A system of congruences mod with is called a Covering System if every Integer satisfies for at least one value of . A covering system in which each integer is covered by just one congruence is called an exact covering system.
See also Covering System
Guy, R. K. ``Exact Covering Systems.'' §F14 in
Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 253-256, 1994.