a(n) = exp(4) * Sum_{k>=0} (-4)^k*k^n/k!.

Terms

a(0) =1a(1) =-4a(2) =12a(3) =-20a(4) =-20a(5) =172a(6) =108a(7) =-2388a(8) =-3220a(9) =47532a(10) =161900a(11) =-1062740a(12) =-8532628a(13) =13623212a(14) =431041132a(15) =1206169260a(16) =-17833021588a(17) =-169685043796a(18) =180187176044