G.f.: Sum_{n>=0} x^n * (1+x)^(n^2) / (1 + x*(1+x)^n)^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =13a(6) =46a(7) =160a(8) =658a(9) =2867a(10) =13481a(11) =68123a(12) =363995a(13) =2059994a(14) =12265480a(15) =76553192a(16) =499565710a(17) =3397751507a(18) =24030183367a(19) =176345076712