculation of the rule for thecoefficient of the amplitude equations provided bythe function BifurcationTheory using the rulesspecifying the required data.

CalculateCoefficient has the following argu-ments:

coefrule denotes a rule for the coefficient of theamplitude equations produced by the functionBifurcationTheory;

urule specifies the names of the state vector com-ponents with their critical values at the bifurca-tion point;

Rrule specifies the names of the bifurcation pa-rameter vector components with their critical val-ues at the bifurcation point;

freqrule specifies the values, wavenumber andfrequency at the bifurcation point;

funcrules denotes an array of rules specifyingall functional and constant objects (array, matri-ces, tensors, etc.) appearing in the output of thefunction BifurcationTheory;

eigenrules denotes an array of rules specifyingvalues of the eigenvectors of the problem;

addrules denotes an array of additional rulesneeded for calculations (for example, restrictionson the parametric deviations); this argument canbe dropped.

The function CalculateCoefficient has the

only option ComplexTheory specifying the namesof the state vector components assumed to becomplex-valued. The default value of the optionis ComplexTheory -> None. Example of the usagein presented in Appendix A.

2.5.2. Lorenz model

The well-known Lorenz system [Lorenz, 1963], thathas been originally suggested as a qualitative model

of cellular convection, exhibits rich dynamic behav-ior including periodic and chaotic motion. It alsopossesses a monotonic bifurcation that correspondsto a primary transition from the quiescent state toconvection. In usual notation, the Lorenz system iswritten as

dx/dt =

(x

y) ,

dy/dt = x(R z) y ,dz/dt = bz + xy .

The linear analysis made with Mathematicawhich is presented in the Appendix A locates thebifurcation point at x = y = z = 0 and R = 1;the eigenvectors are U = {1, 1, 0} and U ={1/, 1, 0}. Calculations of the coefficients per-formed in Appendix A shows that the lowest-orderequation is trivial 1a = 0. The Landau equationhas the form:

2a =

1 + (a3/b + aR2) .

The result proves that the bifurcation is alwayssupercritical at physical (positive) values of theparameters.

2.5.3. Brusselator model

The next example is a well-known Brusselatormodel of chemical oscillations [Nicolis & Prigogine,1977]:

dz/dt = a (1 + b)z + uz2 ,du/dt = bz uz2 .

The real variables u, z denote concentrations of theactivator and inhibitor species. The uniquestationary solution z = a, u = b/a bifurcates intothe limit cycle at b = 1 + a2. Calculations can b emade only after elimination of the parametric de-pendence of the basic solution made by the shiftingof the basic solution to zero. The shifted system

has the following form:

dz/dt = a2u z + bz + 2auz + bz2/a + uz2 ,du/dt = a2u bz 2auz bz2/a uz2 .

The linear analysis of the problem is presented inAppendix B. The eigenvectors at the Hopf bifur-cation point are U = {a/(i a), 1} and U ={(i + a)/a, 1}.


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The calculation produces the following resultsfor the coefficients in the first equation from (33):

c2, 1 =2

1 + ia;

c2, 2(R1) =b1

1 + ia. (40)

Setting b1 = 0 one can proceed to the nextorder:

c3, 1 =2

1 + ia;

c3, 2 =4 6ia 7a2 3ia3 + 4a4

3ia(1 + a2)(1 + ia);

c3, 3(R1, R2) =b2

1 + ia. (41)

The Landau coefficient at the nonlinear termfinally takes a form

4 6ia 7a2 3ia3 + 4a46ia(1 + a2)

;

it is easy to see that its real part simplifies to(1 + 1/2a2)/(1 + a2), and is negative definite,hence, the bifurcation is always supercritical.

2.5.4. Exothermic reaction ina stirred tank reactor

The following example involves somewhat heaviercomputations, and necessitates the use of implicit

functions. Consider a dynamical system describingan exotermic reaction in a continuous stirred tankreactor [Uppal et al., 1974; Pismen, 1986]:

dx/dt = (1 x)ey mx ,g dy/dt = h(1 x)ey my .

(42)

The variables x, y denote the conversion and dimen-sionless temperature, respectively; h is the exother-micity parameter, m is the dimensionless flow rate,and g is the thermal capacitance factor that equalsto unity in an adiabatic reactor and decreases with

intensified cooling. This model exhibits both mono-tonic and Hopf bifurcations. Equilibria of Eq. (42)do not depend on the parameter g, so conditions ofa monotonic bifurcation can depend on two inde-pendent parameters m, h only; the maximal codi-mension is two (a cusp point).

Monotonic bifurcation. Consider first a bifur-cation manifold of codimension one (a fold line)

for a simplified model with g = 1. We localizethis bifurcation as before the bifurcation locusparametrized by the stationary value ys of the vari-able y can be found with the help of the standardMathematica function Solve. This parametrizationis advantageous, since if one of the actual param-eters h or m was used, either two or no solutionswere obtained in different parametric domains, and

the form of the solution was more complicated. Themanifold is given as:

x = 1 1ys

, m =eys

ys 1 , h =y2s

ys 1 .

The admissible values of ys are restricted by thecondition ys > 1. The eigenvectors of the prob-lem can be written as U = {(1 + ys)/y2s , 1} andU = {ys, 1}. The calculation process is similarto that of the preceding subsections with only ex-

ception the bifurcation parameters vector nowhas two components h and m. The lowest-orderamplitude equation takes a form:

1a =eys(2 ys)2(ys 1) a

2 + eysh2 ysm2 . (43)

One can proceed further in order to find correc-tions (parametric deviations of the third-order areset to zero):

2a =eys(3

2y

s)

6(ys 1) a3

+

eys(2 2ys + y2s)

2y2sh2 m2

a . (44)

Cusp point. The loci of fold bifurcation in theparametric plane h, m, drawn as a parametric plot,are shown in Fig. 1. The two branches join witha common tangent at the cusp point. In the vicin-ity of this point, the applicable amplitude equation

will include a restriction on deviations of both pa-rameters that specify the direction of this commontangent. It can be easily seen that the quadraticterm in the lowest-order Eq. (43) vanishes at ys = 2where the cusp point is located. The equation be-comes trivial at h2 = 2m2/e

