a(n) = (4^n + 8)/3.

Terms

a(0) =3a(1) =4a(2) =8a(3) =24a(4) =88a(5) =344a(6) =1368a(7) =5464a(8) =21848a(9) =87384a(10) =349528a(11) =1398104a(12) =5592408a(13) =22369624a(14) =89478488a(15) =357913944a(16) =1431655768a(17) =5726623064a(18) =22906492248a(19) =91625968984a(20) =366503875928