Given a simple graph with $n$ vertices we can define its adjacency matrix as described here. The article further says that -
"The same concept can be extended to multigraphs and graphs with loops by storing the number of edges between each two vertices in the corresponding matrix element, and by allowing nonzero diagonal elements"
So my questions are the following -
Given an $n\times n$ matrix with non-negative integer entries, can it always be thought of as the adjacency matrix of some directed graph (not necessarily simple)?
Similarly if we are given such a matrix which is also symmetric and with non-negative even integer entries in the diagonal (to keep with the convention mentioned in the wikipedia article), can it be thought of as the adjacency matrix of an undirected graph (not necessarily simple)?
I was wondering if the adjacency matrix should satisfy any other properties or is it just any non-negative integer matrix?