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A363988
a(n) = Sum_{k = 0..n} (-4)^(n-k)*binomial(n,k)*binomial(3*n+2*k,2*k)* binomial(2*k,k).
1
1, 16, 828, 53836, 3879404, 296396016, 23517939996, 1916072466688, 159188357217516, 13425731614346272, 1145885291754711328, 98752981735587825288, 8579149369628730276860, 750365776730717473307920, 66009615160057048401092544, 5835864811138398925049262336
OFFSET
0,2
COMMENTS
The sequence of Franel numbers A000172 satisfies the identity A000172(n) = Sum_{k = 0..n} (-4)^(n-k)*binomial(n,k)*binomial(n+2*k,2*k)*binomial(2*k,k). The present sequence comes from a modification of the right-hand side of the identity.
The Franel numbers satisfy the supercongruences u(n*p^r) == u(n*p^(r-1)) (mod p^(3*r)) for all primes p >= 5 and positive integers n and r. We conjecture that the present sequence satisfies the same supercongruences.
FORMULA
a(n) = (-4)^n*hypergeom([-n, (3*n+1)/2, (3*n+2)/2], [1, 1], 1)).
a(n) ~ sqrt(5 + 9*sqrt(3/11)) * 3^(n - 1/2) * (63 + 11*sqrt(33))^n / (Pi * n * 2^(2*n + 3/2)). - Vaclav Kotesovec, Jul 17 2023
MAPLE
seq(add((-4)^(n-k)*binomial(n, k)*binomial(3*n+2*k, 2*k)*binomial(2*k, k), k = 0..n), n = 0..20);
# alternative faster program for large n
seq(simplify((-4)^n*hypergeom([-n, (3*n+1)/2, (3*n+2)/2], [1, 1], 1)), n = 0..20);
MATHEMATICA
Table[(-4)^n*HypergeometricPFQ[{-n, (3*n+1)/2, (3*n+2)/2}, {1, 1}, 1], {n, 0, 20}] (* Vaclav Kotesovec, Jul 17 2023 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Peter Bala, Jul 02 2023
STATUS
approved