A binomial-hypergeometric identity for graph-count coefficients

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For integers g≥1g\geq 1, ℓ≥0\ell\geq 0, and j≥1j\geq 1, let (nr)\binom{n}{r} denote the binomial coefficient, let 2F1{}_2F_1 denote the Gauss hypergeometric function, and let the product over mm be taken for 0≤m≤j−10\leq m\leq j-1. The binomial-hypergeometric identity. The following identity is conjectured:

j! 2ℓ+2g−1((2g−2)+ℓ+jj)2F1(−j,−2j2−2g−(ℓ+j);−1)=∑k=1ℓ+2g(ℓ+2g−1k−1)∏m=0j−12(2m+k).j!\,2^{\ell+2g-1}\binom{(2g-2)+\ell+j}{j}{}_2F_1\left(\begin{matrix}-j,-2j\\2-2g-(\ell+j)\end{matrix};-1\right)=\sum_{k=1}^{\ell+2g}\binom{\ell+2g-1}{k-1}\prod_{m=0}^{j-1}2(2m+k).

The authors verified the identity for 0≤ℓ≤200\leq\ell\leq20 and 1≤g,j≤201\leq g,j\leq20 and leave a general proof for future analysis; it arises by comparing two formulations of the graph counts N4,z(g,j){\mathcal N}_{4,z}(g,j), although equality of the resulting linear combinations does not itself require equality of their coefficients.

References

Primary source

Nicholas Ercolani, Joceline Lega and Brandon Tippings, “Non-recursive Counts of Graphs on Surfaces”, arXiv:2210.00671 (2023).

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