G.f.: x * Product_{k>=1} 1/(1 - a(k)*x^k)^k.

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =63a(5) =396a(6) =2926a(7) =24497a(8) =229757a(9) =2377153a(10) =26917186a(11) =330804783a(12) =4387399275a(13) =62455948949a(14) =950123048257