Evolution Of Special Ruled Surfaces Via The Evolution Of ...five geometric parameters...

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Appl. Math. Inf. Sci. 10, No. 5, 1949-1956 (2016) 1949 Applied Mathematics & Information Sciences An International Journal http://dx.doi.org/10.18576/amis/100536 Evolution Of Special Ruled Surfaces Via The Evolution Of Their Directrices In Euclidean 3-Space E 3 R. A. Hussien and Taha Youssef Math. Dept., Faculty of Science, Assiut Univ., Assiut, 71516, Egypt Received: 4 Jan. 2016, Revised: 20 Apr. 2016, Accepted: 23 Apr. 2016 Published online: 1 Sep. 2016 Abstract: In this paper, evolutions of ruled surfaces that are generated by the normal and binormal vector fields of space curve (normal and binormal surfaces) are presented. These evolutions of the ruled surfaces depend on the evolutions of their directrices. Geometric visualization of these ruled surfaces are presented. In addition, the conditions which make these surfaces of types inextensible, developable and minimal are obtained. Keywords: Curve evolution; Surface evolution; Special Ruled Surfaces; Normal and binormal surface; Inextensible surface. 1 Introduction Recently, the study of the motion of inelastic plane curves has arisen in a number of diverse engineering applications. Chirikjian and Burdick [1] described the motion of a planar hyperredundant (or snake-like) robot as the flow of a plane curve, while Brockett [2] explicitly proposed the idea of an inelastic string machine as a robotic device. Inelastic plane curves, i.e., plane curves whose lengths are preserved. Inextensible curve and surface flows also arise in the context of many problems in computer vision [3], [4] and computer animation [5], and even structural mechanics [6]. There are many applications in image processing and computer vision, such as scale space by linear and nonlinear diffusions [710], image enhancement through anisotropic diffusions [9], [1114] and image segmentation by active contours [1518]. The level set formulation [19] has provided good means to implement these flows. Extending these motions to manifolds embedded in spaces of higher dimensions can be beneficial for many applications. The Subject of how space curves evolve in time is of great interest and has been investigated by many authors. in [20], Hasimoto showed that the nonlinear Schr¨ odinger equation describing the motion of an isolated nonstretching thin vortex filament. Lamb [21] used the Hasimoto transformation to connect other motions of curves to the mKdV and sineGordon equations. Nakayama, et al [22] obtained the sineGordon equation by considering a nonlocal motion. Also, Nakayama and Wadati [23] presented a general formulation of evolving curves in two dimensions and its their connection to mKdV hierarchy. Nassar, et al [2427] studied the evolution of plane curves, the motion of hypersurfaces and the evolution of space curves in R n . R. Mukherjee and R. Balakrishnan [28] applied their method to the sine-Gordon equation and obtained links to five new classes of space curves in addition to the two which were found by Lamb [21]. For each class, they displayed the rich variety of moving curves associated with the onesoliton, the breather, the twosoliton and the solitonantisoliton solutions. In the case of the motion of surfaces, K. Nakayama and M. Wadati [29] formulated the motion of surfaces in 3-dimensional space using differential geometry. They obtained the time evolutions of the metric and the curvaturetensor. D. Y. Kwon and F. C. Park [30], studied the evolution of inelastic plane curves. Also D. Y. Kwon and F.C. Park [31], studied inextensible flows of curves and developable surfaces. T. orpinar et al [32] studied new inextensible flows of tangent developable surfaces in Euclidian 3space E 3 . In this paper, we shall derive a pair of coupled nonlinear partial differential equations (CNLPDEs) governing the time evolution of the curvature and torsion of the evolving curve. Applications to some curves are presented. Then we construct normal and binormal surfaces associated to these curves. Geometric Corresponding author e-mail: rawya [email protected] [email protected] c 2016 NSP Natural Sciences Publishing Cor.

