a(n) = Sum_{k = 0..n} binomial(-n, k) * 2^(n - k).

Terms

a(0) =1a(1) =1a(2) =3a(3) =-2a(4) =19a(5) =-54a(6) =222a(7) =-804a(8) =3075a(9) =-11630a(10) =44458a(11) =-170268a(12) =654766a(13) =-2524508a(14) =9758556a(15) =-37802952a(16) =146724579a(17) =-570450078a(18) =2221230066a(19) =-8660901612a(20) =33811886394a(21) =-132148736148a(22) =517012584036