Independence and chromatic number (and random k-SAT): Sparse Case Dimitris Achlioptas Microsoft.

Independence and chromatic number (and random k-SAT):

Sparse Case

Dimitris AchlioptasMicrosoft

Random graphs

W.h.p.: withprobability thattendsto 1 asn ! 1 .

Let be the moment all vertices have degree 2¿2 ¸

Hamiltonian cycle

Let be the moment all vertices have degree 2

W.h.p. has a Hamiltonian cycle

¿2 ¸

G(n;m = ¿2)

Hamiltonian cycle

W.h.p. it can be found in time

¿2 ¸

G(n;m = ¿2)

O(n3 logn)

[Ajtai, Komlós, Szemerédi 85] [Bollobás, Fenner, Frieze 87]

W.h.p. it can be found in time

¿2 ¸

G(n;m = ¿2)

O(n3 logn)

[Ajtai, Komlós, Szemerédi 85] [Bollobás, Fenner, Frieze 87]

In Hamiltonicity can be decided in G(n;1=2) O(n)expected time.

[Gurevich, Shelah 84]

Hamiltonian cycle

Cliques in random graphs

The largest clique in has size

[Bollobás, Erdős 75] [Matula 76]

2log2 n ¡ 2log2 log2 n§ 1

G(n;1=2)

No maximal clique of size <

G(n;1=2)

log2 n

Can we find a clique of size ?

G(n;1=2)

log2 n

(1+ ²) log2 n

[Karp 76]

Can we find a clique of size ?

G(n;1=2)

log2 n

(1+ ²) log2 n

What if we “hide” a clique of size ?n1=2¡ ²

Two problems for which we know much less.

Chromatic number of sparse random graphs Random k-SAT

Two problems for which we know much less.

Chromatic number of sparse random graphs Random k-SAT

Canonical for random constraint satisfaction:– Binary constraints over k-ary domain

– k-ary constraints over binary domain

Studied in: AI, Math, Optimization, Physics,…

A factor-graph representationof k-coloring

Each vertex is a variable with domain {1,2,…,k}.

Each edge is a constraint on two variables.

All constraints are “not-equal”.

Random graph = each constraint picks two variables at random.

Vertices

SAT via factor-graphs(x12 _ x5 _ x9) ^(x34 _ x21 _ x5) ^ ¢¢¢¢¢¢̂ (x21 _ x9 _ x13)

SAT via factor-graphs

Edge between x and c iff x occurs in clause c.

Edges are labeled +/- to indicate whether the literal is negated.

Constraints are “at least one literal must be satisfied”.

Random k-SAT = constraints pick k literals at random.

Clause nodes

Variable nodes

Clause nodes

(x12 _ x5 _ x9) ^(x34 _ x21 _ x5) ^ ¢¢¢¢¢¢̂ (x21 _ x9 _ x13)

Diluted mean-field spin glasses

Variables

Constraints

Small, discrete domains: spins

Conflicting, fixed constraints: quenched disorder

Random bipartite graph: lack of geometry, mean field

Sparse: diluted

Hypergraph coloring, random XOR-SAT, error-correcting codes…

Random graph coloring:

Background

A trivial lower bound

For any graph, the chromatic number is at least:

Number of vertices

Size of maximum independent set

A trivial lower bound

For any graph, the chromatic number is at least:

Number of vertices

Size of maximum independent set

For random graphs, use upper bound for largest independent set.

¶£ (1¡ p)(

s2) ! 0

An algorithmic upper bound

RepeatPick a random uncolored vertexAssign it the lowest allowed number (color)

Uses 2 x trivial lower bound number of colors

An algorithmic upper bound

RepeatPick a random uncolored vertexAssign it the lowest allowed number (color)

Uses 2 x trivial lower bound number of colors

No algorithm is known to do better

The lower bound is asymptotically tight

As d grows, can be colored using

independent sets of essentially maximum size

[Bollobás 89]

[Łuczak 91]

G(n;d=n)

The lower bound is asymptotically tight

As d grows, can be colored using

independent sets of essentially maximum size

[Bollobás 89]

[Łuczak 91]

G(n;d=n)

Theorem.For everyd > 0, thereexistsan integerk = k(d) such thatw.h.p.thechromaticnumberofG(n;p= d=n)

Only two possible values

is eitherk or k + 1

[Łuczak 91]

Theorem.For everyd > 0, thereexistsan integerk = k(d) such thatw.h.p.thechromaticnumberofG(n;p= d=n)

“The Values”

is eitherk or k + 1

wherek is thesmallestintegers.t. d < 2k logk.