2. This relation definesa direction in the parametric space along which thesecond-order deviation is made. The next order de-viation must be orthogonal to this direction. In the


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5 6 7 8 9 10 11m

3.25

3.5

3.75

4.25

4.5

4.75

5

h

0.4

0.5

0.6

Fig. 1. Loci of fold and Hopf bifurcation in the parametric plane h, m. The three Hopf curves correspond to g = 0.4, 0.5and 0.6. Solid gray lines show the supercritical, and dashed lines, the subcritical bifurcation. Outside the cusped region, theunique stationary state suffers oscillatory instability within the loop of the Hopf bifurcation locus. Within the cusped region,there are three stationary states, of which two, lying on the upper and lower folds of the solution manifold, may be stable.The solutions on the upper fold are unstable below the Hopf bifurcation line.

vicinity of this point the Landau equation is castinto (m3 is set to zero):

2a = e2

6a3 + m2a + e

2h3 . (45)

Hopf bifurcation. The dynamical system (42) canalso undergo a Hopf bifurcation. In the operator

form it can be written as:

G(R)ut = f(u) , (46)

where G(R) denotes a capacitance matrix.The function BifurcationTheory called for

this type of the problem produces the set (33) ofthe amplitude equations (at the Hopf bifurcationpoint) with the following formulae for the coeffi-cients of the first amplitude equation:

c2, 1 = UGU ;

c2, 2(R1) = iwUGRUR1 . (47)The Landau coefficient in the second equation isgiven by (35) with the only replacement of the iden-tity matrix I by the capacitance matrix G. Expres-sion for the coefficient c3, 3(R1, R2) of the linearterm is not shown due to its complexity.

We start the computations from the determina-tion of the Hopf bifurcation manifold. It contains

stationary points of the system at which a matrixfuiwG is singular (w denotes the frequency of thelimit cycle). The manifold is given by the followingset of equations:

ey(h y) my = 0 ,ey(g + y h) + m(1 + g) = 0 .

These equations are solved to express the values

of the parameters m and h through the station-ary value y = y0 and the remaining parameterg = g0; y0, g0 are thus chosen to parameterize the2D Hopf bifurcation manifold in the 3D parametricspace:

h = y0 +g0y0

y0 g0 1 ,

m =g0e

y0

y0 g0 1 .

The loci of Hopf bifurcation in the parametric plane

h, m at several chosen values of g are shown inFig. 1. Outside the cusped region, the unique sta-tionary state suffers oscillatory instability withinthe loop of the Hopf curve (this is possible atg < 1/2 only). Within the cusped region, the solu-tions on the upper fold are unstable below the Hopfbifurcation line. The solutions at the lower foldcan also suffer oscillatory instability at lower valuesof g.


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Now it is possible to compute the frequencyw = ey0

y0 g0y0 1/(1 + g0 y0) and the eigen-

vectors:

U =

g(1 y0 i

y0 g0y0 1 )

y0(y0 1) , 1

;

U

=

(y0 1)g0(1

y0 + i

y0

g0y0

1 )

1 + g0 y0 , 1 .(48)

As it was done above, one can choose the para-metric deviation of the first-order in such a way asto make the first amplitude equation trivial. Thenthe Landau coefficient in the second equation canbe presented after normalization as:

c3, 2/c3, 1 = (y0 g0y0 1)3/2(iy0 i

y0

g0y0

1 )1ey0/12

(12 24g0 24y0 + 40g0y0 4g20y0+ 14y20 21g0y20 + 4g20y20 2y30 + 2g0y30+ i

y0 g0y0 1(12 + 24g0 + 15y0 25g0y0 2g20y0 5y20 + 8g0y20)) .

(49)

First, we take note that only a part of the para-metric plane y0, g0 above the thick solid curve inFig. 2 is actually available, since the condition of

positive oscillation frequency requires

y0 g0y0 1 > 0 . (50)

Within this region, the limit cycle is stable (the bi-furcation is supercritical) if the real part of the coef-ficient at the nonlinear term is negative. Extractingthe real part from (49) brings this condition to theform

3g0 + 2g20 + 2y0 4g0y0 2g20y0 y20 + 2g0y20 1

4(y0 1 g0)(y0 g0y0 1)< 0 . (51)

Noting that the denominator of the above fractionis positive whenever the inequality (50) is verified,one can determine the stability boundary by equat-ing the numerator in (51) to zero, and combiningthe result with (50). The stability boundary in theplane (g0, y0), separating the regions of subcriti-cal and supercritical bifurcation is shown in Fig. 2;

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7g0

0.5

1

1.5

2

2.5

3

3.5

4

y0

sub

sub

super

Fig. 2. The stability boundary in the plane (g0, y0), sepa-rating the regions of subcritical and supercritical Hopf bifur-cation. The unphysical part of the curve below the boundaryof positive frequency is shown by a dashed line. The solidcircle at g0 = 0.5, y0 = 2 marks the double zero point.

the unphysical part of the curve dipping below the

boundary of positive frequency is shown by a dashedline. The region of subcritical bifurcations consistsof two disconnected parts, of which the lower onelies within the region of unique stationary statesand on the lower fold of the solution manifold, andthe upper one, on the upper fold in the region ofmultiple solutions.

3. General Algorithmfor Distributed Systems

3.1. Introduction

The main point of our approach can be formulatedas follows. An algorithm applicable to the nonlin-ear analysis of a number of representative realisticproblems must be as general as possible, i.e. it mustbe designed for classes of problems (say, dynamicalproblems, or reactiondiffusion type problems), oreven better, it should embrace all problems whichcan be solved using standard methods of bifurcationexpansion.

It is clear that the generalization from particu-lar problems to some class of such problems can beperformed using an operator form of the problem.For example, all reactiondiffusion type problemscan be presented in the following operator form:

G(R)

tu(r, t) = D(R)( )u(r, t)

+ f(u(r, t), R) , (52)


14/26


where /t denotes the time differentiation opera-tor, stands for the operator of differentiation overthe spatial variable r; f , u , R are arrays of func-tions, variables and bifurcation parameters of theproblem. G(R) and D(R) denote capacitance anddiffusion matrix respectively, which can depend onthe bifurcation parameters.

