A101609 a(n) = n! * Sum_{k=1..floor(n/2)} 1/k.

0, 2, 6, 36, 180, 1320, 9240, 84000, 756000, 8285760, 91143360, 1173553920, 15256200960, 226040855040, 3390612825600, 56865153945600, 966707617075200, 18112111963545600, 344130127307366400, 7125892746964992000

Offset: 1

Author

Ralf Stephan, Dec 10 2004

Keywords

• nonn
• easy

Crossrefs

Cf. A101610, A101611, A101612, A101613, A000254, A046673, A052517.

Programs

Mathematica

Table[ n! HarmonicNumber[ Floor[ n/2]], {n, 20}] (* Robert G. Wilson v, Dec 11 2004 *)

Formula

a(1) = 0, a(n) = a(n-1)*n + (1 + (-1)^n)*(n-1)!. - Daniel Suteu, Feb 06 2017