Graph Isomorphism is PSPACE-complete
arXiv:0708.4075
Abstract
Combining the the results of A.R. Meyer and L.J. Stockmeyer "The Equivalence Problem for Regular Expressions with Squaring Requires Exponential Space", and K.S. Booth "Isomorphism testing for graphs, semigroups, and finite automata are polynomiamlly equivalent problems" shows that graph isomorphism is PSPACE-complete.