In its turn problem (52) appears to be a par-

ticular case of most general problem which can bewritten as

F(, /t, u(r, t, R), R) = 0 , (53)where F can also include a set of boundary condi-tions imposed on the solution u(r, t, R).

3.2. Linear analysis anddispersion relation

The general standard algorithm for derivation of

amplitude equations starts from the linear analy-sis of an underlying distributed system in the vicin-ity of a basic state. We assume also that for eachvalue R the system admits a stationary spatiallyhomogeneous solution u0(R), which is called ba-sic state. Typically the basic state becomes un-stable in a certain domain of the parametric space,and this instability is usually connected with thebifurcations of new solutions with a more compli-cated spatiotemporal structure. We construct so-lutions of the original problem (53) in the formu = u0(R) + u(r, t, R), where u is a small distur-

bance of the basic state. Substituting the solutionin (53) one arrives in the linear approximation in uto the following:

L(, /t, u0(R), R)u = 0 , (54)where L is a linear operator (Frechet derivative)calculated at the point u = u0 acting on small dis-turbance u. Further on, we shall consider normaldisturbances of the form:

u(r, t, R) = A(k, R)exp(t + ikr) . (55)

The growth rate is generally a complex number.For normal disturbances the evolution equation (54)is reduced to the eigenvalue problem:

L(ik, , u0(R), R)A = 0 , (56)which usually determines a countable set ofbranches of dispersion relation between growthrates n and the wavenumber k = |k|. We assume

that the instability is generated by eigenmodes withthe growth rates j(k, R) = jr(k, R) + ij(k, R).The conditions jr (k, R) = 0 determine instabilityboundaries which are called neutral stability curves.The minima of all functions jr(k, R) are reachedat the same value R = R0 and at (possibly various)values k = kjc .

If j(kjc , R0) = 0, the corresponding insta-

bility is called stationary (monotonous); otherwise,there is an oscillatory (wavy) instability. In eachof these cases, two possibilities arise dependingon the wavenumber critical value: Either the in-stability arises near a nonzero wavenumber kjc(short-wavelength instability), or the instability do-main is localized around kjc = 0 (long-wavelengthinstability).

The linear analysis procedure also includes de-

termination of eigenvectors Uj, Uj of the linear

problem (56) verifying the equation LUj = 0 and

its adjoint counterpart LU

j = 0 needed for fur-ther calculations. Setting aside most complicatedcases of double and triple bifurcation points we,however, permit simple degenerate cases, when eachinstability mode is characterized by a unique set ofwavevector and frequency values.

A general solution ul of the linear problem (54)can be written as a superposition of several normalmodes with jth mode characterized by its scalaramplitude aj, particular values of the wavevectorkj , time frequency j, eigenvector of the mode Ujand its order of smallness dj :

ul = d0a0U0 +

Nj=1

dj (ajUj exp(ikjr0 + ijt0)

+ ajUj exp(ikjr0 ijt0)) , (57)where denotes an operation of the complex con-jugation and denotes an operation of Dirac con-jugation, which in the case of real vector functionF is reduced to the standard complex conjugation.The coefficient a0 = 0 if monotonic long-scale modecharacterized by the zero value of the wavenumberand time frequency is permitted. Each mode givesrise to a corresponding solvability condition whichdetermines a slow spatiotemporal dynamics of themodes amplitude using the consequent orders ofthe multiscale expansion of the problem.

3.3. Multiscale expansion

In order to simplify derivations, we shall eliminatetrivial parametric dependence of the basic state by


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transforming to a new variable u = uu0(R). Theresulting system, F(, /t, u+u0, R) = 0 has thesame form as (53) but contains a modified operator

F(, /t, u, R) = F(, /t, u + u0, R). Since,by definition, u0 satisfies F(, /t, u0, R) = 0,u = 0 is a zero of F(, /t, u, R). Now we candrop the hats over the symbols and revert to theoriginal form (53) while keeping in mind that u = 0is a stationary solution for all R and, consequently,all derivatives FR, FRR, etc. computed at u = 0vanish.

We use the expansions (2) for variables and pa-rameters and introduce a hierarchy of time scalestk, thus replacing the function u by a function ofan array of rescaled time variables. Accordingly,the time derivative is expanded as in (3). The spa-tial derivative is expanded similarly:

= 0 + 1 , (58)

where denotes the spatial scaling which dependson a particular problem; for the sake of simplicity,we shall use only positive integer values of . Sub-stituting expansions (2) and (3) and (58) into theoriginal problem (53) and expanding in , one ar-rives at a set of equations for different orders of .

In the lowest-order we reproduce the equationF(0, 0, u0, R0) = 0 determining the basic solu-tion computed at the critical values of bifurcationparameters. In the next order the linear problem(54) is reproduced in the following form:

Fu(0, 0, u0, R0)u1 = 0 . (59)

The solution u1 consists of normal modes withdj = 1.

In the consequent orders one arrives at theequations of the form:

Fu(0, 0, u0, R0)un = gn , (60)

where gn denotes the nth order inhomogeinity vec-tor. A set of solvability conditions for each of thenormal modes appearing in the linear solution (57)

will define slow spatiotemporal dynamics of the am-plitude of each mode. It can be shown that onlypart of the inhomogeinity gn which projects onthe principal harmonic of a certain mode will con-tribute to the corresponding solvability condition.Denoting the scalar product by angle brackets andthe projection operator on the harmonic exp(i)as P(, exp(i)), one can write the solvability con-dition for jth normal mode in the nth order as

follows:

Uj , P(gn, eikjr0+ijt0) = 0 , (61)

where Uj denotes the eigenvector of the adjoint lin-

ear problem LUj = 0 corresponding to the jthmode.

The linear inhomogeneous problem (60) mustbe solved with respect to un provided solvabilityconditions (61) are satisfied. To this particular so-lution of (60) one must add a linear solution termof the corresponding order of smallness. The com-bined solution is used for the calculations of thenext order inhomogeinity gn+1.

