a(1) = 1; a(n) = Sum_{d|n, d < n} binomial(n,d) * a(d).

Terms

a(0) =1a(1) =2a(2) =3a(3) =16a(4) =5a(5) =96a(6) =7a(7) =1184a(8) =261a(9) =1360a(10) =11a(11) =97428a(12) =13a(13) =24220a(14) =16395a(15) =15267456a(16) =17a(17) =14474736a(18) =19a(19) =251423600a(20) =817971a(21) =7760236a(22) =23a(23) =264344406312a(24) =265675a(25) =135208476a(26) =1223270127a(27) =971632668664a(28) =29a(30) =31