a(n) = A255456(2^n-1).

Terms

a(0) =1a(1) =5a(2) =23a(3) =93a(4) =359a(5) =1335a(6) =4873a(7) =17535a(8) =62601a(9) =222181a(10) =785855a(11) =2772717a(12) =9768351a(13) =34378167a(14) =120910529a(15) =425062511a(16) =1493898001a(17) =5249371781a(18) =18443445415a(19) =64795091709a(20) =227625068503a(21) =799619495287