a(n) = n! / (2 * floor(n/2)!).

Terms

a(0) =1a(1) =3a(2) =6a(3) =30a(4) =60a(5) =420a(6) =840a(7) =7560a(8) =15120a(9) =166320a(10) =332640a(11) =4324320a(12) =8648640a(13) =129729600a(14) =259459200a(15) =4410806400a(16) =8821612800a(17) =167610643200a(18) =335221286400