A255468(2^n-1).

Terms

a(0) =1a(1) =6a(2) =20a(3) =84a(4) =296a(5) =1124a(6) =4056a(7) =15100a(8) =55288a(9) =204740a(10) =753336a(11) =2782588a(12) =10252952a(13) =37831684a(14) =139470616a(15) =514443580a(16) =1896954168a(17) =6996160196a(18) =25799583352a(19) =95147186940a(20) =350881763288