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I am working with graphs with multiple edges and loops, and I want to eliminate all isomorphic graphs from a long list I've generated. The FindGraphIsomorphism function is very nice, but only works for simple graphs. I'm looking to find such a function or its equivalent for multigraphs. For example,

two multigraphs

Is there possibly a third-party package, or maybe even an external program that would find such graph isomorphisms?

Also, is it possible to compare graphics objects in Mathematica, to see whether they are similar? I have tried SameQ, but to no avail.

graphs with self-loops

I am assuming this is the result because Mathematica plots the graphs with floating-point values, so it is impossible to have two identical graphs like this. So is there a function that looks at the similarity between two graphics objects? I think something like that might do the trick for me in most cases.

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You could replace each edge with an edge-vertex-edge path, converting your multigraph into a unique simple graph which you can compare for isomorphism. –  Rahul Oct 14 '12 at 2:53
Clever. I shall give this a go and if successful, make a little wrapper function. –  jlv Oct 14 '12 at 3:05
@Rahul Narain Dang. I was going to suggest that. Now I have to unpat myself on the back. –  Daniel Lichtblau Oct 14 '12 at 16:26

1 Answer 1

You can use this package to call igraph through RLink. igraph does support multi-edges. After setting up the package, do

edgelist1 = {{1, 2}, {2, 3}, {3, 4}, {4, 1}, {1, 2}, {1, 4}}

GraphPlot[Rule @@@ edgelist1]

(* see the multiple edges *)

This is a bit more complex than passing a graph object because in Mathematica you can't construct a graph with multi-edges (as you discovered). So we use IGraphR's intermediary representation:

igobj = RObject[edgelist1, RAttributes["mmaDirectedGraph" :> {False}]]

Then call igraph:

IGraph["graph.isomorphic"][igobj, igobj]

(* ==> True *)
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