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Subhash Khot Georgia TechPowerPoint Presentation

Subhash Khot Georgia Tech

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- Max-Cut:
- Weighted graph H(say weights sum to 1).
- Find a subset of vertices Ato maximize weight ofedges between A and Ac.

A

.097

.183

.059

[Karp’72]:5/6vs.5/6 − 1/poly(n)NP-hard

[Sahni-Gonzalez’76]

[Goemans-Williamson’95]: .878 factor

[Håstad+TSSW’97]: 17/21vs.16/21NP-hard

[Zwick’99/FL’01/CW’04]: 1/2 + (/log(1/))

[KKMO+MOO’05]: UGC-hardness

s

- When OPT is c, can you in poly-time cut s?

1

arccos(1−2c)/

.878 c

Max-Cut-Gain

c

1/2

1

.845

- When OPT is c, can you in poly-time cut s?

Theorem 1:

SDP integrality gap in blue.

Theorem 2:

UGC-hardness there too.

1/2 + (11/13)

Theorem 3:

Other stuff.

1/2 + (2/)

Theorem 4:

1/2 + O(/log(1/))

1/2 + (/log(1/))

c

1/2

1/2 +

- Theme of the paper:
- Semidefinite programming integrality gaps arise naturally in Gaussian space.
- Can be translated into Long Code tests; ) UGC-hardness.

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:
- Goemans-Williamson: “For all H, s¸ blah(c).”
- Proof: Given A, construct A via:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

1. Pick G, rand. n-dim. Gaussian

2. Define A(x) = sgn(G ¢ A(x))

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:
- Feige-Langberg/Charikar-Wirth: “For all H, s¸ blah(c).”
- Proof: Given A, construct A via:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

1. Pick G, rand. n-dim. Gaussian

F

1

2. Define A(x) = sgn(G ¢ A(x))

2. Define A(x) = F(G ¢ A(x))

−1

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:
- Goemans-Williamson: “For all H, s¸ blah(c).”
- Proof: Given A, construct A via:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

1. Pick G, rand. n-dim. Gaussian

2. Define A(x) = sgn(G ¢ A(x))

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

Goemans-Williamson: “For all H, s¸ blah(c).”

Proof: Given A, construct A via:

1. Pick G, rand. n-dim. Gaussian

2. Define A(x) = sgn(G ¢ A(x))

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

Feige-Schechtman: “There exists H s.t. s· blah(c).”

(matches GW for c¸ .845)

Proof: Take V = Rn, w = picking (1−2c)-correlated Gaussians.

Take A(x) = x / || x ||.

Best A is A(x) = sgn(G ¢ x), for any G.

Proof: Symmetrization. [Borell’85]

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

This paper: “There exists H s.t. s· blah(c).”

(essentially matches FL/CW for c = 1/2 + )

Proof: Take V = Rn, w = picking mixture of 2 corr’d Gaussian pairs.

Proof: Take V = Rn, w = picking (1−2c)-correlated Gaussians.

Take A(x) = x / || x ||.

Best A is A(x) = F(G ¢ x), for any G.

Best A is A(x) = sgn(G ¢ x), for any G.

Proof: Symmetrization. [Borell’85]

Proof:

- Semidefinite programming gaps
- Weighted graph: H = (V, w:V£V ! R¸0)
- Assignments: A:V ! [−1,1] vs. A:V ! Bn
- Compare:

(unit n-dim. ball)

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

(x,y) Ã w

A

Feige-Schechtman: “There exists H s.t. s· blah(c).”

(matches GW for c¸ .845)

Proof: Take V = Rn, w = picking (1−2c)-correlated Gaussians.

Take A(x) = x / || x ||.

Best A is A(x) = sgn(G ¢ x), for any G.

Proof: Symmetrization. [Borell’85]

Weighted graph: H = ({−1,1}n, w:V£V ! R¸0)

Weighted graph: H = (V, w:V£V ! R¸0)

Assignments: A:{−1,1}n! [−1,1] vs. Ai(x) = xi

Assignments: A:V ! [−1,1] vs. A:V ! Bn

(unit n-dim. ball)

far from all Dictators

Compare:

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

far from all Dictators

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

i

i

i

(x,y) Ã w

A

Feige-Schechtman: “There exists H s.t. s· blah(c).”

(matches GW for c¸ .845)

Proof: Take V = Rn, w = picking (1−2c)-correlated Gaussians.

Take A(x) = x / || x ||.

Best A is A(x) = sgn(G ¢ x), for any G.

Proof: Symmetrization. [Borell’85]

Weighted graph: H = ({−1,1}n, w:V£V ! R¸0)

Assignments: A:{−1,1}n! [−1,1] vs. Ai(x) = xi

far from all Dictators

Compare:

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

far from all Dictators

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

i

i

i

(x,y) Ã w

KKMO/MOO: “There exists w s.t. s· blah(c).”

Feige-Schechtman: “There exists H s.t. s· blah(c).”

(matches GW for c¸ .845)

Proof: w = picking (1−2c)-correlated bit-strings.

Proof: Take V = Rn, w = picking (1−2c)-correlated Gaussians.

Take A(x) = x / || x ||.

Best A is A(x) = sgn(G ¢ x), for almost any G.

Best A is A(x) = sgn(G ¢ x), for any G.

Proof: Somewhat elaborate reduction to [Borell’85] (“Majority Is Stablest”)

Proof: Symmetrization. [Borell’85]

Weighted graph: H = ({−1,1}n, w:V£V ! R¸0)

Assignments: A:{−1,1}n! [−1,1] vs. Ai(x) = xi

far from all Dictators

Compare:

s:= max E [ (½) − (½) A(x) ¢ A(y) ]

A

(x,y) Ã w

far from all Dictators

c:= max E [ (½) − (½) A(x) ¢ A(y) ]

vs.

i

i

i

(x,y) Ã w

KKMO/MOO: “There exists w s.t. s· blah(c).”

This paper: “There exists w s.t. s· blah(c).”

(essentially matches FL/CW for c = 1/2 + )

(matches GW for c¸ .845)

Proof: w = picking mixture of 2 corr’d bit-string pairs.

Proof: w = picking (1−2c)-correlated bit strings.

Best A is A(x) = F(G ¢ x), for almost any G.

Best A is A(x) = sgn(G ¢ x), for almost any G.

Proof: Somewhat elaborate reduction to [Borell’85] (“Majority Is Stablest”)

Proof: if |ai| is small for each i.

- Conclusion:
- There is something fishy going on.
- What is the connection between SDP integrality gaps and Long Code tests?

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