In this paper, we consider solving the integer linear systems, i.e., given a matrix A ∈ R m×n , a vector b ∈ R m , and a positive integer d, to compute an integer vector x ∈ D n such that Ax ≥ b, where m and n denote positive integers, R denotes the set of reals, andThe problem is one of the most fundamental NP-hard problems in computer science.For the problem, we propose a complexity index η which is based only on the sign pattern of A. For a real γ, let ILS = (γ) denote the family of the problem instances I with η(I) = γ. We then show the following trichotomy: In this paper, we consider the problem of computing an integer vector x ∈ D n such that Ax ≥ b, which we denote by ILS. The ILS problem is one of the most fundamental and important problems in computer science, and have been studied extensively from both theoretical and practical points of view [18,26]. It is known that the ILS problem is strongly NP-hard, and can be solved in polynomial time, if m or n are bounded by some constant [22], or A is totally unimodular and b is integral [15]. When A is quadratic (also called TVPI, i.e., each row of A contains at most two nonzero elements) or Horn (i.e., each row of A contains at most one positive element), the ILS problem is known to be weakly NP-hard, but it can be solved in time polynomial in the input length and d, and hence in pseudo-polynomial time [20,14,29]. The best known bounds for quadratic and Horn systems require O(md) time [2] and O(n 2 md) time, respectively. For unit linear systems, i.e., A ∈ {0, −1, +1} m×n , it is known that the