In the second-order the vector of inhomogeinityg2 can be represented in the form:

g2 = 12

Fuu(0, 0, u0, R0)u1u1

FuR(0, 0, u0, R0)u1R1 , 1Fu(0, 0, u0, R0)1u1 F

ut

(0, 0, u0, R0)1u1 , (62)

where the Kronecker symbol , = 0 at = and , = 1. The resulting set of the conditions inthe second-order is resolved further with respect tothe derivative of the amplitudes 1aj. This resultpresents the set of the amplitude equations of thesecond-order and in its turn is used for derivationof the amplitude equations in the next orders.

The final result is the set of the amplitude equa-tions written in most general form which is valid forthe combination of the normal modes in (57) andspecified value of the spatial variable scaling expo-nent . This set further can be projected ontoa particular class of the problems to produce a re-quired formula. These formulae are applicable toany problem from the selected class. In order tomake actual calculations for a particular problemone must determine the values of the wavevectors,

frequency, eigenvectors, etc. and substitute theminto the abovementioned formulae.

3.4. Function BifurcationTheory

The function BifurcationTheory used for deriva-tion of the normal forms for dynamical systemscan be applied with some additions to distributedsystems.


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If it is required to derive the normal form fora single mode arising at a bifurcation point of thedistributed system, one may use the call

BifurcationTheory[operatoreqn , u, R, t,{r, dim}, spatscale, {U, U},amp, coef, eps, order, wavenumber, freq,opts].

Here r denotes a spatial variable (that can be avector); dim is the space dimension and spatscaledenotes an array consisting of the name of the spa-tial variable and its (integer) scaling exponent .Finally, wavenumber must be set equal to zero fora long-scale instability and to some symbol in theshort-scale case. In the latter case the wavevectorneeded for the calculations is generated automat-ically. The differentiation operators /t and used in the operator equation operatoreqn are rep-resented by the symbols Nabla[t] and Nabla[r],respectively.

A more complicated form must be used in a casewhen there is an angular degeneracy of the normalmodes, namely, when the different modes have thesame values of their parameters but the wavevectorswith equal lengths have different directions. It is ra-tional to specify the mode data using the angularmeasurment for the wavevectors

BifurcationTheory[operatoreqn , u, R, t,{r, dim}, spatscale, {U, U},amps, coef, eps, order, wavenumber, freq,ampout, opts],

where amps consists of the pairs {aj , j} with jdenoting an angle between the wavevector of thejth mode aj and some selected direction. If theargument ampout specifying the names of the am-plitudes for which equations must be generated isomitted, equations for first amplitude only from thearray amps would be produced.

The most general form of the function is usedfor algebraically degenerated cases:

BifurcationTheory[operatoreqn , u, R, t,

{r, dim

}, spatscale, amps,

coef, eps, order, ampout, options]

derives a set of amplitude equations for a dis-tributed system of partial differential equationswritten in an operator form. Here amps is an arrayof elements specifying the normal modes appearingin the linear solution (57). Each item of amps isan array of six elements in the following order amplitude of the mode (aj), mode wavevector (kj),

time frequency (j), eigenvector (Uj), eigenvector

of the adjoint problem (Uj), and order of smallnessof the normal mode (dj). ampout again denotes anarray of the amplitude names for which the ampli-tude equations must be generated.

4. Amplitude Equations for

ReactionDiffusion Problems

4.1. Long-scale instabilities

In this and the next subsections we present the re-sult of derivation of the amplitude equations forreactiondiffusion problem (52) with capacitancematrix G equal to identity matrix I and diffusivitymatrix D which does not depend on the bifurcationparameters.

We start from the case where the instability do-main is localized around kc = 0 restricting ourselves

to nondegenerated nonresonant cases.The operator equation corresponding to theproblem may be written as:

DD.(Nabla[r] . Nabla[r])u + f[u, R] -Nabla[t]u = = 0where DD denotes the diffusivity matrix. Opera-tor Nabla[r] is assumed to be vectorial, hence thescalar Laplacian is written as scalar product of twodifferential operators. The function is called asfollows:

BifurcationTheory[

DD.(Nabla[r].Nabla[r])**u + f[u,R] -

Nabla[t]**u == 0,

u, R, t, {r,2},{r,1},{U,Ut}, a, c, eps, 2,0, 0]

Unfortunately, the version 2.2.2 of Mathematicaused in development of the package has no per-fect typesetting abilities like the version 3.0. Asa result the output of the function is very lengthyand is difficult to understand for the inexperienceduser. Here and further, we present output producedby the function BifurcationTheoryusing standard

textbook notation.The result produced by the function coincides

with that of the monotonic bifurcation of the dy-namical system (36) with the only exception theLandau equation is converted into the GinzburgLandau equation with an additional diffusionalterm of the form:

UDU

UU(11)a . (63)


17/26


Now we turn to the case of the Hopf bifurca-tion for the same problem. The last argument of thefunction BifurcationTheory denoting the limit cy-cle frequency now is set to w rather than zero:

BifurcationTheory[


Nabla[t]**u == 0,

u, R, t, {r,2},{r,1},{U,Ut}, a, c, eps, 2,0, w]

As in the previous case the resulting set of equa-tions is given by (33) with the addition of the samediffusional term.

4.2. Turing bifurcation

Short-scale instabilities are characterized by anonzero value of the wavenumber; hence, severalmodes with different directions of the wavevector

are permitted.This case is characterized by the nonzero value

of the wavenumber and zero frequency, what is re-flected in the function is called:

BifurcationTheory[


Nabla[t]**u == 0,

u, R, t, {r,2},{r,1},{U,Ut},{{a1,0}, {a2, alpha}}, c, eps, 2, k, 0]It corresponds to the case of two normal modes witha wavenumber equal to k a wavevector k1 of thefirst mode a1 is directed along the x-axis in a plane(the dimension of spatial variable is set equal to2); the wavevector k2 of the second mode a2 makesthe angle with k1. We assume that this angle isnot equal to /3 or 2/3 in order to make thesetwo modes nonresonant. The argument specifyingnames of the amplitudes for output is omitted, sothe equations for amplitude a1 are only generatedby default.

The result may be written as:

1a1 =c2, 1, 1c2, 2 (ik11)a1+

c2, 3(R1)

c2, 2 a1 ,

2a1 =c3, 4c3, 3

|a2|2a1+ c3, 5c3, 3

|a1|2a1

+c3, 6(R1, R2)

c3, 3a1+

c3, 1, 1c3, 3

(11)a1

+c3, 1, 2c3, 3

(ik11)2a1+ c3, 2, 1(R1)c3, 3

(ik11)a1 .

