a(n) = floor(n!/3^n).

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =2a(8) =6a(9) =18a(10) =61a(11) =225a(12) =901a(13) =3905a(14) =18226a(15) =91134a(16) =486048a(17) =2754274a(18) =16525645a(19) =104662422a(20) =697749481a(21) =4884246371a(22) =35817806721a(23) =274603184861