## How many $p$-regular graphs with $n$ vertices are there?

Suppose that there are $n$ vertices, we want to construct a regular graph with degree $p$, which, of course, is less than $n$. My question is how many possible such graphs can we get?

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Why the "of course"? – Richard Kent Oct 10 2011 at 19:36
Because he defines "graph" as "simple graph", I am guessing. – Igor Rivin Oct 10 2011 at 19:38
This question appeared before on MO: mathoverflow.net/questions/22089/… – Andy B Oct 11 2011 at 16:03
Thanks for your comment! – John Young Oct 11 2011 at 17:24

McKay and Wormald conjectured that the number of simple $d$-regular graphs of order $n$ is asymptotically $$\sqrt 2 e^{1/4} (\lambda^\lambda(1-\lambda)^{1-\lambda})^{\binom n2}\binom{n-1}{d}^n,$$ where $\lambda=d/(n-1)$ and $d=d(n)$ is any integer function of $n$ with $1\le d\le n-2$ and $dn$ even.

Bender and Canfield, and independently Wormald, proved this for bounded $d$ in 1978, and Bollobás extended this to $d=O(\sqrt{\log n})$ in 1980.

McKay and Wormald proved the conjecture in 1990-1991 for $\min\{d,n-d\}=o(n^{1/2})$ [1], and $\min\{d,n-d\}>cn/\log n$ for constant $c>2/3$ [2]. These remain the best results.

The gap between these ranges remains unproved, though the computer says the conjecture is surely true there too. The formula apart from the $\sqrt2e^{1/4}$ has a simple combinatorial interpretation, and the universality of the constant $\sqrt2e^{1/4}$ is an enigma crying out for an explanation.

Incidentally this conjecture is for labelled regular graphs. For isomorphism classes, divide by $n!$ for $3\le d\le n-4$, since in that range almost all regular graphs have trivial automorphism groups (references on request). For $d=0,1,2,n-3,n-2,n-1$, this isn't true.

[1] Combinatorica, 11 (1991) 369-382. http://cs.anu.edu.au/~bdm/papers/nickcount.pdf

[2] European J. Combin., 11 (1990) 565-580. http://cs.anu.edu.au/~bdm/papers/highdeg.pdf

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There is no closed formula (that anyone knows of), but there are asymptotic results, due to Bollobas, see A probabilistic proof of an asymptotic formula for the number of labelled regular graphs (1980) by B Bollobás (European Journal of Combinatorics) or Random Graphs (by the selfsame Bollobas).

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Looking up OEIS, some related sequences are A005176 for the number of non-isomorphic regular graphs on $n$ vertices, and A005177 for the number non-isomorphic connected regular graphs on $n$ vertices.

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And 'of course', if you really want those graphs you might have a look at genreg by Markus Meringer: http://www.mathe2.uni-bayreuth.de/markus/reggraphs.html

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