(64)

The coefficients c2, 2 = c3, 3 = UU determine

a normalization of the eigenvectors.For the first time scale t1 we have a linear equa-

tion. Its linear term coefficient is proportional tothe first-order deviation of the bifurcation param-eter vector and is given by (26). The differentialterm has a coefficient of the form

c2, 1, 1 = 2U

DU . (65)

Choosing the parametric deviation R1 one may setthe real part of the linear term coefficient equal tozero; then the resulting equation will admit a trav-eling wave solution.

The GinzburgLandau equation at the slowertime scale is most interesting. It contains two non-linear terms describing interaction between the twomodes as well as the self-interaction of the principalmode a1. Interaction coefficient c3, 4 of two modesdepends on the angle between their wavevectors

and is calculated using the formula:c3, 4 = UfuuUR(fu)fuuUU

UfuuUR(fu 2k2D(1 cos ))fuuUU UfuuUR(fu 2k2D(1 + cos ))fuuUU+ UfuuuUUU . (66)

The self-interaction coefficient c3, 5 may befound as half of the value ofc3, 4 calculated at = 0:

c3, 5 =

UfuuU

R(fu)fuuUU

UfuuUR(fu 4k2D)fuuUU/2+ UfuuuUUU/2 . (67)

The linear term coefficient is supplied by (63)with replacement ofR(fu iwI) by R(k2D + fu).

There are three diffusional terms in theGinzburgLandau equation; one is given by (63),another term coefficient c3, 2, 1 depends on the para-metric deviation of the first-order and usuallycan be removed by the appropriate choice of thisdeviation:

c3, 2, 1(R1)

= 2(UR(k2D+fu)fuRUR1)(UDU)/(UU) 2UDR(k2D+fu)fuRUR1+ 2(UR(k2D+fu)DU)(UfuRUR1)/(UU)+ 2UfuRR(k2D+fu)DUR1 . (68)


18/26


The coefficient c3, 1, 2 independent of the para-metric deviation has the following form:

c3, 1, 2 = 4(UR(k2D + fu)DU)(UDU)/(UU)

4UDR(k2D + fu)DU . (69)4.2.1. Hopf bifurcation

Now we turn to the case of the Hopf bifurcation forthe same problem. The resulting set of equationsis given by (65) with slightly changed formulae forthe coefficients in the GinzburgLandau equation.

The modes interaction coefficient c3, 4 is calcu-lated as shown in (66) with the only replacement ofR(fu2k2D(1+cos )) by R(fu2k2D(1+cos )2iwI), where I stands for the identity matrix. Sim-ilarly, in the expression (67) for the self-interactioncoefficient one must replace R(fu4k2D) by R(fu4k2D 2iwI).

The linear term coefficient again is supplied by

(39) with the replacement ofR(fu) by R(fuk2DiwI).

The diffusional coefficient c3, 2, 1 depending onthe parametric deviation is found as (69) with thereplacement ofR(fu k2D) by R(fu k2D iwI).The same replacement is valid for another diffu-sional coefficient c3, 1, 2.

In order to perform calculation of the coeffi-cients of the amplitude equations describing slowdynamics in the vicinity of a bifurcation point ofa particular problem, values of all elements of theoutput of the function BifurcationTheory need to

be specified, substitute them into the formulae andmake a simplification if needed. In this section weillustrate the usage of the above results using twosimple models.

4.3. Brusselator model

The first example is a well-known Brusselator model[Nicolis & Prigogine, 1977] of chemical oscillations:

z/t = a (1 + b)z + uz2 + ()z ,u/t = bz

uz2 + d(

)u .

The variables u, z denote concentrations of the ac-tivator and inhibitor species. This system al-ways has a unique stationary homogeneous solutionz = a, u = b/a. The dispersion relation is found asfollows:

a2 + a2k2 + dk2 bdk2 + dk4

+ iw(1 + a2 b + k2 + dk2) w2 = 0

Now we can trace two possibilities the first is formonotonic bifurcation with frequency w = 0 andthe second for Hopf bifurcation with nonzero w.

4.3.1. Monotonic bifurcation

We start from the determination of the bifurcationcurve describing a dependence of the bifurcation pa-

rameter b on the wavenumber k and then we find theminimum of this curve bcm = (1 + a/

d)2 reached

at kcm = (a2/d)1/4.

It can be seen that the monotonic bifurca-tion corresponds to the short-scale instability. Theeigenvectors of the problem can be found as:

U =

ad

a +

d, 1

;

U =

a +

d

a, 1

.

(70)

Calculation of the coeffcients of amplitudeequations (65) shows that the coefficient c211 of thelinear differential term in the first equation is equalto zero identically, the linear term is proportionalto the bifurcation parameter deviation b1:

1a1 = d

db1

(a +

d)(1 d) a1 .

Setting b1 = 0 we can trivialize the above equation.In the next order equation some of the coeffi-

cients vanish: c3, 2, 1(0) = 0; c3, 1, 1 = 0. The coeffi-cient of the only nonzero diffusional term in (65) isgiven by:

c3, 1, 2 =4d2

a(a +

d),

the linear term coefficient is proportional to second-order parametric deviation:

c3, 6(R2) = d

db2

a +

d.

The dependence of the modes interaction coefficienton is provided by:

c3, 4 =2d2

a(a +

d)2(4cos2 1)2 (8a3 + 7a2

d + 18ad 8d

d

+ 2a2

d(2cos2 + cos 4)+ 4ad(4cos 2 + cos 4)) .

(71)


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It can be easily seen that this coefficient di-verges at cos = 1/2, i.e. at = /3, 2/3. Thereason of the divergence is the existence of the res-onance between the two modes, and for proper de-scription one needs to incorporate the third modewith a wavevector k3 such that k1 + k2 + k3 = 0.The resonant cases are considered below. Finallywe present the self-interaction coefficient:

c3, 5 =d2

9a(a +

d)2(8a3 + 5a2

d + 38ad8d

d) .

