a(n) = 4 * floor(24*2^n/15) = 4*A077854(n).

Terms

a(0) =4a(1) =12a(2) =24a(3) =48a(4) =100a(5) =204a(6) =408a(7) =816a(8) =1636a(9) =3276a(10) =6552a(11) =13104a(12) =26212a(13) =52428a(14) =104856a(15) =209712a(16) =419428a(17) =838860a(18) =1677720a(19) =3355440a(20) =6710884a(21) =13421772a(22) =26843544a(23) =53687088a(24) =107374180a(25) =214748364a(26) =429496728a(27) =858993456a(28) =1717986916a(29) =3435973836