Graph Isomorphism is PSPACE-complete

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 " Iso-morphism testing for graphs, semigroups, and finite automata are polynomiamlly equivalent problems " shows that graph isomorphism is PSPACE-complete. The equivalence problem for regular expressions was shown to be PSPACE-complete by (Meyer and Stockmeyer [2]). Booth [1] has shown that isomorphism of finite automata is equivalent to graph isomorphism. Taking these two results together with the equivalence of regular expressions, right-linear grammars, and finite automata see [3] for example, shows that graph isomorphism is PSPACE-complete. References [1] Booth, K.S. Isomorphism testing for graphs, semigroups, and finite automata are poly-nomiamlly equivalent problems, SIAM J.