A hierarchical sequence (S(W2{2}c) - see A059126).

Terms

a(0) =4a(1) =18a(2) =52a(3) =126a(4) =280a(5) =594a(6) =1228a(7) =2502a(8) =5056a(9) =10170a(10) =20404a(11) =40878a(12) =81832a(13) =163746a(14) =327580a(15) =655254a(16) =1310608a(17) =2621322a(18) =5242756a(19) =10485630a(20) =20971384a(21) =41942898a(22) =83885932a(23) =167772006a(24) =335544160a(25) =671088474a(26) =1342177108a(27) =2684354382a(28) =5368708936a(29) =10737418050