a(n) = (7*2^n - 9 + 5*(-1)^n) / 6.

Terms

a(0) =0a(1) =4a(2) =7a(3) =18a(4) =35a(5) =74a(6) =147a(7) =298a(8) =595a(9) =1194a(10) =2387a(11) =4778a(12) =9555a(13) =19114a(14) =38227a(15) =76458a(16) =152915a(17) =305834a(18) =611667a(19) =1223338a(20) =2446675a(21) =4893354a(22) =9786707a(23) =19573418a(24) =39146835a(25) =78293674a(26) =156587347a(27) =313174698a(28) =626349395a(29) =1252698794