Hardness of integer linear feasibility with cyclic support

Abstract

We prove that integer linear feasibility is strongly $NP$-complete even when the constraint matrix has exactly two nonzero entries in every row and column, with its support forming a single cycle. This holds already for monotone systems (one positive and one negative coefficient in each inequality) with nonzero coefficients of absolute value $2$ or $3$ and right-hand sides in $\{-1,0,1,2\}$. No variable bounds are supplied separately from the system. For systems in which each inequality involves at most two variables, this strengthens known hardness results by restricting all numerical data to fixed finite sets. For systems in which each variable occurs in at most two inequalities, this establishes, to the best of our knowledge, the first $NP$-hardness result for feasibility. We also give negative answers, unless $P=NP$, to questions on polynomial-time solvability of integer programs with bounded coefficients and bounded totally $Δ$-modular treewidth, and on fixed-parameter tractability of integer bound propagation with respect to coefficient and right-hand-side magnitudes.