a(n) = A255470(2^n-1).

Terms

a(0) =1a(1) =6a(2) =24a(3) =100a(4) =396a(5) =1596a(6) =6364a(7) =25500a(8) =101916a(9) =407836a(10) =1631004a(11) =6524700a(12) =26097436a(13) =104392476a(14) =417564444a(15) =1670268700a(16) =6681052956a(17) =26724255516a(18) =106896934684a(19) =427587913500