Self-complementary graph

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Graph is isomorphic to its complement.]
In the mathematical field of graph theory, a self-complementary graph is a graph which is isomorphic to its complement. The simplest non-trivial self-complementary graphs are the path graph and the cycle graph.

Examples

Every Paley graph is self-complementary. For example, the 3 × 3 rook's graph is self-complementary, by a symmetry that keeps the center vertex in place but exchanges the roles of the four side midpoints and four corners of the grid. All strongly [regular graph|strongly regular] self-complementary graphs with fewer than 37 vertices are Paley graphs; however, there are strongly regular graphs on 37, 41, and 49 vertices that are not Paley graphs.
The Rado graph is an infinite self-complementary graph.

Properties

An self-complementary graph has exactly half as many edges of the complete graph, i.e., edges, and it must have diameter either 2 or 3. Since must be divisible by 4, must be congruent to 0 or 1 modulo 4; for instance, a graph cannot be self-complementary.

Computational complexity

The problems of checking whether two self-complementary graphs are isomorphic and of checking whether a given graph is self-complementary are polynomial-time equivalent to the general graph isomorphism problem.