1. SDP Based Approach for Graph Partitioning and Embedding Negative Type Metrics into L 1 Subhash Khot (Georgia Tech) Nisheeth K. Vishnoi (IBM Research and Georgia Tech) CS Perspective Math Perspective Parts I & II

2. CS Story: Sparsity S ScSc Sparsity of a Cut |E(S,S c )| ---------- |S| |S c | Sparsest Cut (SC) cut of minimum sparsity b-Balanced Separator (BS) cut with |S|,|S c | ≥ bn that minimizes |E(S,S c )| b ~ ½

3. Applications Related Measures  Sparsity is often referred to as Graph Conductance  Edge expansion or isoperimetric constant Applications  VLSI Layout  Clustering  Markov Chains  Geometric (Metric) Embeddings

4. Estimating Sparsity λ 2 (G)/n ≤ sparsity(G) ≤ 3/n √λ 2 (G) √∆ λ 2 : spectral gap (second eigenvalue of the Laplacian) Not satisfactory, e.g. n-cycle Approx. Algo. : For any graph G on n vertices, compute a(G), which is within a mult. factor f(n)≥1 of sparsity of G. a(G) ≤ sparsity(G) ≤ a(G) f(n) f(n)=1 is hard What about f(n) = log n or even 10? Hard to compute exactly – compute “approximations”

5. History Algorithms  Spectral Graph partitioning Alon-Milman ’85, Speilman-Teng ’96 (eigenvector based)  O(log n) Leighton-Rao ’88 (Linear Programming (LP) based)  O(log n) London-Linial-Rabinovitch ’94, Aumann-Rabani ’94 (connection to metric embeddings)  O(√log n) Arora-Rao-Vazirani ‘04 (Semi-Definite Programming (SDP) based) Hardness  NP-Hard  Hard to approximate within any constant factor (assuming UGC) Chawla-Krauthgamer-Kumar-Rabani-Sivakumar ‘05, Khot-Vishnoi ‘05

6. Outline of this Talk Part I 1.Graph Partitioning Motivation & History SDP Approach 2.Embedding Negative Type Metrics into L 1 Metric Spaces and Embeddability Cut Cone ≈ L 1 Negative Type Metrics as SDP Solutions LLR/AR Connection Part II 1.Integrality Gap instance Hypercube and Cuts Kahn-Kalai-Linial (isoperimetry of hypercube) The Graph The SDP Solution 2.Conclusion

7. Outline of this Talk Part I 1.Graph Partitioning Motivation & History SDP Approach 2.Embedding Negative Type Metrics into L 1 Metric Spaces and Embeddability Cut Cone ≈ L 1 Negative Type Metrics as SDP Solutions LLR/AR Connection Part II 1.Integrality Gap instance Hypercube and Cuts Kahn-Kalai-Linial (isoperimetry of hypercube) The Graph The SDP Solution 2.Conclusion

8. Quadratic Program for BS  i, v i  {-1,1} Minimize ¼  |v i - v j | 2 ij ε E  |v i - v j | 2  = n 2 i<j Quadratic Program Input: G(V,E) Output: (S,S c ) s.t. |S|,|S c | = n/2 which minimizes |E(S,S c )| Balanced Separator

9. SDP for Balanced Separator  i, v i  {-1,1} Minimize ¼  |v i - v j | 2 ij ε E  |v i - v j | 2  = n 2 i<j Quadratic Program SDP Relaxation  i, v i  R n, ||v i ||=1 Minimize ¼  || v i - v j || 2 ij  E Well-Separatedness  || v i - v j || 2 = n 2 i<j

10. Why is this a Relaxation ? SDP Relaxation G(V,E)  i, v i  R n, ||v i ||=1 Minimize ¼  || v i - v j || 2 ij  E Well-Separatedness  || v i - v j || 2 = n 2 i<j Relaxation u be a unit vector and (S,S c ) |S|, |S c | = n/2 For i є S, v i = u, i є S c, v i = - u ||v i -v j || 2 = 4 · δ S (i,j) Cost of solution = |E(S,S c )| sdp ≤ opt SDP can be computed in polynomial time! Boils down to the spectral approach. Nothing gained(?)

11. Quadratic Program for BS …  i, v i  {-1,1} Minimize ¼  |v i - v j | 2 ij ε E  |v i - v j | 2  = n 2 i<j Triangle Inequality  i, j, k, |v i - v j | 2 + |v j - v k | 2  |v i - v k | 2 (redundant) Quadratic Program Input: G(V,E) Output: (S,S c ) s.t. |S|,|S c | = n/2 which minimizes |E(S,S c )| Balanced Separator

12. SDP for Balanced Separator …  i, v i  {-1,1} Minimize ¼  |v i - v j | 2 ij ε E  |v i - v j | 2  = n 2 i<j Triangle Inequality  i, j, k, |v i - v j | 2 +|v j - v k | 2  |v i - v k | 2 Quadratic Program SDP Relaxation  i, v i  R n, ||v i ||=1 Minimize ¼  || v i - v j || 2 ij  E Well-Separatedness  || v i - v j || 2 = n 2 i<j Triangle Inequality  i, j, k, ||v i - v j || 2 +||v j - v k || 2  ||v i - v k || 2 Still a relaxation …

13. Geometry of Triangle Ineq ≤ 90 o vivi vkvk vjvj each step of length 1 t-steps: length at-most √t Rules out the embedding obtained by the spectral method!

