A255450(2^n-1).

Terms

a(0) =1a(1) =5a(2) =21a(3) =85a(4) =333a(5) =1293a(6) =4997a(7) =19269a(8) =74237a(9) =285885a(10) =1100725a(11) =4237685a(12) =16314029a(13) =62803821a(14) =241772773a(15) =930737317a(16) =3582994525a(17) =13793193757a(18) =53098638101a(19) =204409869525a(20) =786901384973