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

Terms

a(0) =0a(1) =2a(2) =6a(3) =36a(4) =180a(5) =1320a(6) =9240a(7) =84000a(8) =756000a(9) =8285760a(10) =91143360a(11) =1173553920a(12) =15256200960a(13) =226040855040