Transcript of Evolution Of Special Ruled Surfaces Via The Evolution Of ...five geometric parameters...

Page 1: Evolution Of Special Ruled Surfaces Via The Evolution Of ...five geometric parameters {κ,τ,α,β,γ} appearing in the intrinsic Frenet-triad evolution equations (1) and (2) essentially

Appl. Math. Inf. Sci.10, No. 5, 1949-1956 (2016) 1949

Applied Mathematics & Information SciencesAn International Journal

http://dx.doi.org/10.18576/amis/100536

Evolution Of Special Ruled Surfaces Via The EvolutionOf Their Directrices In Euclidean 3-Space E3

R. A. Hussien∗ and Taha Youssef∗

Math. Dept., Faculty of Science, Assiut Univ., Assiut, 71516, Egypt

Received: 4 Jan. 2016, Revised: 20 Apr. 2016, Accepted: 23 Apr. 2016Published online: 1 Sep. 2016

Abstract: In this paper, evolutions of ruled surfaces that are generated by the normal and binormal vector fields of space curve (normaland binormal surfaces) are presented. These evolutions of the ruled surfaces depend on the evolutions of their directrices. Geometricvisualization of these ruled surfaces are presented. In addition, the conditions which make these surfaces of types inextensible,developable and minimal are obtained.

Keywords: Curve evolution; Surface evolution; Special Ruled Surfaces; Normal and binormal surface; Inextensible surface.

1 Introduction

Recently, the study of the motion of inelastic plane curveshas arisen in a number of diverse engineeringapplications. Chirikjian and Burdick [1] described themotion of a planar hyperredundant (or snake-like) robotas the flow of a plane curve, while Brockett [2] explicitlyproposed the idea of an inelastic string machine as arobotic device. Inelastic plane curves, i.e., plane curveswhose lengths are preserved. Inextensible curve andsurface flows also arise in the context of many problemsin computer vision [3], [4] and computer animation [5],and even structural mechanics [6].

There are many applications in image processing andcomputer vision, such as scale space by linear andnonlinear diffusions [7–10], image enhancement throughanisotropic diffusions [9], [11–14] and imagesegmentation by active contours [15–18]. The level setformulation [19] has provided good means to implementthese flows. Extending these motions to manifoldsembedded in spaces of higher dimensions can bebeneficial for many applications.

The Subject of how space curves evolve in time is ofgreat interest and has been investigated by many authors.in [20], Hasimoto showed that the nonlinear Schrodingerequation describing the motion of an isolatednon−stretching thin vortex filament. Lamb [21] used theHasimoto transformation to connect other motions ofcurves to the mKdV and sine−Gordon equations.

Nakayama, et al [22] obtained the sine−Gordon equationby considering a nonlocal motion. Also, Nakayama andWadati [23] presented a general formulation of evolvingcurves in two dimensions and its their connection tomKdV hierarchy. Nassar, et al [24–27] studied theevolution of plane curves, the motion of hypersurfacesand the evolution of space curves inRn. R. Mukherjee andR. Balakrishnan [28] applied their method to thesine-Gordon equation and obtained links to five newclasses of space curves in addition to the two which werefound by Lamb [21]. For each class, they displayed therich variety of moving curves associated with theone−soliton, the breather, the two−soliton and thesoliton−antisoliton solutions. In the case of the motion ofsurfaces, K. Nakayama and M. Wadati [29] formulatedthe motion of surfaces in 3-dimensional space usingdifferential geometry. They obtained the time evolutionsof the metric and the curvature tensor. D. Y. Kwon and F.C. Park [30], studied the evolution of inelastic planecurves. Also D. Y. Kwon and F.C. Park [31], studiedinextensible flows of curves and developable surfaces. T.Korpinar et al [32] studied new inextensible flows oftangent developable surfaces in Euclidian 3−spaceE3.

