Finding total unimodularity in optimization problems solved by linear programs

Authors:
Christoph Dürr;Mathilde Hurand
Affiliations:
Laboratoire de Recherche en Informatique de l'Ecole Polytechnique (LIX), Palaiseau, France;Laboratoire de Recherche en Informatique de l'Ecole Polytechnique (LIX), Palaiseau, France
Venue:
ESA'06 Proceedings of the 14th conference on Annual European Symposium - Volume 14
Year:
2006

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Abstract

A popular approach in combinatorial optimization is to model problems as integer linear programs. Ideally, the relaxed linear program would have only integer solutions, which happens for instance when the constraint matrix is totally unimodular. Still, sometimes it is possible to build an integer solution with same cost from the fractional solution. Examples are two scheduling problems [4,5] and the single disk prefetching/caching problem [3]. We show that problems such as the three previously mentioned can be separated into two subproblems: (1) finding an optimal feasible set of slots, and (2) assigning the jobs or pages to the slots. It is straigthforward to show that the latter can be solved greedily. We are able to solve the former with a totally unimodular linear program, from which we obtain simple combinatorial algorithms with improved worst case running time.