Number of permutations of length n that avoid both 1243 and 2134.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =87a(6) =354a(7) =1459a(8) =6056a(9) =25252a(10) =105632a(11) =442916a(12) =1860498a(13) =7826120a(14) =32956964a(15) =138911074a(16) =585926818a(17) =2472923499a(18) =10442263142a(19) =44112331275a(20) =186413949540a(21) =788000866243