Universal relations and #P-completeness

This paper follows the methodology introduced by Agrawal and Biswas in [Manindra Agrawal, Somenath Biswas, Universal relations, in: Structure in Complexity Theory Conference, 1992, pp. 207–220], based on a notion of universality for the relations associated with NP-complete problems. The purpose was to study NP-complete problems by examining the effects of reductions on the solution sets of the associated witnessing relations. This provided a useful criterion for NP-completeness while suggesting structural similarities between natural NP-complete problems. We extend these ideas to the class #P. The notion we find also yields a practical criterion for #P-completeness, as illustrated by a varied set of examples, and strengthens the argument for structural homogeneity of natural complete problems.