We generalize the Ihara-Selberg zeta function to hypergraphs in a natural way. Hashimoto's factorization results for biregular bipartite graphs apply, leading to exact factorizations. For (d, r)-regular hypergraphs, we show that a modified Riemann hypothesis is true if and only if the hypergraph is Ramanujan in the sense of Winnie Li and Patrick Solé. Finally, we give an example to show how the generalized zeta function can be applied to graphs to distinguish non-isomorphic graphs with the same Ihara-Selberg zeta function.

In her Ph.D. Thesis, Czarneski began a preliminary study of the coefficients of the reciprocal of the Ihara zeta function of a finite graph. We give a survey of the results in this area and then give a complete characterization of the coefficients. As an application, we give a (very poor) bound on the number of Eulerian circuits in a graph. We also use these ideas to compute the zeta function of graphs which are cycles with a single chord. We conclude by posing several questions for future work.

We enumerate all connected graphs with minimal vertex degree 2 on at most 11 vertices and determine their Ihara zeta functions. We also count the number of such graphs for which there is another graph with the same zeta function. We then use these graphs to conjecture properties of the graphs determined by the zeta function. In addition, we study switching constructions, proposed by Godsil and McKay, to determine whether they preserve the zeta function. We show that GM switching is not strong enough to preserve the zeta function, but GM* switching, following the notation of Haemers and Spence, does preserve the zeta function.

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