a(n) = 3^n*Bell(n).

Terms

a(0) =1a(1) =3a(2) =18a(3) =135a(4) =1215a(5) =12636a(6) =147987a(7) =1917999a(8) =27162540a(9) =416236401a(10) =6848207775a(11) =120206639790