The Parametric Frobenius Problem

Abstract: Given relatively prime positive integers a_1,...,a_n, the Frobenius number is the largest integer that cannot be written as a nonnegative integer combination of the a_i. We examine the parametric version of this problem: given a_i=a_i(t) as functions of t, compute the Frobenius number as a function of t. A function f is a quasi-polynomial if there exists a period m and polynomials f_0,...,f_{m-1} such that f(t)=f_{t mod m}(t) for all positive integers t. We conjecture that, if the a_i(t) are polynomials (or quasi-polynomials) in t, then the Frobenius number agrees with a quasi-polynomial, for sufficiently large t. We prove this in the case where the a_i(t) are linear functions, and also prove it in the case where n (the number of generators) is at most 3.