a(n) = A253086(2^n-1).

Terms

a(0) =1a(1) =4a(2) =12a(3) =48a(4) =144a(5) =440a(6) =1816a(7) =6816a(8) =25816a(9) =101856a(10) =397944a(11) =1573464a(12) =6274560a(13) =25071152a(14) =100180872a(15) =400625360