A255446(2^n-1).

Terms

a(0) =1a(1) =5a(2) =21a(3) =79a(4) =291a(5) =1043a(6) =3725a(7) =13205a(8) =46793a(9) =165471a(10) =585191a(11) =2068315a(12) =7310897a(13) =25837453a(14) =91315845a(15) =322715687a(16) =1140515523a(17) =4030652683a(18) =14244666301a(19) =50341596733a(20) =177910950553a(21) =628749519543