In this paper, we shall derive a pair of couplednonlinear partial differential equations (CNLPDEs)governing the time evolution of the curvature and torsionof the evolving curve. Applications to some curves arepresented. Then we construct normal and binormalsurfaces associated to these curves. Geometric

∗ Corresponding author e-mail:rawya−[email protected] [email protected]

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visualization of the normal and binormal surfaces aredisplayed via solving the Gauss-Weingarten equations fora specified coefficients of the first and secondfundamental forms using fundamental theorem ofsurfaces. The essence of this paper is that, we linked themotion of surfaces with the motion of curves, i.e., if thecurve moves, then the normal and binormal surfacesmove.

The article is organized as follows. In section1.1, weintroduce the time−evolution equations that are satisfiedby the intrinsic quantities of curves. Also, we deriveCNPDEs which formulate the problem directly in termsof the curvatures and obtained the exact solution for them.In subsection1.2, we determine the curve from itscurvatures. In section1.3, we introduce some applicationsof the curve evolution specified by its local geometry. Insection2, we introduce differential geometry of surfaces.In section3, we introduce the geometric properties ofnormal and binormal surfaces. In section4, wereconstruct the surfaces from the coefficients of the thefirst and second fundamental forms via numericalintegration of the Gauss-Weingarten equations andplotted them.

1.1 Time-evolution equations

In this section we briefly review the main results for theevolution of space curves as presented in [28], and extendthese results to derive time−evolution equations that issatisfied by the intrinsic quantities of the curve. Let usconsider a curve embedded in three-dimensional spacedescribed in parametric form by a position vectorr = r(s), s being the usual arclength variable. The unittangent vectort = r s, the principal normaln and thebinormalb form an orthonormal triad of unit vectors thatsatisfy the Frenet–Serret equations [33]:

ts = κn,ns = −κ t + τb, (1)

bs = −τn.

Here and hereafter, the subscripts denote partialderivatives.κ andτ are the curvature and torsion of thecurve.

If this curve moves with timet, then all quantities inEq. (1) become functions of boths and t. The generaltemporal evolution in which the triad{t,n,b} remainsorthonormal adopts the following form [34]

tt = αn+βb,nt = −αt + γb, (2)

bt = −β t − γn.

As is clear, the parametersα, β and γ (which are thevelocities of the moving frame{t,n,b}) determine themotion of the curve.

On requiring the compatibility conditions

tts = tst, nts = nst, bts = bst. (3)

Apply the compatibility conditions Eq. (3) to the systems(1), (2), then

κt = αs− τβ ,τt = γs+κβ , (4)

βs = κγ − τα.

The temporal evolution of the curvatureκ and the torsionτ of the curve may now be expressed in terms of thecomponents of velocity{α,β} which can be written ascoupled nonlinear partial differential equations as follows,

κt = αs−β τ,

τt = (βs+ τα

κ)s+κβ .

(5)

From these equations we note that the componentγ in Eq.(2) does not affect the final shape of the evolving curve.For a given {α,β ,γ}, the motion of the curve isdetermined from these equations. Mathematica packagesoftware (computational software program used inscientific, engineering, mathematical fields and otherareas of technical computing) was used for solving theEqs.(5) which applies the tanh−and sech−methods [35].The outline for given{α,β ,γ} is that we get{κ ,τ}.

1.2 Determining a parametrized curve from itscurvature and torsion

One of the basic problems in geometry is to determineexactly the geometric quantities which distinguish onefigure from another. For example, line segments areuniquely determined by their lengths, circles by theirradii, triangles by side-angle-side, etc. It turns out thatthis problem can be solved in general for sufficientlysmooth regular curves. We will see that a regular curve isuniquely determined by two scalar quantities, calledcurvature and torsion, as functions of the naturalparameter, which follows from the next theorem.

Theorem 1(Fundamental existence and uniquenesstheorem for space curves). Let κ(s) and τ(s) bearbitrary continuous functions on a≤ s ≤ b. then thereexists, except for position in space , one and only onespace curve C for whichκ(s) is the curvature,τ(s) is thetorsion and s is a natural parameter along C [36].

