Longest Induced Cycles in Circulant Graphs

Abstract

In this paper we study the length of the longest induced cycle in the unit circulant graph $X_n = Cay({\Bbb Z}_n; {\Bbb Z}_n^*)$, where ${\Bbb Z}_n^*$ is the group of units in ${\Bbb Z}_n$. Using residues modulo the primes dividing $n$, we introduce a representation of the vertices that reduces the problem to a purely combinatorial question of comparing strings of symbols. This representation allows us to prove that the multiplicity of each prime dividing $n$, and even the value of each prime (if sufficiently large) has no effect on the length of the longest induced cycle in $X_n$. We also see that if $n$ has $r$ distinct prime divisors, $X_n$ always contains an induced cycle of length $2^r+2$, improving the $r \ln r$ lower bound of Berrezbeitia and Giudici. Moreover, we extend our results for $X_n$ to conjunctions of complete $k_i$-partite graphs, where $k_i$ need not be finite, and also to unit circulant graphs on any quotient of a Dedekind domain.