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Focused local search for random 3-satisfiability

Sakari Seitz1,2, Mikko Alava2 and Pekka Orponen1

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A local search algorithm solving an NP-complete optimization problem can be viewed as a stochastic process moving in an 'energy landscape' towards eventually finding an optimal solution. For the random 3-satisfiability problem, the heuristic of focusing the local moves on the currently unsatisfied clauses is known to be very effective: the time to solution has been observed to grow only linearly in the number of variables, for a given clauses-to-variables ratio α sufficiently far below the critical satisfiability threshold αc≈4.27. We present numerical results on the behaviour of three focused local search algorithms for this problem, considering in particular the characteristics of a focused variant of simple Metropolis dynamics. We estimate the optimal value for the 'temperature' parameter η for this algorithm, such that its linear time regime extends as close to αc as possible. Similar parameter optimization is performed also for the well-known WalkSAT algorithm and for the less studied, but very well performing focused record-to-record travel method. We observe that with an appropriate choice of parameters, the linear time regime for each of these algorithms seems to extend well into ratios α>4.2—much further than has so far been generally assumed. We discuss the statistics of solution times for the algorithms, relate their performance to the process of 'whitening', and present some conjectures on the shape of their computational phase diagrams.


Keywords

analysis of algorithms

stochastic search

energy landscapes (experiment)

typical-case computational complexity

PACS

02.50.Ey Stochastic processes

02.60.Pn Numerical optimization

05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

MSC

65Kxx Mathematical programming, optimization and variational techniques

68P10 Searching and sorting

60Gxx Stochastic processes

68Q25 Analysis of algorithms and problem complexity (See also 68W40)

Subjects

Computational physics

Statistical physics and nonlinear systems

Dates

Issue 06 (June 2005)

Received 28 January 2005, accepted for publication 24 May 2005

Published 14 June 2005



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