In the next subsection, we shall show how to recreatecurves in the space from their curvature and torsion vianumerical integration of Frenet−Seret equations up to itsposition in space.

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1.3 Applications

In this subsection, we consider some applications of thecurve evolution specified by its local geometry. The set offive geometric parameters{κ ,τ,α,β ,γ} appearing in theintrinsic Frenet-triad evolution equations (1) and (2)essentially describes a moving curve.

1.3.1 Case (1)

For a curve moving in the space by the velocities

{α,β ,γ}= {κ ,κs,κss+ τκ

κ}. (6)

The evolution equations for the curvature and the torsionof the curve given from Eq. (5) as follows,

κt = κs−κsτ,

τt = (κss+ τκ

κ)s+κκs.

(7)

The general solutions to this system are given by

κ1(s, t) = 2c2sech(c1t + c2s+ c3),

τ1(s, t) =c2− c1

c2, c2 ≥ 0,

(8)

κ2(s, t) = c4,

τ2(s, t) = c5 f (c6t + c6s+ c7),(9)

whereci ,(i = 1, ...,7) are arbitrary real constants andfis an arbitrary function. As the first solution, it is easilyverified that the set{κ ,τ,α,β ,γ} satisfies Eq. (4).

The figures1, 4, 7 and10 represent snapshots of theevolving space curve obtained by solving theFrenet−Serret Eq. (1) for a specified curvature andtorsion using Mathematica [37]. Any moving space curvecan be studied from two perspectives, namely the shapeof the curve and the evolution of the curve. At every fixedtime t, we clearly have a representation of thecorresponding static space curve at that instant. Theprogram [37], as it stands, generates static space curves. Itwas extended slightly to generate the evolution of thespace curves with timet.

In all the figures1, 4, 7 and10, we have used the totalcurve length of 20 (−10≤ s≤ 10). In practice, the rangeof variation oft must remain much smaller than that ofsso that the length of the curve suffices to display thecomplete geometric structure corresponding to thesolution concerned.

If we putc1 = 0.4,c2 = 0.5,c3 = 0 in Eq. (8), thenκ =sech(0.4t +0.5s),τ = 0.2. we see thatκ → 0 ass→±∞.Thus, for large values of s, the curve straightens out at bothends as shown in Fig.1.

1.3.2 Case (2)

We consider the case that the velocities are given by

{α,β ,γ}= {0,κ ,κs

κ}. (10)

From Eq. (5) it follows that the evolution equations for thecurvature and the torsion of the curve are given as follows,

κt =−κτ,

τt = (κs

κ)s+κ2.

(11)

The general solution to this system is given by

κ(s, t) =√

c21+ c2

2sech(c1t + c2s+ c3),

τ(s, t) = c1 tanh(c1t + c2s+ c3).(12)

wherec1,c2,c3 are arbitrary real constants. If we takec1 =1,c2 = 1 andc3 = 0 in Eq. (12), thenκ =

√2sech(t + s)

andτ(s, t) = tan(t +s). We see thatκ → 0 ass→±∞ andτ →±1 ass→±∞. Thus, for large values ofs, the curvestraightens out at both ends as shown in Fig.10.

In the next section we mention some basic facts in thegeneral theory of surfaces useful for the rest of the paper.

2 Differential geometry of surfaces

Let x = x(s, t) denote the position vector of a genericpoint P on a surface S inR3. Then, the vectorsxs andxtare tangential toS at P, at such points at which they arelinearly independent,

N =xs∧xt

|xs∧xt |, (13)

determines the unit normal vector toS. The first andsecond fundamental forms ofSare defined by

I = 〈dx.dx〉= g11ds2+2g12dsdt+g22dt2,II = 〈−dx.dN〉= L11ds2+2L12dsdt+L22dt2,

(14)

where,

g11= 〈xs,xs〉, g12 = 〈xs,xt〉, g22 = 〈xt ,xt〉L11= 〈xss,N〉, L12= 〈xst,N〉, L22= 〈xtt ,N〉,

(15)where〈,〉 is the Euclidean scaler product.

