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  • $\begingroup$ Could you explain a bit how undirRule works? $\endgroup$ Commented Apr 13, 2013 at 21:29
  • $\begingroup$ @PinoAir It sorts the vertices in undirected edges in canonical order so that there is no difference between 1 <->2 and 2<->1. If you don't do this you might not recognize two vertices as the same just because their nodes are ordered differently. I don't do this for directed edges as I assume that order matters there. $\endgroup$ Commented Apr 13, 2013 at 21:56
  • $\begingroup$ I believe you should be able to write: undirRule = a_UndirectedEdge :> Sort[a] -- Sort works on heads besides List. (Sorry for the non-vote but I cannot test this as you know.) $\endgroup$ Commented Apr 13, 2013 at 22:45
  • $\begingroup$ @Sjoerd Unfortunately this is incorrect, unless you consider the graphs to be labelled (but in that case isomorphism is not an issue). Consider: big = Graph[{1 <-> 2, 2 <-> 3, 1 <-> 3, 3 <-> 4}], small = Graph[{1 <-> 2, 1 <-> 4}]. small is clearly a subgraph of big in the usual unlabelled sense, but the function says it isn't. $\endgroup$ Commented Apr 13, 2013 at 23:30
  • $\begingroup$ @PinoAir You should clarify if you mean the labelled or unlabelled case. When talking about isomorphism, people usually mean the unlabelled case as the labelled one is trivial. $\endgroup$ Commented Apr 13, 2013 at 23:31