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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =6a(5) =30a(6) =90a(7) =630a(8) =2520a(9) =22680a(10) =113400a(11) =1247400a(12) =7484400a(13) =97297200a(14) =681080400a(15) =10216206000a(16) =81729648000