Numerator of n!/3^n.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =8a(5) =40a(6) =80a(7) =560a(8) =4480a(9) =4480a(10) =44800a(11) =492800a(12) =1971200a(13) =25625600a(14) =358758400a(15) =1793792000a(16) =28700672000a(17) =487911424000a(18) =975822848000