a(n) = A255295(2^n-1).

Terms

a(0) =1a(1) =6a(2) =24a(3) =92a(4) =340a(5) =1236a(6) =4452a(7) =15956a(8) =57028a(9) =203508a(10) =725604a(11) =2585876a(12) =9212932a(13) =32818740a(14) =116898468a(15) =416365652a(16) =1482959428a(17) =5281740660a(18) =18811402980a(19) =66998214548a(20) =238618498180a(21) =849854020788