I'm reading these lecture notes and I am confused about the use of the phrase "natural operator" and "natural quadratic" on page three.
While the adjacency matrix is the most natural matrix to associate with a graph, I also ο¬nd it the least useful. Eigenvalues and eigenvectors are most meaningful when used to understand a natural operator or a natural quadratic form. The adjacency matrix provides neither.
Can someone explain to me what these phrases mean? Does "natural" refer to $\mathbb{N}$?