A255277(2^n-1).

Terms

a(0) =1a(1) =7a(2) =27a(3) =113a(4) =447a(5) =1743a(6) =6789a(7) =26371a(8) =102399a(9) =397425a(10) =1542327a(11) =5984815a(12) =23222957a(13) =90110355a(14) =349647247a(15) =1356699401a(16) =5264252887a(17) =20426289087a(18) =79257818197a(19) =307535089027