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## Theoretical Computer Science

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Institute of Theoretical Computer Science   Mai 11, 2012 Dr. B. G¨ artner, Prof. J. Matouˇsek and S. Stich  Approximation Algorithms and Semideﬁnite Programming FS12 Homework 4  Course Webpage:  http://www.ti.inf.ethz.ch/ew/lehre/ApproxSDP12/ Due date:  Mai 25, 2012, 10h15. ã  Solutions are to be handed in typed in L A TEX. ã  Please bring a print-out of your solution with you to the lecture. If you cannot attend, you mayalternatively send your solution as a PDF to  stich@inf.ethz.ch . ã  Proofs are to be formally correct, complete and clearly explained. Be short and precise! Exercise 1 (Maximizing quadratic forms)  (12.5 Points) [Exercise 10.3]   Let  A  ∈  R n × n be a positive deﬁnite matrix and consider the optimization problem OPT  := max  x T  A x :  x  ∈ {− 1 , 1 } n  .  (1)We relax this problem to the vector program SDP  := max  i,j a ij v T i  v j  :   v i   = 1 ,i  = 1 ,...,n  .  (2)Let  v ∗ 1 ,..., v ∗ n  be an optimal solution of (2). Then a feasible solution  y  ∈  R n to (1) can be obtainedfrom the  v ∗ i ’s by the random hyperplane rounding (again as in Goemans-Williamson). The goal of thisexercise is to show that the expected approximation ratio of this algorithm is at least  2 π  ≈  0 . 636619 ... .a) Let  X   ∈  R n × n be a positive semideﬁnite matrix,  | x ij | ≤  1 for all 1  ≤  i,j  ≤  n  and let us deﬁnearcsin[ X  ] := (arcsin  x ij ). Show that arcsin[ X  ]  − X     0.b) Starting from an optimal solution  v ∗ 1 ,..., v ∗ n  of (2), calculate the expected value of the roundedsolution  y  and conclude that  OPT  ≥  2 π SDP .c) Let  A  as above. Show that the value  OPT  of the optimization problem (1) equals OPT  = max  2 πA ã  arcsin[ X  ]:  X     0 ,X  ii  = 1 ,i  = 1 ,...,n  .  (3) Hint - Exercise 1.a)  Consider the Taylor series of arcsin x  −  x  and use the following fact (which youdon’t need to prove):For matrices  X,Y   ∈  R n × n , deﬁne the entrywise – or  Hadamard   – product  X   ◦ Y   := ( x ij y ij ). Schur’s Theorem:  If   X     0 and  Y     0, then  X   ◦ Y     0.  Exercise 2 (Coloring 3-Colorable Graphs)  (12.5 Points) In this exercise we will derive a coloring algorithm that uses ˜ O ( n 0 . 387 ) colors to color 3-colorable graphs.Note that this algorithms performs worse than the KMS-scheme from the lecture. We use semideﬁniteprogramming to ﬁrst obtain a  semicoloring   of the graph. A semicoloring is a coloring of the nodes of a  n vertex graph  G  such that at most  n/ 4 edges have endpoints of the same color (i.e. at least  n/ 2 verticesare colored such that any edge between them has endpoints that are colored diﬀerently).Let  G  = ( V,E  ) be a 3-colorable graph on  n  vertices,  m  edges and with maximal degree ∆. Consider thefollowing vector program, where we have a vector  v i  for every vertex  i  ∈  V  : SDP  := min  t :  v T i  v j  ≤  t, ∀{ i,j } ∈  E,  v i   = 1 ,i  = 1 ,...,n  .  (4)We already know (cf. Section 3.7) that for every 3-colorable graph  G  there is a feasible solution to (4)with value  SDP  ≤ − 12 . Let  v ∗ 1 ,..., v ∗ n  be an optimal solution of (2). We can obtain a (semi)coloring bythe following rounding procedure: Chose  r  = 2 + log 3  ∆ random unit vectors  r 1 ,..., r r . The  r  vectorsdeﬁne 2 r diﬀerent regions into which the vectors  v ∗ i  can fall: one region for each possibility of whether r T i  v ∗ j  ≥  0 or  r T i  v ∗ j  <  0 for all  i  = 1 ,...,r . We then color the vectors in each region with a distinct color.a) Suppose you have an algorithm that produces a semicoloring for  G  with  k  colors. Show how we canobtain a valid coloring of   G  using  O ( k log n ) colors.b) Prove that for the above rounding scheme and every edge  e  =  { i,j }  of   G : Pr [ i  and  j  get the same color]  ≤  19∆  . c) Conclude that the rounding scheme produces a semicoloring of   G  with 4∆ log 3  2 ≈  4∆ 0 . 613 colors withprobability at least 1 / 2.d) Use Wigderson’s trick to obtain an algorithm that semicolors every 3-colorable graph with  O ( n log 6  2 )colors with probability at least 1/2.
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