Duplicate of A080674.

Terms

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