14. Integrality Gap: Upper Bound Arora-Rao-Vazirani ’04: O(√log n) for Sparsest Cut, Balanced Separator sdp within a factor of O(√log n) of the opt Integrality gap: max over all graphs on n vertices, the ratio of opt/sdp (as a function of n) ARV conjectured that the integrality gap is upper bounded by some constant (independent of n) Lack of any counterexample!

15. Outline of this Talk Part I 1.Graph Partitioning Motivation & History SDP Approach 2.Embedding Negative Type Metrics into L 1 Metric Spaces and Embeddability Cut Cone ≈ L 1 Negative Type Metrics as SDP Solutions LLR/AR Connection Part II 1.Integrality Gap instance Hypercube and Cuts Kahn-Kalai-Linial (isoperimetry of hypercube) The Graph The SDP Solution 2.Conclusion

16. Math Story: Metric Embeddings Metric is a distance function d on [n] x [n] s.t. d(i, j) + d(j, k)  d(i, k) (triangle inequality) Metric d embeds into metric  with distortion   1 if there is a map φ s.t.  i,j d(i, j)   (φ(i), φ(j))   d(i, j) ( distances are preserved upto a factor of  )

17. Negative Type Metrics (squared- L 2 ) d on {1,2,…,n} is of negative type if there are vectors v 1, v 2, …, v n Such that d(i, j) = || v i - v j || 2 satisfies triangle inequality Same as:  i, j, k, || v i - v j || 2 + || v j - v k || 2  || v i - v k || 2 NEG = class of such metrics. arise as SDP solutions L 1  NEG

18. Embedding NEG into L 1 Conjecture: (Explicit by Goemans, Linial abt ’95) Every NEG metric embeds into L 1 with O(1) (constant) distortion What’s the connection to sparsity ?

19. Cuts and L 1 Metrics p S : non-negative real for every subset S of [n] d(i,j) :=  p S δ S (i,j) Fact : d is isometrically embeddable in L 1 Further : Every L 1 embeddable metric on n-points can be written as non-negative linear sum of cut-metrics on {1,…,n} Cut-Metrics on {1,2,…,n} δ S (i, j) = 1 if i, j are separated by (S,S c ) = 0 otherwise S ScSc

20. Sparsest Cut ≈ Optimizing Over L 1 [Aumann Rabani 98, Linial London Rabinovich ’94] LP-relaxation over all METRICS [Bourgain ’85] Every n-point metric embeds into L 1 with O(log n) distortion O(log n) factor approximation for Sparsity!  i~j δ S (i, j) -------- --  i<j δ S (i, j) Minimize S  V Minimize d is L 1  i~j d(i, j) -------- --  i i<j d(i, j) =

21. Metric Embeddings & Sparsity Optimizing over cuts ≈ Optimizing over L 1 metrics SDP solution ≈ Optimizing over NEG Goemans-Linial/ARV Conjecture: NEG embeds in L 1 with O(1) distortion/ Integrality Gap is O(1) Implies O(1) approx. algo for estimating sparsity!

22. Integrality Gap: Lower Bound Sparsest Cut, Balanced Separator  log log n integrality gap instance Khot-Vishnoi ’05, Krauthgamer-Rabani ‘06 Devanur-Khot-Saket-Vishnoi ‘06 Disproves the GL/ARV Conjecture Previous best lower bound: 1.16 [Zatloukal ’04]

23. Outline of this Talk Part I 1.Graph Partitioning Motivation & History SDP Approach 2.Embedding Negative Type Metrics into L 1 Metric Spaces and Embeddability Cut Cone ≈ L 1 Negative Type Metrics as SDP Solutions LLR/AR Connection Part II 1.Integrality Gap instance Hypercube and Cuts Kahn-Kalai-Linial (isoperimetry of hypercube) The Graph The SDP Solution 2.Conclusion

24. Outline of this Talk Part I 1.Graph Partitioning Motivation & History SDP Approach 2.Embedding Negative Type Metrics into L 1 Metric Spaces and Embeddability Cut Cone ≈ L 1 Negative Type Metrics as SDP Solutions LLR/AR Connection Part II 1.Integrality Gap instance Hypercube and Cuts Kahn-Kalai-Linial (isoperimetry of hypercube) The Graph The SDP Solution 2.Conclusion

