G.f.: (1 + Sum_{ i >= 0 } 2^i*x^(2^(i+1)-1)) / (1-x)^3.

Terms

a(0) =1a(1) =4a(2) =9a(3) =18a(4) =31a(5) =48a(6) =69a(7) =98a(8) =135a(9) =180a(10) =233a(11) =294a(12) =363a(13) =440a(14) =525a(15) =626a(16) =743a(17) =876a(18) =1025a(19) =1190a(20) =1371a(21) =1568a(22) =1781a(23) =2010a(24) =2255a(25) =2516a(26) =2793a(27) =3086a(28) =3395a(29) =3720