isVexillary -- whether a permutation is vexillary, i.e. 2143-avoiding

Synopsis

Usage:

isVexillary w
Inputs:
- w, a list,
Outputs:
- a Boolean value,

Description

Given a permutation in 1-line notation, checks if the permutation is vexillary, i.e. $2143$-avoiding. A permutation $w$ is $2143$-avoiding if there do not exist indices $i < j < k < l$ such that $w_j < w_i < w_l < w_k$.

i1 : w = {7,2,5,8,1,3,6,4}

o1 = {7, 2, 5, 8, 1, 3, 6, 4}

o1 : List

i2 : isVexillary w

o2 = false

i3 : v = {1,6,9,2,4,7,3,5,8}

o3 = {1, 6, 9, 2, 4, 7, 3, 5, 8}

o3 : List

i4 : isVexillary v

o4 = true

Ways to use isVexillary :

isVexillary(List)

For the programmer

The object isVexillary is a method function.