Number of 8 up, 8 down permutations of [n].

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =1a(8) =1a(9) =1a(10) =9a(11) =45a(12) =165a(13) =495a(14) =1287a(15) =3003a(16) =6435a(17) =12870a(18) =207350a(19) =1782638a(20) =10852478a(21) =52422578a(22) =213586658a(23) =762311978a(24) =2445069626a(25) =7177825511a(26) =167072419775