Examples

If , w.h.p. the chromatic number is or .d = 7 4 5

Examples

If , w.h.p. the chromatic number is or .

If , w.h.p. the chromatic number is

d = 1060

377145549067226075809014239493833600551612641764765068157

3771455490672260758090142394938336005516126417647650681576

Theorem.If (2k ¡ 1)lnk < d < 2k lnk thenw.h.p.thechromaticnumberof G(n;d=n) is k + 1.

One value

Theorem.If (2k ¡ 1)lnk < d < 2k lnk thenw.h.p.thechromaticnumberof G(n;d=n) is k + 1.

If , then w.h.p. the chromatic number isd = 10100

22306189349652674971022028151567123496495382061668383284446576222083269567040304449938600863080217

One value

Random k-SAT:

Background

Random k-SAT

Fix a set of n variables

Among all possible k-clauses select m

uniformly and independently. Typically .

Example ( ) :

X = f x1;x2; : : : ;xng

(x12 _ x5 _ x9) ^(x34 _ x21 _ x5) ^ ¢¢¢¢¢¢̂ (x21 _ x9 _ x13)

m = rn

Generating hard 3-SAT instances

[Mitchell,

Selman,

Levesque 92]

Generating hard 3-SAT instances

[Mitchell,

Selman,

Levesque 92]

The critical point appears to be around r ¼4:2

The satisfiability threshold conjecture

For every , there is a constant such that

limn!1

Pr[F k(n;rn) is satis¯able] =½

1 if r = rk ¡ ²0 if r = rk + ²

k ¸ 3 rk

The satisfiability threshold conjecture

For every , there is a constant such that

limn!1

Pr[F k(n;rn) is satis¯able] =½

1 if r = rk ¡ ²0 if r = rk + ²

k ¸ 3 rk

For every , k ¸ 3

k< rk < 2k ln2

Unit-clause propagation

Repeat– Pick a random unset variable and set it to 1– While there are unit-clauses satisfy them– If a 0-clause is generated fail

Unit-clause propagation

Repeat– Pick a random unset variable and set it to 1– While there are unit-clauses satisfy them– If a 0-clause is generated fail

UC finds a satisfying truth assignment if

[Chao, Franco 86]

The probability of satisfiability it at most

An asymptotic gapµ ¶ · µ ¶ ¸

1¡12k

¶ r ¸n

! 0 for r ¸ 2k ln2µ ¶ · µ ¶ ¸

1¡12k

¶ r ¸n

! 0 for r ¸ 2k ln2

An asymptotic gap

Since mid-80s, no asymptotic progress over

k< rk < 2k ln2

Getting to within a factor of 2

a randomk-CNF formulawith m = rnclausesw.h.p.hasa complementarypairof satisfyingtruthassignments.

r < 2k¡ 1 ln2¡ 1 :

Theorem:For all k ¸ 3 and

The trivial upper bound is the truth!

r · 2k ln2¡ k2 ¡ 1

Theorem:For all k ¸ 3, a randomk-CNF formulawithm = rn clausesis w.h.p.satis¯able if

k 3 4 5 7 20 21Upper bound 4:51 10:23 21:33 87:88 726;817 1;453;635

Our lower bound 2:68 7:91 18:79 84:82 726;809 1;453;626Algorithmic lower bound 3:52 5:54 9:63 33:23 95;263 181;453

Some explicit bounds for the k-SAT threshold

The second moment method

Pr[X > 0]¸E[X ]2

E[X 2]

otherwise let = 0, so that

For any non-negative r.v. X,

Proof: Let Y = 1 if X > 0, andY = 0 otherwise.By Cauchy-Schwartz,

E[X ]2 = E[X Y ]2 · E[X 2]E[Y 2] = E[X 2]Pr[X > 0] :

Ideal for sums

If X = X 1+X 2+¢¢¢ then

E[X ]2 =X

E[X i ]E[X j ]

E[X 2] =X

E[X i X j ]

Ideal for sums

If X = X 1+X 2+¢¢¢ then

E[X ]2 =X

E[X i ]E[X j ]

E[X 2] =X

E[X i X j ]

Example:

The X i correspondto the¡n

¢potential q-cliquesin G(n;1=2)

Dominant contribution fromnon-ovelappingcliques

General observations

Method works well when the Xi are like

“needles in a haystack”

Lack of correlations rapid drop in

influence around solutions

Lack of correlations rapid drop in

influence around solutions

Algorithms get no “hints”

Let X be the # of satisfying truth assignments

The second moment method for random k-SAT

Let X be the # of satisfying truth assignments£

For everyclause-density r > 0, thereis ¯ = ¯ (r) > 0 such that

E[X ]2

E[X 2]< (1¡ ¯ )n

The number of satisfying truth assignments has

huge variance.

