Computing with an algebraic-perturbation variant of Barvinok’s algorithm

Abstract

We present a variant of Barvinok’s algorithm for computing a short rational generating function for the integer points in a nonempty pointed polyhedron P:={x∈Rn:Ax≤b} given by rational inequalities. A main use of such a rational generating function is to count the number of integer points in P.Our variant is based on making an algebraic perturbation of the right-hand side b∈Qm of the inequalities, replacing each bi with bi+τi, where τ is considered to be an arbitrarily small positive real indeterminate. Hence, elements of our right-hand side vector become elements of the ordered ring Q[τ] of polynomials in τ. Denoting the algebraically-perturbed polyhedron as P(τ)⊂R[τ]n, we readily see that: (i) P(τ) is always full dimensional, (ii) P(τ) is always simple, and (iii) P(τ) contains the same integer points as P. Unlike other versions of Barvinok’s algorithm, we will have to do some arithmetic in Q[τ]. However, because of (i) we will not need to preprocess our input polyhedron if it is not full dimensional, and because of (ii) we will not need to triangulate tangent cones at the vertices of the polyhedron.We give the details of our perturbation variant of Barvinok’s algorithm, describe a proof-of-concept implementation developed in Mathematica, and present results of computational experiments.