Numbers of the form (4^n + 4^(n-1) + ... + 1) + (n mod 2).

Terms

a(0) =6a(1) =21a(2) =86a(3) =341a(4) =1366a(5) =5461a(6) =21846a(7) =87381a(8) =349526a(9) =1398101a(10) =5592406a(11) =22369621a(12) =89478486a(13) =357913941a(14) =1431655766a(15) =5726623061a(16) =22906492246a(17) =91625968981a(18) =366503875926