A255443(2^n-1).

Terms

a(0) =1a(1) =5a(2) =19a(3) =73a(4) =271a(5) =993a(6) =3643a(7) =13309a(8) =48655a(9) =177805a(10) =649699a(11) =2374129a(12) =8674999a(13) =31698625a(14) =115826659a(15) =423229469a(16) =1546477863a(17) =5650815053a(18) =20648028987a(19) =75447710625a(20) =275685244735