A254866 a(n) = (n!!)^n.

1, 1, 4, 27, 4096, 759375, 12230590464, 140710042265625, 472769874482845188096, 601016336033894136931640625, 697127546117424200558837760000000000, 153133225508583375568553948649382367138671875, 91653624689233987245068783089656480594395136000000000000

Offset: 0

Author

Vaclav Kotesovec, Feb 19 2015

Keywords

• nonn
• easy

Links

• Eric Weisstein, Double Factorial, (MathWorld)

Crossrefs

Cf. A006882, A036740, A185141.

Cf. A000165, A001147.

Programs

Mathematica

Table[(n!!)^n, {n, 0, 15}]

Formula

a(n) ~ Pi^(n/2) * n^(n*(n+1)/2) / exp(n^2/2 - 1/6) if n is even.

a(n) ~ 2^(n/2) * n^(n*(n+1)/2) / exp(n^2/2 - 1/6) if n is odd.