A279886 a(n) = A057863(n+1) * Sum_{k=0..n}(k! / (2*k+1)!!).

1, 4, 66, 7200, 6917400, 72442188000, 9822893856930000, 19942863749656848000000, 687732249521129504041350000000, 450437284567157389148103391935000000000, 6194243041031315772374678081343893262937500000000

Offset: 0

Author

Daniel Suteu, Dec 21 2016

Keywords

• nonn
• easy

Links

• Daniel Suteu, Table of n, a(n) for n = 0..29

Crossrefs

Cf. A057863.

Programs

Mathematica

Table[Product[(2 k - 1)!!, {k, n + 1}] Sum[(j!/(2 j + 1)!!), {j, 0, n}], {n, 0, 10}] (* Michael De Vlieger, Dec 22 2016 *)

PARI

a(n) = prod(k=0,n,prod(i=0, k, 2*i+1))*sum(k=0,n,k!/prod(i=0,k,2*i+1));

Formula

a(n) ~ Pi/2 * A057863(n+1).

a(0) = 1, a(n) = a(n-1) * (2*n+1)!! + n! * A057863(n).