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

Terms

a(0) =0a(1) =1a(2) =5a(3) =21a(4) =85a(5) =341a(6) =1365a(7) =5461a(8) =21845a(9) =87381a(10) =349525a(11) =1398101a(12) =5592405a(13) =22369621a(14) =89478485a(15) =357913941a(16) =1431655765a(17) =5726623061a(18) =22906492245a(19) =91625968981a(20) =366503875925