Notion of symmetry of vertices in a graph

graph theory

This is a beginner question in graph theory. I have an undirected graph. I want to colour one node. Sometimes colouring two different nodes would lead to the "same" graph. Eg. for a square with diagonal there are only two non-equivalent ways. What do I call two or more nodes which are "symmetric" under the colouring of one, and how do I find such subsets say, from the adjacency matrix? Many thanks for pointing out the correct terminology.

Best Answer

If coloring two different vertices gives the same graph upon isomorphism, this means that the two vertices are in the same orbit of the automorphism group. So your subsets are the orbits of the automorphism group. You can compute the orbits by computing the group automorphism of your graph. This is not a trivial problem (There is no polynomial algorithm found for it, even though it is believed not to be NP-complete), but there are practical algorithms, for example nauty's library. This library directly gives you the orbits.