E.g.f.: 1/Product_{m>0} (1+(-x)^m/m!).

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =21a(5) =96a(6) =520a(7) =3795a(8) =32053a(9) =284368a(10) =2763876a(11) =30648465a(12) =373339824a(13) =4833294389a(14) =67167087793