a(n) = (3^n/n!) * Product_{k=0..n-1} (3*k - 2).

Terms

a(0) =1a(1) =-6a(2) =-9a(3) =-36a(4) =-189a(5) =-1134a(6) =-7371a(7) =-50544a(8) =-360126a(9) =-2640924a(10) =-19806930a(11) =-151252920a(12) =-1172210130a(13) =-9197341020a(14) =-72921775230a(15) =-583374201840