tailieunhanh - Báo cáo toán học: "Random k-SAT: the limiting probability for satisfiability for moderately growing k"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: Random k-SAT: the limiting probability for satisfiability for moderately growing k. | Random k-SAT the limiting probability for satisfiability for moderately growing k Amin Coja-Oghlan Alan Frieze Department of Mathematical Sciences Carnegie Mellon University PittsbUrgh PA 15213 USA. acoghlan@ alan@ Submitted Sep 11 2007 Accepted Jan 17 2008 Published Feb 4 2008 Mathematics Subject Classihcation 05C88 Abstract We consider a random instance Im Im n k of k-SAT with n variables and m clauses where k k n satishes k log2 n 1. Let m 2k n In 2 c for an absolute constant c. We prove that TW. T -e c lim Pr Im is satishable e nn 1 Introduction An instance of k-SAT is defined by a set of variables V f X1 x2 . xng and a set of clauses C1 C2 . Cm. We will let clause Ci be a sequence Ai 1 Ai 2 . Ai k where each literal Ai i is a member of L V u V where V fx1 x2 . xng. In our random model each Ai l is chosen independently and uniformly from L. 1 We denote the resulting random instance by Im Im n k. Supported by DFG COJ 646. Supported in part by NSF grant CCF-0502793 1 We are aware that this allows clauses to have repeated literals or instances of x x. The focus of the paper is on k O ln n although the main result is valid for larger k. Thus most clauses will not have repeated clauses or contain a pair x x. THE ELECTRONIC JOURNAL OF COMBINATORICS 15 2008 N2 1 Random k-SAT has been well studied to say the least see the references in 6 . If k 2 then it is known that there is a satisfiability threshold at around m n. More precisely if e 0 is fixed and I is a random instance of 2-SAT then lim Pr Im n 2 is satisfiable nn Í0 m 1 e n m 1 e n Thus random 2-SAT is now pretty much understood. For k 3 the story is very different. It is now known that a threshold for satisfiability exists in some not completely satisfactory sense Friedgut 5 . There has been considerable work on trying to find estimates for this threshold in the case k 3 see the references in 6 . Currently the best lower bound for the threshold is due to Hajiaghayi and Sorkin