Number of canonical Gray cycles of length 2n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =6a(4) =4a(5) =22a(6) =96a(7) =1344a(8) =672a(9) =3448a(10) =12114a(11) =158424a(12) =406312a(13) =4579440a(14) =37826256a(15) =906545760a(16) =362618304a(17) =1784113248a(18) =5576251408a(19) =68998853808a(20) =154939065862