spectrum -- Returns the spectrum of a graph

The spectrum of a graph G is the set of the eigenvalues of the adjacency matrix A corresponding to G. For simple graphs, these eigenvalues are all real since A must be symmetric. The user should be aware that Macaulay 2 does not give exact values for these eigenvalues, they are numerical approximations, but it is still a good tool to use to check if two graphs are isomorphic; isomorphic graphs share the same spectrum although the converse is not necessarily true.