Let
be the vertex set of a simple
graph and
its edge set. Then a graph isomorphism from a simple
graph
to a simple graph
is a bijection
such that
iff
(West 2000, p. 7).
If there is a graph isomorphism for to
, then
is said to be isomorphic to
, written
.
There exists no known P algorithm for graph isomorphism testing, although the problem has also not been shown to be NP-complete. As a result, the special complexity class graph isomorphism complete is sometimes used to refer to the problem of graph isomorphism testing.