a(n) = A255464(2^n-1).

Terms

a(0) =1a(1) =6a(2) =22a(3) =90a(4) =358a(5) =1434a(6) =5734a(7) =22938a(8) =91750a(9) =367002a(10) =1468006a(11) =5872026a(12) =23488102a(13) =93952410a(14) =375809638a(15) =1503238554a(16) =6012954214a(17) =24051816858a(18) =96207267430a(19) =384829069722