A graph Γ is called an n-Cayley graph over a group G if there exists a semiregular subgroup of Aut(Γ) that is isomorphic to G with n orbits (of equal size). This is one of the generalizations of Cayley graphs. In this paper, we show that a sequence of n-Cayley graphs over solvable groups cannot form an expander family. We also show that a connected graph being an n-Cayley graph over G is equivalent to being a Galois covering with Galois group G.
Nao Toyama (Fri,) studied this question.