Number of permutations in S_n avoiding the patterns 1342 and 2143.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =88a(6) =368a(7) =1584a(8) =6968a(9) =31192a(10) =141656a(11) =651136a(12) =3023840a(13) =14166496a(14) =66876096a(15) =317809216a(16) =1519163456a(17) =7299577216a(18) =35237444736a(19) =170812433536a(20) =831127053696