Number of permutations of length n which avoid the patterns 2134, 3421.

Terms

a(0) =1a(1) =2a(2) =6a(3) =22a(4) =86a(5) =330a(6) =1206a(7) =4174a(8) =13726a(9) =43134a(10) =130302a(11) =380414a(12) =1078270a(13) =2978814a(14) =8046590a(15) =21311486a(16) =55468030a(17) =142147582a(18) =359268350a(19) =896794622a(20) =2213543934a(21) =5408292862a(22) =13091995646a(23) =31424774142a(24) =74845257726a(25) =176991240190a(26) =415789744126a(27) =970830381054