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How does one go about systematically constructing a self-complementary graph, on say 8 vertices?

[Added: Maybe everyone else knows this already, but I had to look up my guess to be sure it was correct: a self-complementary graph is a simple graph which is isomorphic to its complement. --PLC]

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2 Answers

up vote 6 down vote accepted

Here's a nice little algorithm for constructing a self-complementary graph from a self-complementary graph $H$ with $4k$ or $4k+1$ vertices, $k = 1, 2, ...$ (e.g., from a self-complementary graph with $4$ vertices, one can construct a self-complementary graph with $8$ vertices; from $5$ vertices, construct one with $9$ vertices).

See this PDF on constructing self-complementary graphs.

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This is a beautiful algorithm, thank you. – John May 23 '11 at 0:40
@John, so all you wanted was one self-complementary graph of each (possible) order? I thought you wanted all of them! – Gerry Myerson May 23 '11 at 2:31
@Gerry Yikes: that would certainly be an "efficiency" challenge, indeed! I came across a "comprehensive" "manual" on self-complementary graphs (200+ pages)...but intriguing from what I gleaned. I'll link it in my answer above. – amWhy May 23 '11 at 2:40
@Gerry, I looked at my question again and I agree, my original wording was vague. I've edited the question slightly to reflect on what I meant. – John May 23 '11 at 3:45

Systematically is easy; systematically and efficiently, I don't know. It's easy to work out how many edges such a graph must have, that's a start. There's also some information at http://oeis.org/A000171

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