a(n) = 4^n + 2^(n+1).

Terms

a(0) =3a(1) =8a(2) =24a(3) =80a(4) =288a(5) =1088a(6) =4224a(7) =16640a(8) =66048a(9) =263168a(10) =1050624a(11) =4198400a(12) =16785408a(13) =67125248a(14) =268468224a(15) =1073807360a(16) =4295098368a(17) =17180131328a(18) =68720001024a(19) =274878955520