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a(n) = binomial(6*n, n) * hypergeom([-5*n, -n], [-6*n], -1).
5

%I #11 Jan 09 2024 11:09:16

%S 1,11,221,4991,118721,2908411,72616013,1837271615,46943003137,

%T 1208483403179,31297149356221,814471993937855,21281058718583873,

%U 557930580979801755,14669716953106628781,386675596518995000191,10214494658006725840897,270345191656309313382475

%N a(n) = binomial(6*n, n) * hypergeom([-5*n, -n], [-6*n], -1).

%C In general, for k > 1, binomial(k*n, n) * hypergeom([(1-k)*n, -n], [-k*n], -1) ~ sqrt((k + (k^2 - k + 1) / sqrt(k^2 - 2*k + 2)) / (4*(k-1)*Pi*n)) * ((A341476(k) + A341477(k)*sqrt((k-1)^2 + 1)) / (k-1)^(k-1))^n.

%H Michael De Vlieger, <a href="/A341491/b341491.txt">Table of n, a(n) for n = 0..697</a>

%H Lin Yang, Yu-Yuan Zhang, and Sheng-Liang Yang, <a href="https://doi.org/10.1016/j.laa.2023.12.021">The halves of Delannoy matrix and Chung-Feller properties of the m-Schröder paths</a>, Linear Alg. Appl. (2024).

%F a(n) ~ sqrt((6 + 31/sqrt(26))/(20*Pi*n)) * (42671 + 8346*sqrt(26))^n / 5^(5*n).

%t Table[Binomial[6*n, n] * Hypergeometric2F1[-5*n, -n, -6*n, -1], {n,0,20}]

%Y Cf. A001850, A026000, A026001, A331329, A341476, A341477.

%K nonn

%O 0,2

%A _Vaclav Kotesovec_, Feb 13 2021