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ExteriorLength on Permutation ​

The longest proper prefix whose relative order is that of the suffix of the same length.

On Permutation

Called as CombinatorialStat(x, "ExteriorLength"). The head's page has its examples.

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FindStat St001052

The length of the exterior of a permutation.

ObjectValue
[1,2]1
[2,1]1
[1,2,3]2
[1,3,2]1
[2,1,3]1
[2,3,1]1
[3,1,2]1
[3,2,1]2

Data from FindStat, the combinatorial statistic finder; the page at findstat.org/St001052 has the rest.