Plactic monoid

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.

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.

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.

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 ${\displaystyle {\frac {n(n+1)}{2}}}$.

• Chinese monoid

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