Physics – Condensed Matter – Disordered Systems and Neural Networks
Scientific paper
2007-11-25
Physics
Condensed Matter
Disordered Systems and Neural Networks
10 pages, 5 figures, Fig. 4 in reduced quality to reduce size, Proceedings of the International Workshop on Statistical-Mechan
Scientific paper
10.1088/1742-6596/95/1/012011
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.
Hartmann Alexander K.
Mann Alexander
Radenbach Wolfgang
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