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

Terms

a(0) =3a(1) =1a(2) =1a(3) =1a(4) =1a(5) =3a(6) =6a(7) =12a(8) =27a(9) =59a(10) =133a(11) =307a(12) =725a(13) =1743a(14) =4251a(15) =10509a(16) =26285a(17) =66430a(18) =169450a(19) =435848a(20) =1129505a(21) =2947131a(22) =7737583a(23) =20430377a(24) =54226471a(25) =144621405a(26) =387420489a(27) =1042127936a(28) =2813988985a(29) =7625597484