(72)

4.3.2. Oscillatory bifurcation

The minimum of the bifurcation curve bco = 1 + a2

is found at kco = 0, which means that the oscilla-tory bifurcation with frequency of the limit cycle isw = a that corresponds to the long-scale instability.The eigenvectors of the problem can be found as:

U = a

a i , 1

;

U =

a + i

a, 1

. (73)

The calculation produces Eqs. (33) with co-efficients given by (40) and (42) with the addi-tional diffusional term with coefficient equal to((d + 1) + ia(d 1))/2.

It can also be noted here that by equating thecritical values of the bifurcation parameter b formonotonic and oscillatory cases it is possible to findthe critical value ac = 2d/(d 1) at which the bi-furcation is degenerate, i.e. both Hopf and Turingmodes are permitted. This degenerate case will bedescribed below.

4.4. Two-level laser model

The simple model of a two-level laser [Coullet et al.,1989] describes the dynamics of the electric field in-teracting with an active media. The model equa-tions are the following:

At = ia()A + (P A) ,Pt = (1 + i)P (R n)A , (74)nt = bn + 1/2(AP + AP) .

The variables A, P, n denote complex amplitudeof the electrical field, complex media polariza-tion density and real atomic population inversion,respectively.

This model exhibits short-scale oscillatory in-stability in the vicinity of the basic nonlasing solu-tion A0 = P0 = n0 = 0. It must be noted that inorder to make calculations one need to complete thesystem (75) with complex conjugation of first twoequations for A, P. As a result the state vector{A, A, P, P, n} appears to have five componentsinstead of the original three.

4.4.1. Oscillatory bifurcation

The bifurcation curve has a form:

R = 1 +

ak2

1 +

2. (75)

The lowest minimum (for positive a and )of this curve lies at kc =

/a and it equals to

R0 = 1, the critical frequency wc = . The eigen-vectors of the problem are given by:

U = {0, 1, 0, 1, 0} ; U =

0,1

, 0, 1, 0

.

(76)

In this case one does not need to shift the basicsolution because it does not depend on the pa-rameters of the problem. For the calculation ofthe coefficients of the amplitude equations in thiscase one needs additional preparatory step due tothe complex nature of the original problem. In theformulae for the coefficients for short-scale insta-bilities one may find the notion U denoting theso-called Dirac conjugated eigenvector U. Thesevectors present particular values of the state vec-tor, which in our case has two complex conjugatedvariable components. The construction of the vec-tor U out of U may be written symbolically in theform:

U = G(U) , (77)where denotes usual complex conjugation andG is an operation of mutual interchange of the vec-tor components corresponding to the complex con-jugated variables. In our case the complex conju-gated variables A, A have to be the first positionsin the state vector, while P, P are at the nexttwo places. As a result the vector U is constructedas:

U = {1, 0, 1, 0, 0} .The lowest-order equation for one of two ex-

cited modes with amplitudes a1, a2 and wavevectors


20/26


k1, k2 is written as:

1a1 = 2a(k11)a1 + a1R11 +

, (78)

and may be simplified by setting R1 = 0.It appears that the term describing the mode

mode interaction in the GinzburgLandau equation

in our case does not depend on the value of the anglebetween mode wavevectors and is given by c3, 4 =2/b; the self-interaction coefficient c3, 5 = 1/b.Two out of three diffusional coefficients in the sec-ond amplitude equation are not zeros:

c3, 1, 1 = ia

; c3, 1, 2 = 4a2

(1 + )(1 + i) .(79)

Finally the amplitude equation may be written inthe form:

2a1 = b(1 + )

(2|a2|2 + |a1|2)a1

4a2

(1 + )(1 + i) (k11)2a1

ia1 +

(11)a1 + R2(1 + )

a1 . (80)

5. Amplitude Equations forConvective Problems

5.1. Convective instabilities

The convective problems are characterized by thepresence of the additional convective term inEq. (52) and can be written in the following form[Rovinsky & Menzinger, 1992, 1993]:

tu(r, t) = D( )u(r, t) + V(n )u(r, t)

+ f(u(r, t), R) , (81)

where n denotes a constant vector determining thedirection of flow and the matrix V sets the veloc-ities values; it is assumed that the velocities areof the order . Then for the Turing instabilityone must call the function BifurcationTheory asfollows:

BifurcationTheory[

DD.(Nabla[r].Nabla[r])**u + f[u,R] +eps V.({1,0}.Nabla[r])**u -Nabla[t]**u == 0,

u, R, t, {r,2},{r,1},{U,Ut},{{a1,0}, {a2, alpha}}, c, eps, 2, k, 0]Here the velocity direction is chosen parallel to thex-axis. Note that the small parameter of expansioneps in this case appears explicitly in the operatorequation describing the problem. The result pro-duced may be written as:

1a1 =c2, 1, 1c2, 2 (ik

11)a1 +c2, 3c2, 2 a

1 +c2, 3(R1)

c2, 2 a1 ,

2a1 =c3, 4c3, 3

|a2|2a1 + c3, 5c3, 3

|a1|2a1 + c3, 6c3, 3

a1

+c3, 6(R1, R2)

c3, 3a1 +

c3, 1, 1c3, 3

(11)a1

+c3, 1, 2c3, 3

(ik11)2a1 + c3, 2, 1c3, 3

(ik11)a1

+c3, 2, 1(R1)

c3, 3(ik11)a1 + c3, 2, 2

c3, 3(n1)a1 .

(82)

It can be noted that the set of equations is verysimilar to Eq. (65) with the addition of some terms.Here we write the expressions for the coefficientswhich are changed or added comparing with coef-ficients of (65). The additional linear term in thefirst equation is due to the velocity matrix V andits coefficient is given by:

c2, 3 = ikUVU .

Similarly, there are two linear terms in the secondequation, their coefficients now depend on V:

c3, 6 = k2UVR(fu k2D)VU k2(UVR(fu k2D)VU)(UVU)/(UU) ,

c3, 6(R1, R2) = (UR(fu k2D)fuRUR1)(UfuRUR1)/(UU) + UfuRUR2 UfuRR(fu k2D)fuRUR1R1 + UfuRRUR1R1/2 ikUVR(fu k2D)fuRUR1 + ik(UVR(fu k2D)fuRUR1)(UVU)/(UU)+ ik(UVR(fu k2D)VU)(UfuRUR1)/(UU) ikUfuRR(fu k2D)VUR1 .

