A255460(2^n-1).

Terms

a(0) =1a(1) =6a(2) =22a(3) =82a(4) =306a(5) =1142a(6) =4250a(7) =15806a(8) =58730a(9) =218142a(10) =810074a(11) =3007886a(12) =11167914a(13) =41463774a(14) =153942330a(15) =571535534a(16) =2121906442a(17) =7877856190a(18) =29247532442a(19) =108585059214a(20) =403135208426