Modular difference logic is hard
Nikolaj Bjørner, Andreas R. Blass, Yuri G. Gurevich, Madan Musuvathi · arXiv (Cornell University) · 2008
In connection with machine arithmetic, we are interested in systems of constraints of the form x + k \leq y + k'. Over integers, the satisfiability problem for such systems is polynomial time. The problem becomes NP complete if we restrict attention to the residues for a fixed modulus N.