a(n) = Product_{i=1..n} floor(4*i/3).

Terms

a(0) =1a(1) =2a(2) =8a(3) =40a(4) =240a(5) =1920a(6) =17280a(7) =172800a(8) =2073600a(9) =26956800a(10) =377395200a(11) =6038323200a(12) =102651494400