a(n) = A253100(2^n-1).

Terms

a(0) =1a(1) =6a(2) =24a(3) =96a(4) =372a(5) =1416a(6) =5340a(7) =20040a(8) =75012a(9) =280392a(10) =1047324a(11) =3910440a(12) =14597508a(13) =54485736a(14) =203357724a(15) =758969736a(16) =2832570372a(17) =10571410056a(18) =39453266460a(19) =147242049000a(20) =549515715972