Number of 6 up, 6 down permutations of [n].

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =1a(8) =7a(9) =28a(10) =84a(11) =210a(12) =462a(13) =924a(14) =11220a(15) =74283a(16) =354563a(17) =1363571a(18) =4482323a(19) =13058891a(20) =226630385a(21) =2101302530a(22) =13794660782a(23) =71790720848a(24) =314709809876