Number of 7 up, 7 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) =8a(10) =36a(11) =120a(12) =330a(13) =792a(14) =1716a(15) =3432a(16) =48477a(17) =368797a(18) =2002429a(19) =8685469a(20) =31964725a(21) =103593205a(22) =303129685a(23) =6156709438a(24) =66139517554a(25) =498859498016