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Solution-space structure of (some) optimization problems

A K Hartmann1, A Mann2 and W Radenbach3

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We study numerically the cluster structure of random ensembles of two NP-hard optimization problems originating in computational complexity, the vertex-cover problem and the number partitioning problem. We use branch-and-bound type algorithms to obtain exact solutions of these problems for moderate system sizes. Using two methods, direct neighborhood-based clustering and hierarchical clustering, we investigate the structure of the solution space. The main result is that the correspondence between solution structure and the phase diagrams of the problems is not unique. Namely, for vertex cover we observe a drastic change of the solution space from large single clusters to multiple nested levels of clusters. In contrast, for the number-partitioning problem, the phase space looks always very simple, similar to a random distribution of the lowest-energy configurations. This holds in the ''easy''/solvable phase as well as in the ''hard''/unsolvable phase.


PACS

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

05.20.-y Classical statistical mechanics

05.50.+q Lattice theory and statistics (Ising, Potts, etc.)

02.60.Pn Numerical optimization

Subjects

Computational physics

Statistical physics and nonlinear systems

Dates

Issue 1 (2008)



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