Huge tables and multicommodity flows are fixed parameter tractable via unimodular integer Caratheodory

Abstract

The three-way table problem is to decide if there exists an l x m x n table satisfying given line sums, and find a table if there is one. It is NP-complete already for l=3 and every bounded integer program can be isomorphically represented in polynomial time for some m and n as some 3 x m x n table problem. Recently, the problem was shown to be fixed-parameter tractable with parameters l,m. Here we extend this and show that the huge version of the problem, where the variable side n is a huge number encoded in binary, is also fixed-parameter tractable with parameters l,m. We also conclude that the huge multicommodity flow problem with m suppliers and a huge number n of consumers is fixed-parameter tractable parameterized by the numbers of commodities and consumer types. One of our tools is a theorem about unimodular monoids which is of interest on its own right. The monoid problem is to decide if a given integer vector is a finite nonnegative integer combination of a given set of integer vectors, and find such a decomposition if one exists. We consider sets given implicitly by an inequality system. For such sets, it was recently shown that in fixed dimension the problem is solvable in polynomial time with degree which is exponential in the dimension. Here we show that when the inequality system which defines the set is defined by a totally unimodular matrix, the monoid problem can be solved in polynomial time even in variable dimension.