Cores of imprimitive symmetric graphs of order a product of two distinct primes

Ricky Rotheram, Sanming Zhou

A retract of a graph $\Gamma$ is an induced subgraph $\Psi$ of $\Gamma$ such that there exists a homomorphism from $\Gamma$ to $\Psi$ whose restriction to $\Psi$ is the identity map. A graph is a core if…