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(PARI) a(n) = sum(k=0, n, (-1)^(n-k) * binomial(n+3*k, 4*k) * binomial(5*k, k) / (4*k+1)); \\ Michel Marcus, Nov 14 2021
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a(n) = (-1)^5*F([1/5, 2/5, 3/5, 4/5, (1+n)/3, (2+n)/3, (3+n)/3, -n], [1/4, 1/2, 1/2, 3/4, 3/4, 1, 5/4], 3^3*5^5/2^16), where F is the generalized hypergeometric function. - Stefano Spezia, Nov 14 2021
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allocated for Ilya Gutkovskiy
G.f. A(x) satisfies: A(x) = 1 / ((1 + x) * (1 - x * A(x)^4)).
1, 0, 1, 4, 21, 114, 651, 3844, 23301, 144169, 906866, 5782350, 37289431, 242793439, 1593918916, 10538988984, 70121101825, 469133993094, 3154115695476, 21299373321344, 144402246424591, 982506791975780, 6706724412165956, 45917245477282994
0,4
a(n) = Sum_{k=0..n} (-1)^(n-k) * binomial(n+3*k,4*k) * binomial(5*k,k) / (4*k+1).
nmax = 23; A[_] = 0; Do[A[x_] = 1/((1 + x) (1 - x A[x]^4)) + O[x]^(nmax + 1) // Normal, nmax + 1]; CoefficientList[A[x], x]
Table[Sum[(-1)^(n - k) Binomial[n + 3 k, 4 k] Binomial[5 k, k]/(4 k + 1), {k, 0, n}], {n, 0, 23}]
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Ilya Gutkovskiy, Nov 13 2021
approved
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allocated for Ilya Gutkovskiy
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