Top degree coefficients of the denumerant

For a given sequence α=[α 1 ,α 2 ,⋯,α N ,α N+1 ] of N+1 positive integers, we consider the combinatorial function E(α)(t) that counts the nonnegative integer solutions of the equation α 1 x 1 +α 2 x 2 +⋯+α N x N +α N+1 x N+1 =t, where the right-hand side t is a varying nonnegative integer. It is well-known that E(α)(t) is a quasipolynomial function of t of degree N. In combinatorial number theory this function is known as the denumerant. Our main result is a new algorithm that, for every fixed number k, computes in polynomial time the highest k+1 coefficients of the quasi-polynomial E(α)(t) as step polynomials of t. Our algorithm is a consequence of a nice poset structure on the poles of the associated rational generating function for E(α)(t) and the geometric reinterpretation of some rational generating functions in terms of lattice points in polyhedral cones. Experiments using a MAPLE implementation will be posted separately.