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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =12a(5) =60a(6) =120a(7) =840a(8) =1680a(9) =15120a(10) =30240a(11) =332640a(12) =665280a(13) =8648640a(14) =17297280a(15) =259459200a(16) =518918400a(17) =8821612800a(18) =17643225600a(19) =335221286400a(20) =670442572800