a(n) = (4^n + 2)/3.

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =86a(5) =342a(6) =1366a(7) =5462a(8) =21846a(9) =87382a(10) =349526a(11) =1398102a(12) =5592406a(13) =22369622a(14) =89478486a(15) =357913942a(16) =1431655766a(17) =5726623062a(18) =22906492246a(19) =91625968982a(20) =366503875926