Vexillary permutation

In mathematics, a vexillary permutation is a permutation "μ" of the positive integers containing no subpermutation isomorphic to the permutation (2143); in other words, there do not exist four numbers "i" < "j" < "k" < "l" with "μ"("j") < "μ"("i") < "μ"("l") < "μ"("k"). They were introduced by . The word "vexillary" means flag-like, and comes from the fact that vexillary permutations are related to flags of modules.
 showed that vexillary involutions are enumerated by Motzkin numbers.