a(n) = 2(m!)^2 for n = 2m and m!(m+1)! for n = 2m+1.

Terms

a(0) =2a(1) =1a(2) =2a(3) =2a(4) =8a(5) =12a(6) =72a(7) =144a(8) =1152a(9) =2880a(10) =28800a(11) =86400a(12) =1036800a(13) =3628800a(14) =50803200a(15) =203212800a(16) =3251404800a(17) =14631321600a(18) =263363788800