A255298(2^n-1).

Terms

a(0) =1a(1) =4a(2) =16a(3) =58a(4) =204a(5) =714a(6) =2492a(7) =8682a(8) =30228a(9) =105226a(10) =366276a(11) =1274922a(12) =4437692a(13) =15446554a(14) =53765916a(15) =187147146a(16) =651418116a(17) =2267444842a(18) =7892485300a(19) =27472040138a(20) =95624259340a(21) =332847482970