Graph Isomorphism is a common topic in the conference. Next to factoring, graph isomorphism is the most well-studied problem in NP not known to be in P or NP-complete. Graph isomorphism sits in co-AM, i.e. there is a two-round interactive proof system for showing that two graphs are not isomorphic. The best deterministic algorithm uses time exponential in (n log n)^{0.5}.

Jacobo Toran today gave a talk on the hardness of graph isomorphism. He showed that given a black box for GI, one can compute the number of accepting paths in a directed graph, a class known as #L functions or equivalently the determinant of an integer matrix. He also showed that matching reduces to graph isomorphism. Whether every language in P is log-space reducible to graph isomorphism remains an interesting open question.

Later in the conference V. Arvind will present his result that GI is in SPP, or more specifically that one can determine graph isomorphism in P^{SAT} where every query made to the SAT oracle has at most one satisfying assigment.

Graph Isomorphism is the current algorithmic challenge for quantum computers. It is an example of the hidden subgroup problem, a special case of which was used for Shor's factoring algorithm. Perhaps the interaction between the classical and quantum theorists at this conference may help find a efficient quantum algorithm for GI.

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