a(n) = (4^n + 20)/3.

Terms

a(0) =7a(1) =8a(2) =12a(3) =28a(4) =92a(5) =348a(6) =1372a(7) =5468a(8) =21852a(9) =87388a(10) =349532a(11) =1398108a(12) =5592412a(13) =22369628a(14) =89478492a(15) =357913948a(16) =1431655772a(17) =5726623068a(18) =22906492252a(19) =91625968988a(20) =366503875932