a(n) = 4 * floor(23*2^n/15).

Terms

a(0) =4a(1) =12a(2) =24a(3) =48a(4) =96a(5) =196a(6) =392a(7) =784a(8) =1568a(9) =3140a(10) =6280a(11) =12560a(12) =25120a(13) =50244a(14) =100488a(15) =200976a(16) =401952a(17) =803908a(18) =1607816a(19) =3215632a(20) =6431264a(21) =12862532a(22) =25725064a(23) =51450128a(24) =102900256a(25) =205800516a(26) =411601032a(27) =823202064a(28) =1646404128a(29) =3292808260