25. Recall: Integrality Gap Lower Bound Sparsest Cut, Balanced Separator  log log n integrality gap instance Khot-Vishnoi ’05, Krauthgamer-Rabani ’06, Devanur-Khot-Saket-Vishnoi ‘06 Disproves the GL/ARV Conjecture Previous best lower bound: 1.16 [Zatloukal ’04]

26. Integrality Gap: Lower Bound Thm: Construct graph G({1,…,n},E) and unit vector assignment i -> v i є R n s.t. 1.G is an “expander”: every ¼ balanced cut has Ω(|E|(log log n)/log n) edges ( Kahn-Kalai-Linial) 2.“Low” SDP solution: O(|E|/log n) 3.Well-Separatedness: Σ i<j ||v i -v j || 2 = n 2 4.Triangle inequality: d(i,j):=||v i -v j || 2 is a metric Integrality gap: Ω(log log n)

27. Starting Point: Hypercube(!) H={-1,1} k n = 2 k (1,1,1)(1,1,1) (-1,1,1) (1,1,-1) (1,-1,1) (1,-1,-1) (-1,-1,1) (-1,1,-1) (-1,-1,-1)

28. Hypercube … H={-1,1} k Advantages? Understand cuts in H : tools from Fourier Analysis Vertex is a vector in R k : starting point for SDP solution But … Hypercube has “small” balanced cuts : coordinate cuts have 1/k fraction of the edges

29. Cuts in Hypercube: Coordinate # of edges = |E(H)|/k Edges across pairs of vertices differing in i-th bit

30. Cuts in Hypercube … decompose into coordinate cuts any balanced cut has a coord. cut which contributes E(H)/k 2 edges

31. Kahn-Kalai-Linial any balanced cut has a coordinate cut which contributes E(H) (log k)/k 2 edges

32. Increasing Size of Balanced Cuts consider balanced cuts in which coordinates are indistinguishable (w.r.t. to their contribution to the cut) can be achieved by symmetrizing the hypercube! each coordinate contributes equally: total E(H) (log k)/k edges

33. e.g. 4-dim hypercube (1,1,1,1) (-1,-1,-1,-1) (1,1,1,-1) (-1,1,1,1) (1,-1,1,1) (1,1,-1,1) (-1,-1,-1,1) (1,-1,-1,-1) (-1,1,-1,-1) (-1,-1,1,-1) (1,1,-1,-1) (-1,1,1,-1) (-1,-1,1,1) (1,-1,-1,1) (1,-1,1,-1) (-1,1,-1,1)

34. More Formally … H={-1,1} k with a rotation group acting on its coordinates Partitions H into equivalence classes V 1,…,V n Each V i is a vertex. Edges are hypercube edges G(V,E), |E(G)|=|E(H)|, k ~ log n Balanced cuts in G correspond to balanced cuts in H KKL: any balanced cut (in H) has “a” coordinate cut which contributes E(H) (log k)/k 2 edges to the cut. Group is transitive: “every” coordinate cut has the same contribution Any balanced cut in G has ≥ E(G) (log k)/k = E(G) (log log n)/(log n)

35. Integrality Gap: Lower Bound Thm: Construct graph G({1,…,n},E) and unit vector assignment i -> v i є R n s.t. 1.G is an “expander”: every ¼ balanced cut has Ω(|E|(log log n)/log n) edges ( Kahn-Kalai-Linial) 2.“Low” SDP solution: O(|E|/log n) 3.Well-Separatedness: Σ i<j ||v i -v j || 2 = n 2 4.Triangle inequality: d(i,j):=||v i -v j || 2 is a metric Integrality gap: Ω(log log n)

36. SDP Solution (1,1,1,1) (-1,-1,-1,-1) (1,1,1,-1) (-1,1,1,1) (1,-1,1,1) (1,1,-1,1) (-1,-1,-1,1) (1,-1,-1,-1) (-1,1,-1,-1) (-1,-1,1,-1) (1,1,-1,-1) (-1,1,1,-1) (-1,-1,1,1) (1,-1,-1,1) (1,-1,1,-1) (-1,1,-1,1)

37. Formally: SDP Solution Vertex: equivalence class: x 1, x 2, …, x k (rotations) Vector: (1/√k) · Σ j x j Observations: Edge across two nodes differing in one bit Contribution to sdp ~ 1/k. sdp ≤ |E(G)|/k = |E(G)|/(log n) Triangle Inequality: (little bit of work and case analysis!) For most classes {x 1, …, x k } is “nearly orthogonal” Hidden: Gram-Schmidt Orthogonalization, Tensoring, Well-separatedness

38. Conclusion (Simple) log log n integrality gap for SC/BS Close the gap between log log n and  log n ? [Lee-Naor ’06, Cheeger-Kleiner ’06] Another counter-example. May give (log n) c Thank you!