The Gauss equations associated withsare [33]:

xss= Γ 111xs+Γ 2

11xt +L11N,

xst = Γ 112xs+Γ 2

12xt +L12N,

xtt = Γ 122xs+Γ 2

22xt +L22N.

(16)

The Weingarten equations are

Ns =g12L12−g22L11

gxs+

g12L11−g11L12

gxt ,

Nt =g12L22−g22L12

gxs+

g12L12−g11L22

gxt ,

(17)

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where

g= det(gi j ) = g11g22−g212. (18)

The quantitiesΓ ki j are called the Christoffel symbols of the

second kind, and they are given by

Γ ki j =

12

gkl(∂

∂ui gl j +∂

∂u j gil −∂

∂ul gi j ), (19)

wherex1 = s , x2 = t ,

I = gikdx jdxk (20)

and

g jkgkl = δ ij . (21)

In the above, the Einstein convention of summation overrepeated indices has been adopted.

The Gaussian curvatureκg and the mean curvatureκmare

κg =Lg=

L11L22−L212

g11g22−g212

, (22)

κm =L11g22−2L12g12+L22g11

2g, (23)

where

L = det(Li j ) = L11L22−L212. (24)

Applying the compatibility conditions(xss)t = (xst)sand(xst)t = (xtt)s to the linear Gauss system (16). Thenwe have the Gauss and Mainardi-Codazzi system

L = g11((Γ 122)s− (Γ 1

12)t +Γ 122Γ 1

11+Γ 222Γ 1

12−Γ 112Γ 1

12−Γ 212Γ 1

22)

+g12((Γ 222)s− (Γ 2

12)t +Γ 122Γ

211−Γ 1

12Γ2

12),

(25)

∂L11

∂ t− ∂L12

∂s= L11Γ 1

12+L12(Γ 212−Γ 1

11)−L22Γ 211,

∂L12

∂ t− ∂L22

∂s= L11Γ 1

22+L12(Γ 222−Γ 1

12)−L22Γ 212.

(26)

Theorem 2.(Fundamental existence and uniquenesstheorem Of Surfaces) Let g11,g12 and g22 be functions of sand t of class C2 and let L11,L12 and L22 be functions of sand t of class C1 all defined on an open set containing(s0, t0) such that for all (s, t),(i) g11g22−g2

12> 0, g11 > 0, and g22 > 0(ii) g11,g12,g22,L11,L12 and L22 satisfy the compatibilityequations (25),(26). Then there exists a patchx = x(s, t)of class C3 defined in a neighborhood of(s0, t0) for whichg11,g12,g22,L11,L12 and L22 are the first and secondfundamental coefficients. The surface represented byx = x(s, t) is unique except for position in space [36].

3 Geometric properties of the normal andbinormal surfaces

A ruled surface is a surface that can be swept out bymoving a line in space. It therefore has a parameterizationof the form

x(s,u) = r(s)+uℓ(s), (27)

wherer is called the ruled surface directrix (also calledthe base curve) andℓ is the director curve. The straightlines themselves are called rulings. The rulings of a ruledsurface are asymptotic curves. Furthermore, the Gaussiancurvature on a ruled regular surface is everywherenonpositive. If the curvesr = r(s) andℓ= ℓ(s) move withtime t, then Eq. (27) becomes

x(s,u, t) = r(s, t)+uℓ(s, t) (28)

3.1 Normal surface

We may take each of the frame vectors of the curver andreapply it as the vectorℓ to describe a ruled surfaceuniquely determined by the shape of the curve and itsvariations in space. If the generatrix vectorℓ is the normaln of the curver , then Eq. (28) define a normal surfacewith the parametric representation

x(s,u, t) = r(s, t)+un(s, t). (29)

Definition 1.A surface evolutionx(u,s, t) and its flow∂x∂ t

are said to be inextensible if its first fundamental quantities{g11,g12,g22} satisfies [31]

∂g11

∂ t=

∂g12

∂ t=

∂g22

∂ t= 0. (30)

The tangent vectors for the surfaceSare

xs(s,u, t) = (1−uκ)t(s, t)+ τub(s, t),xu(s,u, t) = n(s, t).

