Covering systems where the prime divisors of all moduli are only $2$, $3$, or $5$

Abstract

We try to find all quadruples of positive integers $(m,a,b,c)$ with $a\ge b\ge c$ such that there exists a distinct covering system with minimum modulus $m$ and least common multiple of the moduli $2^a{3}^b{5}^c$. We obtain complete description of all such quadruples when $m=2,3,4,5$, or $6$, except when $m=6$ and $b=c=1$. We also show that if the LCM of the moduli has only $2$, $3$, or $5$ as prime divisors, then $m\le 9$ and construct a distinct covering system with $m=8$, $a=8$, $b=3$, and $c=2$. When a covering system exists for a quadruple $(m,a,b,c)$ we provide an example. Nonexistence of covering systems is established via integer programming or by using a new estimate on the density of a set covered by a system of congruences.