a(n) = (4/3)*(4^n - 1).

Terms

a(0) =0a(1) =4a(2) =20a(3) =84a(4) =340a(5) =1364a(6) =5460a(7) =21844a(8) =87380a(9) =349524a(10) =1398100a(11) =5592404a(12) =22369620a(13) =89478484a(14) =357913940a(15) =1431655764a(16) =5726623060a(17) =22906492244a(18) =91625968980a(19) =366503875924