(31)

Using Eqs. (13) and (31), then the unit normal vector fieldonShas the form

N =xu∧xs

|xu∧ r s|=

(uκ −1)b+ τut√

(uκ −1)2+ τ2u2. (32)

Using Eq. (15), then the coefficients of the firstfundamental form are

g11 = (1−uκ)2+(τu)2,

g12 = 0, (33)

g22 = 1.

Using the above system and Eq. (30), one can see that thenormal surface is inextensible if

−2uκt(1−uκ)+2u2ττt = 0. (34)

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Using Eq. (15), then the coefficients of the secondfundamental form are

L11 =−u2κsτ +uτs(uκ −1)√

(uκ −1)2+u2τ2,

L12 =−τ

(uκ −1)2+u2τ2, (35)

L22 = 0.

So, the Gaussian curvatureκg and the mean curvatureκmare

κg =−τ2

((1−uκ)2+u2τ2))2 ,

κm =−u2κsτ +uτs(uκ −1)

2((1−uκ)2+ τ2u2)3/2.

(36)

From the above equations, the normal surfacex(s,u, t) isdevelopable (κg = 0) if

τ = 0, (37)

and minimal(κm = 0) if

−u2κsτ +uτs(uκ −1) = 0, (38)

a short calculation shows that the above equation can bewritten as

u2 ∂∂s

(τκ)− τs

κ2 = 0 (39)

3.2 Binormal surface

If the generatrix vectorℓ is the binormalb of the curver ,then from Eq.28, we get the binormal surface. Theparametric representation for this surface is

x(s,v, t) = r(s, t)+ vb(s, t). (40)

The tangent vectors forSare

xs(s,v, t) = t(s, t)− τvn(s, t),xv(s,v, t) = b(s, t).

(41)

The unit normal vector field on the surfaceS is given by

N =n+ τvt√1+ τ2v2

. (42)

A short calculation shows that the coefficients of the firstfundamental form are

g11 = 1+ v2τ2, g12= 0, g22 = 1.

Using the above system and Eq. (30), one can see that thebinormal surface is inextensible if

∂τ∂ t

= 0, so τ = τ0 = constant (we chooseτ0 = 0).

(43)

Using Eq. (15), then the coefficients of the secondfundamental form are

L11 =τ2v2κ +κ − τsv√

1+ v2τ2, L12 =

−τ√1+ v2τ2

, L22 = 0.

So, the Gaussian curvatureκg and the mean curvatureκmare

κg =−τ2

(1+ τ2v2)2 , κm =τ2v2κ +κ − τsv

2(1+ τ2v2)3/2. (44)

From the above equations, the binormal surfacex(s,v, t) isdevelopable (κg = 0) iff

τ = 0, (45)

and minimal (κm = 0) iff

τ2v2κ +κ − τsv= 0 (46)

4 Geometric visualization of the normal andbinormal surfaces and its generator

In this section, we shall display the evolution of Normaland Binormal Surfaces depends on the evolution of theirdirectrices. The surfaces below represent snapshots of theevolving normal and binormal surfaces obtained bysolving the Gauss-Weingarten Eqs. (16) for specified firstand second fundamental forms using Mathematica [38].Any moving surface can be studied from twoperspectives, namely, the shape of the surface and theevolution of the surface. At every fixedt, we clearly havea representation of the corresponding (static) surface atthat instant. The program [38] as it stands generates staticsurfaces. It was extended slightly to generate theevolution of the surfaces witht. We used the curvatureand torsion of the curves which obtained in section (1)which represents the base curves of the normal andbinormal surfaces.

