The subsequence A253071(2^n-1).

Terms

a(0) =1a(1) =7a(2) =21a(3) =95a(4) =333a(5) =1319a(6) =4837a(7) =18447a(8) =68733a(9) =259447a(10) =972565a(11) =3661535a(12) =13756333a(13) =51754567a(14) =194586181a(15) =731919279a(16) =2752461533a(17) =10352254743a(18) =38932913525a(19) =146424889471a(20) =550683608589