a(n) = Sum[2^(A047260(i)-1), {i,1,n}].

Terms

a(0) =1a(1) =9a(2) =25a(3) =57a(4) =121a(5) =633a(6) =1657a(7) =3705a(8) =7801a(9) =40569a(10) =106105a(11) =237177a(12) =499321a(13) =2596473a(14) =6790777a(15) =15179385a(16) =31956601a(17) =166174329a(18) =434609785a(19) =971480697a(20) =2045222521a(21) =10635157113a(22) =27815026297a(23) =62174764665a(24) =130894241401a(25) =680650055289