The number of permutations that avoid the pattern 122'1', that is, out of four consecutive elements in a permutation we never have the situation that the first two elements form an ascent while the last two elements form a descent.
A173385
The number of permutations that avoid the pattern 122'1', that is, out of four consecutive elements in a permutation we never have the situation that the first two elements form an ascent while the last two elements form a descent.
Terms
- a(0) =1a(1) =1a(2) =2a(3) =6a(4) =18a(5) =66a(6) =252a(7) =1176a(8) =5768a(9) =34216a(10) =209552a(11) =1521696a(12) =11196768a(13) =96160416a(14) =825730752a(15) =8183634816a(16) =80315504768a(17) =902135948416
External references
- oeis: A173385