Ihara zeta function, coefficients of Maclaurin series, and Ramanujan graphs

Abstract: Let $X$ denote a connected $(q+1)$-regular undirected graph of finite order $n$. The graph $X$ is called Ramanujan whenever $$ |\lambda|\leq 2q^{\frac{1}{2}} $$ for all nontrivial eigenvalues $\lambda$ of $X$. We consider the variant $\Xi(u)$ of the Ihara zeta function $Z(u)$ of $X$ defined by \begin{gather*} \Xi(u)^{-1} = \left{ \begin{array}{ll} (1-u)(1-qu)(1-q^{\frac{1}{2}} u)^{2n-2}(1-u^2)^{\frac{n(q-1)}{2}} Z(u) \qquad &\hbox{if $X$ is nonbipartite}, (1-q^2u^2) (1-q^{\frac{1}{2}} u)^{2n-4} (1-u^2)^{\frac{n(q-1)}{2}+1} Z(u) \qquad &\hbox{if $X$ is bipartite}. \end{array} \right. \end{gather*} The function $\Xi(u)$ satisfies the functional equation $\Xi(q^{-1} u^{-1})=\Xi(u)$. Let ${h_k}{k=1}^\infty$ denote the number sequence given by $$ \frac{d}{du}\ln \Xi(q^{-\frac{1}{2}}u) =\sum{k=0}^\infty h_{k+1} u^k. $$ In this paper we establish the equivalence of the following statements: (i) $X$ is Ramanujan; (ii) $h_k\geq 0$ for all $k\geq 1$; (iii) $h_{k}\geq 0$ for infinitely many even $k\geq 2$. Furthermore we derive the Hasse--Weil bound for the Ramanujan graphs.