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 1: Time evolution of the curveκ = sech(0.4t +0.5s),τ = 0.2

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(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 2: Time evolution of the normal surfaces

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 3: Time evolution of the binormal surfaces

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 4: Time evolution of the curveκ = 0.5,τ = sin(t + s)

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 5: Time evolution of the normal surfaces

5 Discussion

1.In Figure 1, we obtained the time evolution of the curveκ = sech(0.4t+0.5s),τ = 0.2. We found thatκ → 0 ast →±∞ andτ →±1 ast →±∞. Thus, for large valuesof t, the curve straightens out at both ends.

2.In Figure 4, we obtained the time evolution of the curveκ = 0.5,τ = sin(t + s). We found that thatτ →±1 as

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 6: Time evolution of the binormal surfaces

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 7: Time evolution of the curveκ = 1,τ = tanh(t + s)

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 8: Time evolution of the normal surfaces

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 9: Time evolution of the binormal surfaces

t →±∞. Thus, for large values oft, the curve collapsesinto helix.

3.In Figure 7, we obtained the time evolution of the curveκ = 1,τ = tanh(t + s). We found that thatκ → 1 ast →±∞ andτ →±1 ast →±∞. Thus, for large valuesof t, the curve collapses into helix.

4.In Figure 10, we obtained the time evolution of thecurve κ =

√2sech(t + s),τ = tanh(s+ t). We found

thatκ → 0 ast →±∞ andτ →±1 ast →±∞. Thus,

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(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 10: Time evolution of the curveκ =√

2sech(t +s),τ = tanh(s+ t)

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 11: Time evolution of the normal surfaces

(a) t=0 (b) t=1 (c) t=2 (d) t=3

Fig. 12: Time evolution of the binormal surfaces

for large values oft, the curve straightens out at bothends.

6 Conclusions

In this paper, we conclude that the componentγ in Eqs.(2) does not affect the final shape of the evolving curve.For ruled surfaces, we present an evolution of the ruledsurfaces generated by the normal and binormal of spacecurve. This evolution depends on the evolution of thedirectrix. The results that we obtained are:

1.Normal surfaces are developable ifτ = 0, minimal if−u2κsτ + uτs(uκ − 1) = 0, and inextensible if−2uκt(1−uκ)+2u2ττt = 0.

2.Binormal surfaces are developable ifτ = 0, minimal ifτ2v2κ + κ − τsv = 0, τ = 0, and inextensible if∂τ

∂ t =0, soτ = τ0 = constant (we choseτ0 = 0).

In Fig.(10), (11) and (12), we found thatκ → 0 ass→±∞,τ →±1 ass→±∞. When we solve the system of Frenet-Serret, the torsion does not affect on the final shape of theevolving curve as shown in Fig.(10). But when we solvethe system of Gauss-Weingarten, the curvature does notaffect on the final shape of the evolving surface as shownin Figs. (11) and (12).

We linked the motion of surfaces with the motion ofcurves i.e., if the curve moves, then the normal andbinormal surfaces moves. Geometric visualization ofthese surfaces are displayed.

Acknowledgement

The authors would like to thank the referees for theirhelpful comments and suggestions.

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R. A. Hussien isa lecturer in MathematicsDepartment, Facultyof Science, Assiut University,Egypt. Her research interestsare in differential geometry,variational problemsand evolution of curvesand surfaces.

Taha Youssef is Ph.D. student in MathematicsDepartment, Facultyof Science, Assiut University,Egypt. His Master ison ”Geometric Visualizationof Evolution of Curves andSurfaces Prescribed by theirCurvatures”. His researchinterests are in differential

geometry and evolution of curves and surfaces.

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