Iwasawa theory for branched ${\mathbb{Z}}_p$-towers of finite graphs and Ihara zeta and $L$-functions

We revisit the theory of Ihara $L$-functions in the context initially studied by Bass and Hashimoto and more recently by Zakharov. In particular, we study if the Artin formalism is satisfied by these $L$-functions. As an application, we give a proof of the analogue of Iwasawa’s asymptotic class number formula for the $p$-part of the number of spanning trees in branched ${\mathbb{Z}}_p$-towers of finite connected graphs using Ihara zeta and $L$-functions. Moreover, we relate a generator for the characteristic ideal of the finitely generated torsion Iwasawa module that governs the growth of the $p$-part of the number of spanning trees in such towers with Ihara $L$-functions.