Uniform asymptotic expansion for regular graph counts

Let nn and dd be integers with 1dn21\leq d\leq n-2 and dndn even, and set λ=d/(n1)\lambda=d/(n-1) and Λ=λ(1λ)\varLambda=\lambda(1-\lambda). Let RG(n,d)\operatorname{RG}(n,d) denote the number of dd-regular graphs on nn vertices, and let p1,,p7p_1,\ldots,p_7 be the polynomials specified above. Uniform asymptotic expansion for regular graph counts. Uniformly for all such dd and nn,

RG(n,d)=(λλ(1λ)1λ)(n2)(n1d)nexp(j=17pj(Λ)Λjnj1+O(1d3(nd)3n)).\operatorname{RG}(n,d) = \Big(\lambda^\lambda (1-\lambda)^{1-\lambda}\Big)^{\binom n2} \binom{n-1}{d}^{n} \exp\biggl(\,\sum_{j=1}^7 \frac{p_j(\varLambda)}{\varLambda^j n^{j-1}} + O\bigg(\frac{1}{d^3(n-d)^3n}\bigg)\biggr).

This conjecture proposes a uniform approximation for the number of regular graphs across the full permitted degree range, improving earlier finite-term calculations and including an explicit error term.

Sources & referencesView supporting material

Primary source

Mikhail Isaev, “A tail bound for cumulant series for complex functions of independent random variables”, arXiv:2508.16952 (2025).

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