If n mod 4 = 2 or n mod 4 = 3 then a(n) = 0 else let m=floor(n/4), then a(n) = (2*m)!/m!.

Terms

a(0) =1a(1) =1a(2) =0a(3) =0a(4) =2a(5) =2a(6) =0a(7) =0a(8) =12a(9) =12a(10) =0a(11) =0a(12) =120a(13) =120a(14) =0a(15) =0a(16) =1680a(17) =1680a(18) =0a(19) =0a(20) =30240a(21) =30240a(22) =0a(23) =0a(24) =665280a(25) =665280a(26) =0a(27) =0a(28) =17297280a(29) =17297280