A355990 a(n) = n! / (6 * floor(n/3)!).

1, 4, 20, 60, 420, 3360, 10080, 100800, 1108800, 3326400, 43243200, 605404800, 1816214400, 29059430400, 494010316800, 1482030950400, 28158588057600, 563171761152000, 1689515283456000, 37169336236032000, 854894733428736000, 2564684200286208000

Offset: 3

Author

Seiichi Manyama, Jul 22 2022

Keywords

• nonn
• easy

Crossrefs

Column 3 of A355996.

Cf. A355988, A355989, A355991,

Programs

Mathematica

a[n_] := n!/(6 * Floor[n/3]!); Array[a, 22, 3] (* Amiram Eldar, Jul 22 2022 *)

PARI

a(n) = n!/(6*(n\3)!);

PARI

my(N=30, x='x+O('x^N)); Vec(serlaplace((1-x^3)*(exp(x^3)-1)/(6*(1-x))))

Formula

E.g.f.: (1 - x^3) * (exp(x^3) - 1)/(6 * (1 - x)).

a(n) = A355988(n)/6.