Cayley graphs of given degree and diameter for cyclic, Abelian, and metacyclic groups

Let CC(d,2) and AC(d,2) be the largest order of a Cayley graph of a cyclic and an Abelian group, respectively, of diameter 2 and a given degree d. There is an obvious upper bound of the form CC(d,2)≤AC(d,2)≤d2/2+d+1. We prove a number of lower bounds on both quantities for certain infinite sequences of degrees d related to primes and prime powers, the best being CC(d,2)≥(9/25)(d+3)(d−2) and AC(d,2)≥(3/8)(d2−4). We also offer a result for Cayley graphs of metacyclic groups for general degree and diameter.