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

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =16a(5) =21a(6) =102a(7) =109a(8) =565a(9) =826a(10) =4913a(11) =4924a(12) =28036a(13) =28049a(14) =378218a(15) =427646a(16) =1841532a(17) =1841549a(18) =29704312a(19) =29704331a(20) =182590131a(21) =194454702a(22) =3660242371a(23) =3660242394a(24) =17058569521a(25) =17059419626a(26) =308650641577a(27) =311298706504