(83)


21/26


There is also additional differential term with the coefficient given by:

c3, 2, 1 = 2ik(UR(k2D + fu)VU)(UDU)/(UU) 2ikUDR(k2D + fu)VU

+ 2ik(UR(k2D + fu)DU)(UVU)/(UU) 2ikUVR(k2D + fu)DU . (84)The long-scale Hopf instability case is solved by following lines:

BifurcationTheory[DD.(Nabla[r].Nabla[r])**u + f[u,R] +eps V.({1,0}.Nabla[r])**u- Nabla[t]**u == 0,u, R, t, {r,2},{r,1},{U,Ut}, a, c, eps, 2, 0, w]The resulting GinzburgLandau equation reads:

2a =c3, 4c3, 3

|a|2a + c3, 5(R1, R2)c3,3

a +c3, 1, 1c3, 3

(11)a + c3, 2, 1c3, 3

(n1)a . (85)

Here are the coefficients of the equation:

c3, 3 = UU ,

c3, 4 = UfuuuUUU/2 UfuuUR(fu)fuuUU

UfuuUR(fu 2iwI)fuuUU/2 ,c3, 1, 1 = U

DU ,

c3, 2, 1 = UVU . (86)

The linear term coefficient c3, 5(R1) is given byEq. (36).

6. Resonant and Degenerate Cases

In this section we review the most difficult cases ofbifurcation analysis resonant and algebraicallydegenrate bifurcations of distributed systems. Weconsider the three-mode resonance in the reactiondiffusion problem and demonstrate the ability of thefunction BifurcationTheory to cope with degener-ate case of simultaneous existence of the Hopf andTuring bifurcations. A simple example of calcula-tion of the amplitude equation coefficients for Brus-selator model is presented. Some nontrivial exam-

ples of usage the function BifurcationTheory fordescription of resonant patterns in nonlinear opticsare found in [Rubinstein & Pismen, 1997, 1998].

6.1. Three-wave resonance inturing bifurcation

The short-scale instabilities can demonstrate reso-nance of normal modes, i.e. resonance of their fre-

quencies wj and wavevectors kj:

j

wj = 0;

j

kj = 0 . (87)

The maximal number of the resonant modes is notlimited, the minimal is equal to three. The sim-plest resonant situation arises with three monotonicmodes with the wavevectors kj satisfying the con-dition: k1 + k2 + k3 = 0, i.e. the angle between theadjacent modes is 2/3. If two out of these threemodes are chosen, calculations show that the coef-ficient of the modes interaction term given by (66)

diverges [cf. Eq. (72)], and, hence, the third modemust be incorporated in order to describe the reso-nance properly.

6.1.1. General results

The function BifurcationTheory enables to gen-erate the normal forms for resonances, in this casethe values of the angles specifying the directions ofthe wavevectors must be set explicitly:

BifurcationTheory[


Nabla[t]**u == 0,

u, R, t, {r,2},{r,1},{U,Ut},{{a1,0}, {a2, 2 Pi/3}, {a3, 4 Pi/3}}, c,eps, 2, k, 0]

Here we present the results produced by the func-tion BifurcationTheory for the three modes reso-nance. These formulae may appear helpful for cal-culations even by hand.


22/26


The normal forms for the amplitude a1 is given by (54) with additional resonant terms:

1a1 =c2, 1, 1c2, 2

(ik11)a1 + c2, 4(R1)c2, 2

a1 +c2, 3c2, 2

a2a3 ,

2a1 =c3, 7c3, 5

|a3|2a1 + c3, 9c3, 5

|a2|2a1 + c3, 11c3, 5

|a1|2a1 + c3, 6(R1)c3, 5

a2a3 +

c3, 12(R1, R2)

c3, 5a1

+

c3, 1, 1

c3, 5 (11)a1 +c3, 1, 2

c3,5 (ik11)

2

a1 +

c3, 4, 1(R1)

c3, 5 (ik11)a1 +

c3, 2, 1

c3, 5 a

2(ik11)a

3

c3, 2, 2c3, 5

a2(ik31)a3 +c3, 3, 1c3, 5

a3(ik11)a2 c3, 3, 2c3, 5

a3(ik21)a2 .

(88)

Below there are formulae for the coefficients ofthe resonant terms. The quadratic term a2a

3 ap-

pearing in the equation at the first time scale hasthe coefficient given by:

c2, 3 = UfuuUU . (89)

The GinzburgLandau equation contains threecubic terms the self-interaction term does notchange upon the addition of the third wave; two

terms describing interaction of the principal har-monic with the two others have equal coeffcientsc3, 7 = c3, 9 given by (66) with an additional term:

2(UfuuUU)(UfuuR(fu k2D)UU)/(UU) .

(90)

There is a quadratic term a

2a

3in the Ginzburg

Landau equation for which the coefficient is givenby:

c3, 6(R1) = UfuuRUUR1 + (U

fuuUU)(UR(fu k2D)fuRUR1)/(UU)

+ 4(UfuRUR1)(UR(fu k2D)fuuUU)/(UU) 3UfuuUR(fu k2D)fuRUR1 . (91)

There are also four differential second-order terms in the equation; their coefficients are given below.

c3, 2, 1 = c3, 3, 1 = 2(UfuuUU)(U

R(fu k2D)DU)/(UU) UDR(fu k2D)fuuUU , (92)

c3, 2, 2 = c3, 3, 2 = 4(UDU)(U

R(fu

k2D)fuuUU)/(U

U)

2UfuuU

R(fu

k2D)fuRDU . (93)

6.1.2. Calculations for brusselator model

The calculation of the above coefficients made withthe help of the function CalculateCoefficientproduces the following results. The coefficient of theresonant quadratic term in the lower-order equationis found as:

c2, 3 =2d

d(

d a)d + a

.

The modes interaction coefficients are equal to:

c3, 7 = c3, 9 =d2

a(

d + a)3(d 1)2

(3a4+8a2d+6a4d+d220a2d2

3a4d2+6d3+16a2d33d4+2a

d

(2a2+d2a2d6d2+d3)) .

