E.g.f. exp(x+x^2/2+x^4/24).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =11a(5) =31a(6) =106a(7) =372a(8) =1499a(9) =6211a(10) =28606a(11) =135356a(12) =697357a(13) =3688049a(14) =20935006a(15) =121837276a(16) =753159801a(17) =4767863657a(18) =31807384354a(19) =217048147396