a(n) = exp(3) * Sum_{k>=0} (-3)^k*k^n/k!.

Terms

a(0) =1a(1) =-3a(2) =6a(3) =-3a(4) =-21a(5) =24a(6) =195a(7) =-111a(8) =-3072a(9) =-4053a(10) =57003a(11) =277854a(12) =-697539a(13) =-12261567a(14) =-29861778a(15) =371727465a(16) =3511027599a(17) =2028432480a(18) =-188521156857