The coefficients of the second-order differentialterms are:

c3, 2, 1 = c3, 3, 1 =4d2

a(

d + a)2(d 1) (d + a2d a2 ad

d) ,

c3, 2, 2 = c3, 3, 2 =4d3

a(

d + a)2(1 d) (1 + add) .

Finally the quadratic term coefficient in theGinzburgLandau equation takes the form:

c3, 6(R1) =2d2

db1

a(

d + a)3(d 1)2 (2a2 a2d a2d2

3d +

d(ad2 + 3ad a)) .


23/26


6.2. Four-wave resonance inoscillatory instability

A more complicated case of the resonance arisesfor short-scale oscillatory instability. The simplestwaves configuration contains four waves with equalfrequencies w and the wavevectors making the fol-lowing angles with the x-axis: 0, , , +. In this

case the GinzburgLandau equation in addition tothe terms describing two-modes interaction, a termpertaining to interaction of three modes will alsoappear:

1a1 =c2, 1, 1c2, 2

(ik11)a1+ c2, 3(R1)c2, 2

a1 ,

2a1 =c3, 4c3, 3

|a4|2a1+ c3, 6c3, 3

|a3|2a1+ c3, 7c3, 3

|a2|2a1

+c3, 8c3, 3

|a1|2a1+ c3, 5c3, 3

a2a4a3

+c3, 9(R1, R2)

c3, 3a1+

c3, 1, 1c3, 3

(11)a1

+c3, 1, 2c3, 3

(ik11)2a1+ c3, 2, 1(R1)c3, 3

(ik11)a1 .(94)

The interaction coefficient c3, 7 between two modesa1 and a2 with corresponding wavevectors k1, k2making the angle is given by expression:

c3, 7 = UfuuUR(

fu)

fuuUU

UfuuUR(fu 2k2D(1 cos ))fuuUU UfuuUR(fu 2k2D(1 + cos ) 2iwI)fuuUU + UfuuuUUU . (95)

The expressions for coefficients c3, 6, c3, 4 can be pro-duced out of the above by the replacements and + , respectively. The self-interactioncoefficient c3, 8 is found as half of the value of c3, 7at = 0.

Finally, we present the expression for the three-modes interaction coefficient:

c3, 5 = UfuuUR(fu 2k2D(1 + cos ))fuuUU UfuuUR(fu 2iwI)fuuUU UfuuUR(fu 2k2D(1 cos ))fuuUU+ UfuuuUUU . (96)

The calculation of the coeffcients using theabove results for the model of two-level laser showsthat the interaction coefficients are independent ofthe value of the angle : c3, 4 = c3, 5 = c3, 6 =c3, 7 = 1/b; the self-interaction coefficient c3, 8 =1/(2b).

6.3. Algebraic degeneracy HopfTuring bifurcation

Here we present an example of usage of the functionBifurcationTheory for analysis of algebraically de-generated cases. Consider a situation where bothHopf and Turing bifurcations are permitted simul-taneously. It may occur if the minima of the bi-furcation curves for long-scale oscillatory (klo =0, w = wlo = 0) and for short-scale monotonic(k = ksm = 0, wsm = 0) bifurcations are at thesame level.

The function is called as follows:

BifurcationTheory[DD.(Nabla[r].Nabla[r])**u + f[u,R] -Nabla[t]**u == 0,u, R, t, {r,2}, {r,2}, {{a1,{k,0},0,U,Ut,3},{a2,{0,0},w,V,Vt,2}}, c, eps, 5, {a1,a2}]Here U,Ut denote the eigenvectors corresponding tothe Turing mode (its amplitude a1 is of the third-order of smallness); V,Vt correspond to the Hopfmode with second-order amplitude a2. Scaling ex-ponent in (58) = 2. The last argument of the

function shows that the normal forms are producedfor both modes.

The resulting equations are:

3a1 =c6, 2(R3)

c6, 1a1 ,

4a2 =c6, 4c6, 3

|a2|2a2 + c6, 5(R2, R4)c6, 3

a2

+c6, 1, 1c6, 3

(11)a2 .

(97)

The normalization coefficients are given by:

c6, 1 = UU ,

c6, 3 = VV .

The linear term coefficient c6, 2(R3) of the Turingmode is found as:

c6, 2(R3) = UfuRUR3 .


24/26


The coefficients in the normal form for the Hopfmode are given by Eqs. (35), (36) and (63) withreplacements U V, U V, R2 R4, R1 R2.

Acknowledgment

L. M. Pismen, acknowledges the support by theFund for Promotion of Research at the Technion.

ReferencesCoullet, P., Gil, L. & Rocca, F. [1989] Optical vortices,

Opt. Commun. 73, 403408.Guckenheimer, J. & Holmes, P. [1983] Nonlinear Oscilla-

tions, Dynamical Systems and Bifurcations of VectorFields (Springer, Berlin).

Guckenheimer, J., Myers, M. & Sturmfels, B. [1997]Computing Hopf bifurcations I, SIAM J. Num.Anal. 34, 121.

Haken, H. [1987] Advanced Synergetics (Springer,Berlin).

Hale, J. K. & Kocak, H. [1991] Dynamics and Bifurca-tions(Springer, Berlin).

Lorenz, E. N. [1963] Deterministic nonperiodic flow,J. Atmos. Sci. 20, 130141.

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Appendix

Appendices contain the auxillary material showing

usage of Mathematica for localization of the basicstates and subsequent linear analysis of systems un-der investigation. Some examples of calculation ofthe coefficients of amplitude equations for particu-lar systems are presented.

A. Linear Analysis and AmplitudeEquations for Lorenz Model

We start with defining an array f containing theright-hand sides of the above equations:

In[2]:=Lorenz = {-sigma x + sigma y, RR x - x z -y, x y - b z};Here x, y, z denote the components of the vec-tor u and RR stands for the bifurcation parameterR. The bifurcation point of the problem is deter-mined as a point where the Jacobian matrix is sin-gular. We find the Jacobian matrix of the problemLorenzmatr and then write a set of equations de-termining bifurcation points:

In[3]:=

Lorenzmatr = Outer[D,Lorenz,{x,y,z}];Lorenzbifcond =Joi

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