a(n) = A246031(2^n-1).

Terms

a(0) =1a(1) =26a(2) =124a(3) =1400a(4) =10000a(5) =89504a(6) =707008a(7) =5924480a(8) =47900416a(9) =393069824a(10) =3189761536a(11) =25963397888a(12) =210468531712