Tally NP Sets and Easy Census Functions

Abstract

We study the question of whether every P set has an easy (i.e., polynomial-time computable) census function. We characterize this question in terms of unlikely collapses of language and function classes such as the containment of #P_1 in FP, where #P_1 is the class of functions that count the witnesses for tally NP sets. We prove that every #P_{1}^{PH} function can be computed in FP^{#P_{1}^{#P_{1}}}. Consequently, every P set has an easy census function if and only if every set in the polynomial hierarchy does. We show that the assumption of #P_1 being contained in FP implies P = BPP and that PH is contained in MOD_{k}P for each k \geq 2, which provides further evidence that not all sets in P have an easy census function. We also relate a set's property of having an easy census function to other well-studied properties of sets, such as rankability and scalability (the closure of the rankable sets under P-isomorphisms). Finally, we prove that it is no more likely that the census function of any set in P can be approximated (more precisely, can be n^{\alpha}-enumerated in time n^{\beta} for fixed \alpha and \beta) than that it can be precisely computed in polynomial time.