Topological Distance
•Idea: find the largest common subgraph, in this case, subtree.
•The sum of the eigenvalues of a tree adjacency matrix are invariant to similarity transforms, meaning any consistent re-ordering of the tree.
•So, color all vertexes with a vector made up of the eigenvalue sums of its children sorted by value: χ(u) in Rδ(G)-1
•Closer vectors indicate closer isometries.