A273983 a(n) = ((4*n)!! - (4*n-1)!!)/(4*n+1).

1, 31, 2745, 487935, 145769625, 65830256415, 41892106080825, 35736278004165375, 39370290736153001625, 54420772423242699849375, 92234193751998833171261625, 188098544080793843475953349375, 454418941572893462364414856265625, 1283429428883663190972186961851609375

Offset

1,2

Comments

Sequence is inspired by A273889. The same argument in A273889 can be used here to prove the expression evaluates to integers.

Links

Chai Wah Wu, Table of n, a(n) for n = 1..202

Prog

(Python)
doublefac=lambda x:1 if x<2 else x*doublefac(x-2)
for i in range(200):
    print(i, (doublefac(4*i)-doublefac(4*i-1))//(4*i+1))
# Brian Cheung, Jun 15 2016

Crossrefs

Cf. A273889, A007912.

Sequence in context: A302137 A136245 A262642 * A173563 A190527 A106205

Adjacent sequences: A273980 A273981 A273982 * A273984 A273985 A273986

Keyword

nonn

Author

Chai Wah Wu, Jun 05 2016

Status

approved