Hofmeister introduced the notion of a concrete (resp., concrete regular) covering of a graph G and gave formulas for enumerating the isomorphism classes of concrete (resp., concrete regular) coverings of G [Ars Combin., 32 (1991), pp. 121–127; SIAM J. Discrete Math., 8 (1995), pp. 51–61]. In this paper, we show that the number of the isomorphism classes of… (More)