A255454(2^n-1).

Terms

a(0) =1a(1) =5a(2) =19a(3) =73a(4) =263a(5) =973a(6) =3531a(7) =12965a(8) =47251a(9) =173009a(10) =631631a(11) =2310273a(12) =8440503a(13) =30859717a(14) =112776995a(15) =412263309a(16) =1506786763a(17) =5507802233a(18) =20131419959a(19) =73585107241a(20) =268963530095a(21) =983115443149