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

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =32a(5) =64a(6) =384a(7) =768a(8) =6144a(9) =12288a(10) =122880a(11) =245760a(12) =2949120a(13) =5898240a(14) =82575360a(15) =165150720a(16) =2642411520a(17) =5284823040a(18) =95126814720a(19) =190253629440