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

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =64a(5) =402a(6) =2972a(7) =24884a(8) =233224a(9) =2413402a(10) =27321706a(11) =335811420a(12) =4453678055a(13) =63403359154a(14) =964550068984