Adjacency Spectrum and Wiener Index of the Essential Ideal Graph of a Finite Commutative Ring $\mathbb{Z}_{n}$
Let $R$ be a commutative ring with unity. The essential ideal graph $\mathcal{E}_{R}$ of $R$, is a graph with a vertex set consisting of all nonzero proper ideals of \textit{R} and two vertices $I$ and $K$ are adjacent if and only if $I+ K$ is an essential ideal. In this paper, we study the adjacency spectrum of the essential ideal graph of the finite commutative ring $\mathbb{Z}_{n}$, for $n=\{p^{m}, p^{m_{1}}q^{m_{2}}\}$, where $p,q$ are distinct primes, and $m,m_{1}, m_2\in \mathbb N$. We show that $0$ is an eigenvalue of the adjacency matrix of $\mathcal{E}_{\mathbb{Z}_{n}}$ if and only if either $n= p^2$ or $n$ is not a product of distinct primes. We also determine all the eigenvalues of the adjacency matrix of $\mathcal{E}_{\mathbb{Z}_{n}}$ whenever $n$ is a product of three or four distinct primes. Moreover, we calculate the topological indices, namely the Wiener index and hyper-Wiener index of the essential ideal graph of $\mathbb{Z}_{n}$ for different forms of $n$