G.f.: C(x)*C(2*x^2)*C(3*x^3)*..., where C(x) is the g.f. for A000108.

Terms

a(0) =1a(1) =1a(2) =4a(3) =10a(4) =33a(5) =81a(6) =282a(7) =762a(8) =2599a(9) =7979a(10) =27343a(11) =89371a(12) =315256a(13) =1078498a(14) =3857048a(15) =13651786a(16) =49475282a(17) =178736186a(18) =655247192a(19) =2401663838a(20) =8883371016a(21) =32906649488a(22) =122619768860a(23) =457836275272