a(n) = 4 * floor(9*2^n/5).

Terms

a(0) =4a(1) =12a(2) =28a(3) =56a(4) =112a(5) =228a(6) =460a(7) =920a(8) =1840a(9) =3684a(10) =7372a(11) =14744a(12) =29488a(13) =58980a(14) =117964a(15) =235928a(16) =471856a(17) =943716a(18) =1887436a(19) =3774872a(20) =7549744a(21) =15099492a(22) =30198988a(23) =60397976a(24) =120795952a(25) =241591908a(26) =483183820a(27) =966367640a(28) =1932735280a(29) =3865470564