Bartholdi zeta functions of graph coverings

We give a decomposition formula for the Bartholdi zeta function of a regular covering of a graph G. Furthermore, we define an L-function of G, and give a determinant expression of it. As a corollary, we obtain a decomposition formula for the Bartholdi zeta function of a regular covering of G by L-functions of G. Also, we present another proof of the determinant expression of the L-function of G.