a(n) = 8^n + 2^(n+1).

Terms

a(0) =3a(1) =12a(2) =72a(3) =528a(4) =4128a(5) =32832a(6) =262272a(7) =2097408a(8) =16777728a(9) =134218752a(10) =1073743872a(11) =8589938688a(12) =68719484928a(13) =549755830272