a(n) = denominator(n!/floor(n/2)!^4).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =2a(6) =9a(7) =9a(8) =288a(9) =32a(10) =400a(11) =400a(12) =43200a(13) =43200a(14) =1058400a(15) =70560a(16) =18063360a(17) =18063360a(18) =6584094720a(19) =6584094720a(21) =156764160000