E[X ]2

E[X 2]< (1¡ ¯ )n

¾2 f0;1gn

The number of satisfying truth assignments has

huge variance.

E[X ]2

E[X 2]< (1¡ ¯ )n

The satisfying truth assignments do not form a

“uniformly random mist” in

Let be the number of

satisfied literal occurrences in F under σ

H (¾;F )

where satisfies . (1+ °2)k¡ 1(1¡ °2) = 1° < 1

To prove 2k ln2 – k/2 - 1

H (¾;F )

Let be defined as

X (F ) =X

1¾j=F °H (¾;F )

1¾j=c °H (¾;c)

X = X (F )

To prove 2k ln2 – k/2 - 1

H (¾;F )

Let be defined as

X (F ) =X

1¾j=F °H (¾;F )

1¾j=c °H (¾;c)

X = X (F )

General functions

Given any t.a. σ and any k-clause c let

be the values of the literals in c under σ.

v = v(¾;c) 2 f¡ 1;+1gk

General functions

Given any t.a. σ and any k-clause c let

be the values of the literals in c under σ.

We will study random variables of the form

where is an arbitrary functionf : f¡ 1;+1gk ! R

v = v(¾;c) 2 f¡ 1;+1gk

f (v(¾;c))

f (v) = 1 for all v

f (v) =

(0 if v = (¡ 1; ¡ 1; : : : ;¡ 1)

1 otherwise

f (v) =

0 if v = (¡ 1;¡ 1; : : : ;¡ 1) orif v = (+1;+1; : : : ;+1)

1 otherwise

# of satisfying

truth assignments

=) # of “Not All Equal”

truth assignments

f (v(¾;c))

f (v) = 1 for all v

f (v) =

(0 if v = (¡ 1; ¡ 1; : : : ;¡ 1)

1 otherwise

f (v) =

0 if v = (¡ 1;¡ 1; : : : ;¡ 1) orif v = (+1;+1; : : : ;+1)

1 otherwise

# of satisfying

truth assignments

# of satisfying

truth assignments

whose complement

is also satisfying

Overlap parameter = distance

Overlap parameter is Hamming distance

For any f, if agree on variables¾;¿ z = n=2h i

Ehf (v(¾;c))f (v(¿;c))

£f (v(¾;c))

£f (v(¿;c))

For any f, if agree on variables¾;¿ z = n=2h i

Ehf (v(¾;c))f (v(¿;c))

£f (v(¾;c))

£f (v(¿;c))

For any f , if agree on variables, let¾;¿ z = ®n

Cf (z=n) ´ Ehf (v(¾;c))f (v(¿;c))

Independence

Contribution according to distance

Entropy vs. correlation

E[X 2] = 2nnX

¶Cf (z=n)m

E[X ]2 = 2nnX

¶Cf (1=2)m

For every function f :

Recall: µ

®®(1¡ ®)1¡ ®

£ poly(n)

Independence

Contribution according to distance

®=z/n

The importance of being balanced

An analytic condition:

C0f (1=2)= 0 ( )

v2f¡ 1;+1gk

f (v)v = 0=) the s.m.m. fails

®=z/n

7< r < 11

NAE 5-SAT

NAE 5-SAT7< r < 11

C0f (1=2)= 0 ( )

v2f¡ 1;+1gk

C0f (1=2)= 0 ( )

v2f¡ 1;+1gk

f (v)v = 0

C0f (1=2)= 0 ( )

v2f¡ 1;+1gk

A geometric criterion:

C0f (1=2)= 0 ( )

v2f¡ 1;+1gk

f (v)v = 0

(+1,+1,…,+1)(-1,-1,…,-1)

Constant

Complementary

Balance & Information Theory

Want to balance vectors in “optimal” way

Information theory

maximize the entropy of the subject to

=)f (v)

f (¡ 1;¡ 1; : : : ; ¡ 1)= 0andX

v2f¡ 1;+1gk

f (v)v = 0

Information theory

maximize the entropy of the subject to

=)f (v)

f (¡ 1;¡ 1; : : : ; ¡ 1)= 0andX

v2f¡ 1;+1gk

f (v)v = 0

Lagrange multipliers the optimal f is

for the unique that satisfies the constraints

f (v) = ° # of +1sin v

Information theory

maximize the entropy of the subject tof (v)

f (¡ 1;¡ 1; : : : ; ¡ 1)= 0andX

v2f¡ 1;+1gk

f (v)v = 0

(+1,+1,…,+1)(-1,-1,…,-1)

Heroic

Random graph coloring

Threshold formulation

Theorem.A randomgraphwithn verticesandm = cn edgesis w.h.p.k-colorableif

c · k logk ¡ logk ¡ 1 :

c ¸ k logk ¡ 12 logk.

and whp non-k-colorable if

Main points

Non-k-colorability:

k-colorability:

Proof. Applysecondmomentmethod to thenumber of balanced k-coloringsof G(n;m).

