Zeta functions of oriented line graphs of graph coverings

In this paper, we obtain a decomposition formula for the zeta function of an oriented line graph of a regular covering of a graph G. Furthermore, we introduce an L-function of an oriented line graph L⇒(G) of G, and give a decomposition formula for the zeta function of an oriented line graph of a regular covering of G by L-functions of L⇒(G). As a by-product, we obtain an L-function version of Bass's result.