Algorithmic barriers from phase transitions
Abstract
For many random Constraint Satisfaction Problems, by now, we have asymptotically tight estimates of the largest constraint density for which they have solutions. At the same time, all known polynomialtime algorithms for many of these problems already completely fail to find solutions at much smaller densities. For example, it is wellknown that it is easy to color a random graph using twice as many colors as its chromatic number. Indeed, some of the simplest possible coloring algorithms already achieve this goal. Given the simplicity of those algorithms, one would expect there is a lot of room for improvement. Yet, to date, no algorithm is known that uses $(2\epsilon) \chi$ colors, in spite of efforts by numerous researchers over the years. In view of the remarkable resilience of this factor of 2 against every algorithm hurled at it, we believe it is natural to inquire into its origin. We do so by analyzing the evolution of the set of $k$colorings of a random graph, viewed as a subset of $\{1,...,k\}^{n}$, as edges are added. We prove that the factor of 2 corresponds in a precise mathematical sense to a phase transition in the geometry of this set. Roughly, the set of $k$colorings looks like a giant ball for $k \ge 2 \chi$, but like an errorcorrecting code for $k \le (2\epsilon) \chi$. We prove that a completely analogous phase transition also occurs both in random $k$SAT and in random hypergraph 2coloring. And that for each problem, its location corresponds precisely with the point were all known polynomialtime algorithms fail. To prove our results we develop a general technique that allows us to prove rigorously much of the celebrated 1step ReplicaSymmetryBreaking hypothesis of statistical physics for random CSPs.
 Publication:

arXiv eprints
 Pub Date:
 March 2008
 arXiv:
 arXiv:0803.2122
 Bibcode:
 2008arXiv0803.2122A
 Keywords:

 Mathematics  Combinatorics;
 Mathematics  Probability;
 05C80
 EPrint:
 extended abstract