Proof. The¬probability that¬there exists¬any k-coloring¬is at¬most

Let Xσ be the indicator that the balanced

k-partition σ is a proper k-coloring.

We will prove that if then for all

there is a constant

such that

This implies that is k-colorable w.h.p.

¾X ¾

Theorem.A randomgraphwithn verticesandm = cn edgesis w.h.p.k-colorableif

c · k logk ¡ logk ¡ 1 :D = D(k) > 0

E[X 2] < D E[X ]2

G(n;cn)

sum over all of .

For any pair of balanced k-partitions

let aijn be the # of vertices having

color i in σ and color j in τ.

¾;¿ E[X ¾X ¿ ]

E[X 2] =

Pr[¾and¿ areproper] =

@1¡2k

Examples

When are uncorrelated, is

the flat matrix

Balance is doubly-stochastic.=) A

Examples

When are uncorrelated, is

the flat matrix

As align,

tends to the

identity matrix

Balance is doubly-stochastic.=) A

A matrix-valued overlap

A 2B k

µ ¶ µ X ¶

E[X 2] =X

A 2B k

¶ µ1¡

over doubly-stochastic matrices .k £ k A = (aij )

which is controlled by the maximizer of

A 2B k

µ ¶ µ X ¶

E[X 2] =X

A 2B k

¶ µ1¡

X µ X ¶

aij logaij + c logµ

X µ X ¶

aij logaij + c logµ

over doubly-stochastic matrices .k £ k A = (aij )

which is controlled by the maximizer of

A 2B k

µ ¶ µ X ¶

E[X 2] =X

A 2B k

¶ µ1¡

Entropy decreases away from the flat matrix

– For small , this loss overwhelms the sum of squares gain– But for large enough ,….

The maximizer jumps instantaneously from flat, to a matrix where vertices capture the majority of mass

Proved this happens only for

aij logaij + c¢1k

c > k logk ¡ logk ¡ 1

aij logaij + c¢1k

The maximizer jumps instantaneously from flat, to a matrix where entries capture majority of mass

aij logaij + c¢1k

The maximizer jumps instantaneously from flat, to a matrix where entries capture majority of mass

aij logaij + c¢1k

c > k logk ¡ logk ¡ 1 This jump happens only after

Proof overview

Proof. Compare the value at the flat matrix with upper bound for everywhere else derived by:

Proof overview

1. Relax to singly stochastic matrices.

Proof overview

2. Prescribe the L2 norm of each row ρi.

Proof overview

3. Find max-entropy, f(ρi), of each row given ρi.

Proof overview

4. Prove that f’’’ > 0.

Proof overview

5. Use (4) to determine the optimal distribution of the ρi given their total ρ.

Proof overview

5. Use (4) to determine the optimal distribution of the ρi given their total ρ.

6. Optimize over ρ.

Theorem.For everyintegerd > 0, w.h.p.thechromaticnumberof a randomd-regulargraph

is eitherk, k + 1,or k + 2

wherek is thesmallestintegers.t. d < 2k logk.

Random regular graphs

ai logai

ai = 1

a2i = ½

Maximize

subject to

for some 1=k< ½< 1

A vector analogue(optimizing a single row)

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

Maximize

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

For k = 3the maximizer is(x;y;y) wherex > y

Maximize

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

Maximize

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

Maximize

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

Maximize

ai logai

ai = 1

a2i = ½

subject to

for some 1=k< ½< 1

For the maximizer isk > 3

(x;y;: : : ;y)

Maximize

Create a composite image of an object that:

– Minimizes “empirical error” Typically, least-squares error over luminance

– Maximizes “plausibility” Typically, maximum entropy

Maximum entropy image restoration

Structure of maximizer helps detect stars in astronomy

Maximum entropy image restoration

The End

Independence and chromatic number (and random k-SAT): Sparse Case Dimitris Achlioptas Microsoft.

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