# Plactic monoid

In mathematics, the plactic monoid is the monoid of all words in the alphabet of positive integers modulo Knuth equivalence. Its elements can be identified with semistandard Young tableaux. It was discovered by Donald Knuth (1970) (who called it the tableau algebra), using an operation given by Craige Schensted (1961) in his study of the longest increasing subsequence of a permutation.

It was named the "monoïde plaxique" by Lascoux & Schützenberger (1981), who allowed any totally ordered alphabet in the definition. The etymology of the word "plaxique" is unclear; it may refer to plate tectonics ("tectonique des plaques" in French), as elementary relations that generate the equivalence allow conditional commutation of generator symbols: they can sometimes slide across each other (in apparent analogy to tectonic plates), but not freely.

## Definition

The plactic monoid over some totally ordered alphabet (often the positive integers) is the monoid with the following presentation:

• The generators are the letters of the alphabet
• The relations are the elementary Knuth transformations yzx = yxz whenever x < y ≤ z and xzy = zxy whenever x ≤ y < z.

## Knuth equivalence

Two words are called Knuth equivalent if they represent the same element of the plactic monoid, or in other words if one can be obtained from the other by a sequence of elementary Knuth transformations.

Several properties are preserved by Knuth equivalence.

• If a word is a reverse lattice word, then so is any word Knuth equivalent to it.
• If two words are Knuth equivalent, then so are the words obtained by removing their rightmost maximal elements, as are the words obtained by removing their leftmost minimal elements.
• Knuth equivalence preserves the length of the longest nondecreasing subsequence, and more generally preserves the maximum of the sum of the lengths of k disjoint non-decreasing subsequences for any fixed k.

Every word is Knuth equivalent to the word of a unique semistandard Young tableau (this means that each row is non-decreasing and each column is strictly increasing). So the elements of the plactic monoid can be identified with the semistandard Young tableaux, which therefore also form a monoid.

## Tableau ring

The tableau ring is the monoid ring of the plactic monoid, so it has a Z-basis consisting of elements of the plactic monoid, with the same product as in the plactic monoid.

There is a homomorphism from the plactic ring on an alphabet to the ring of polynomials (with variables indexed by the alphabet) taking any tableau to the product of the variables of its entries.

## Growth

The generating function of the plactic monoid on an alphabet of size n is

${\displaystyle \Gamma (t)={\frac {1}{(1-t)^{n}}}{\frac {1}{(1-t^{2})^{n(n-1)/2}}}\ }$

showing that there is polynomial growth of dimension ${\displaystyle {\frac {n(n+1)}{2}}}$.

## References

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• Lascoux, Alain; Schützenberger, Marcel-P. (1981), "Le monoïde plaxique" (PDF), Noncommutative structures in algebra and geometric combinatorics (Naples, 1978), Quaderni de La Ricerca Scientifica, 109, Rome: CNR, pp. 129–156, MR 0646486
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• Schensted, C. (1961), "Longest increasing and decreasing subsequences", Canadian Journal of Mathematics, 13: 179–191, doi:10.4153/CJM-1961-015-3, ISSN 0008-414X, MR 0121305
• Schützenberger, Marcel-Paul (1997), "Pour le monoïde plaxique", Mathématiques, Informatique et Sciences Humaines (140): 5–10, ISSN 0995-2314, MR 1627563