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

Terms

a(0) =1a(1) =-3a(2) =-9a(3) =-45a(4) =-270a(5) =-1782a(6) =-12474a(7) =-90882a(8) =-681615a(9) =-5225715a(10) =-40760577a(11) =-322379109a(12) =-2579032872a(13) =-20830650120a(14) =-169621008120