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

Terms

a(0) =4a(1) =15a(2) =40a(3) =93a(4) =202a(5) =423a(6) =868a(7) =1761a(8) =3550a(9) =7131a(10) =14296a(11) =28629a(12) =57298a(13) =114639a(14) =229324a(15) =458697a(16) =917446a(17) =1834947a(18) =3669952a(19) =7339965a(20) =14679994a(21) =29360055a(22) =58720180a(23) =117440433a(24) =234880942a(25) =469761963a(26) =939524008a(27) =1879048101a(28) =3758096290a(29) =7516192671