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تسجيل مستخدم جديد$X$-Ramanujan Graphs
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Let $X$ be an infinite graph of bounded degree; e.g., the Cayley graph of a free product of finite groups. If $G$ is a finite graph covered by $X$, it is said to be $X$-Ramanujan if its second-largest eigenvalue $lambda_2(G)$ is at most the spectral radius $rho(X)$ of $X$, and more generally $k$-quasi-$X$-Ramanujan if $lambda_k(G)$ is at most $rho(X)$. In case $X$ is the infinite $Delta$-regular tree, this reduces to the well known notion of a finite $Delta$-regular graph being Ramanujan. Inspired by the Interlacing Polynomials method of Marcus, Spielman, and Srivastava, we show the existence of infinitely many $k$-quasi-$X$-Ramanujan graphs for a variety of infinite $X$. In particular, $X$ need not be a tree; our analysis is applicable whenever $X$ is what we call an additive product graph. This additive product is a new construction of an infinite graph $mathsf{AddProd}(A_1, dots, A_c)$ from finite atom graphs $A_1, dots, A_c$ over a common vertex set. It generalizes the notion of the free product graph $A_1 * cdots * A_c$ when the atoms $A_j$ are vertex-transitive, and it generalizes the notion of the universal covering tree when the atoms $A_j$ are single-edge graphs. Key to our analysis is a new graph polynomial $alpha(A_1, dots, A_c;x)$ that we call the additive characteristic polynomial. It generalizes the well known matching polynomial $mu(G;x)$ in case the atoms $A_j$ are the single edges of $G$, and it generalizes the $r$-characteristic polynomial introduced in [Ravichandran16, Leake-Ravichandran18]. We show that $alpha(A_1, dots, A_c;x)$ is real-rooted, and all of its roots have magnitude at most $rho(mathsf{AddProd}(A_1, dots, A_c))$. This last fact is proven by generalizing Godsils notion of treelike walks on a graph $G$ to a notion of freelike walks on a collection of atoms $A_1, dots, A_c$.
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