a(n) = sum(2^(A047240(i)-1), i=1..n).

Terms

a(0) =1a(1) =3a(2) =35a(3) =99a(4) =227a(5) =2275a(6) =6371a(7) =14563a(8) =145635a(9) =407779a(10) =932067a(11) =9320675a(12) =26097891a(13) =59652323a(14) =596523235a(15) =1670265059a(16) =3817748707a(17) =38177487075a(18) =106896963811a(19) =244335917283