The complexity of computing the size of an interval

Abstract

We study the complexity of counting the number of elements in intervals of feasible partial orders. Depending on the properties that partial orders may have, such counting functions have different complexities. If we consider total, polynomial-time decidable orders then we obtain exactly the P functions. We show that the interval size functions for polynomial-time adjacency checkable orders are tightly related to the class FPSPACE(poly): Every FPSPACE(poly) function equals a polynomial-time function subtracted from such an interval size function. We study the function DIV that counts the nontrivial divisors of natural numbers, and we show that DIV is the interval size function of a polynomial-time decidable partial order with polynomial-time adjacency checks if and only if primality is in polynomial time. © 2011 Springer-Verlag Berlin Heidelberg.