G.f.: Sum_{n>=0} (1 - (1-x)^n)^n.

Terms

a(0) =1a(1) =1a(2) =4a(3) =23a(4) =176a(5) =1697a(6) =19805a(7) =271669a(8) =4285195a(9) =76430799a(10) =1521161530a(11) =33422603485a(12) =803584699252