Isomorphism testing for graphs of bounded genus

We present an algorithm which determines isomorphism of graphs in v<supscrpt>O(g)</supscrpt>steps where v is the number of vertices and g is the genus of the graphs. In [FMR 79] an algorithm was presented for embedding graph on surfaces of genus g in v<supscrpt>O(g)</supscrpt> steps. Here we show how to extend this algorithm to isomorphism testing for graphs of small genus. This result is noteworthy for at least two reasons. First, this extends the polynomial time isomorphism results for the plane [HT 72] and also the projective plane [L 80] to arbitrary surfaces. Second, this gives one of the few known natural decompositions of the isomorphism problem into an infinite hierarchy of problems P<subscrpt>o</subscrpt>,P<subscrpt>1</subscrpt>,... such that isomorphism testing of problems in P<subscrpt>1</subscrpt> is decidable in time v<supscrpt>O(i)</supscrpt>.