# nLab semistandard Young tableau

Contents

### Context

#### Combinatorics

combinatorics

enumerative combinatorics

graph theory

rewriting

### Polytopes

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category: combinatorics

#### Representation theory

representation theory

geometric representation theory

# Contents

## Idea

In combinatorics a (semi-)standard Young tableau is a labelling of the boxes of a Young diagram with positive natural numbers (a Young tableau) satisfying extra conditions, at the minimum that labels do not decrease to the right and do increase downwards.

The number of (semi-)standard Young tableau of given shape (underlying Young diagram) govern various objects in the representation theory of the symmetric- and general linear group.

## Definition

###### Definition

(Young diagram)
For $n \in \mathbb{N}$ a positive natural number, a Young diagram with $n$ boxes is a partition of $n$, hence a sequence of weakly decreasing positive natural numbers

$(\lambda_1 \geq \lambda_2 \geq \cdots \geq \lambda_{rows(\lambda)} ) \,, \;\; \lambda_i \in \mathbb{N}_+$

whose sum is $n$:

(1)$n \;=\; \underset{i}{\sum} \lambda_i \,,$

###### Definition

Given a Young diagram/partition $\lambda$ (Def. ), a semistandard Young tableau $T$ of shape $\lambda$

• is an indexed set of positive natural numbers of the following form (a Young tableau):

$\big( T_{i, j} \in \mathbb{N}_+ \big)_{ {1 \leq i \leq rows(\lambda)} \atop { 1 \leq j \leq \lambda_i } } \,;$
• such that these labels are:

1. weakly increasing to the right/along rows;

(2)$\underset{ i }{\forall} \;\;\;\;\;\; j_1 \lt j_2 \;\Rightarrow\; T_{i, j_1} \leq T_{i, j_2}$
2. strictly increasing downwards/along columns:

(3)$\underset{ j }{\forall} \;\;\;\;\;\; i_1 \lt i_2 \;\Rightarrow\; T_{i_1, j} \lt T_{i_2, j}$

###### Remark

Beware that in a semistandard Young tableau (Def. ):

• the label $T_{1, 1}$ in the top left box is not required to be 1;

• in some applications (e.g. (7) below), though not in general, the labels are in addition required to be bounded $T_{i, j} \leq k$.

###### Definition

(standard Young tableau)
A semistandard Young tableau (Def. ) is called a standard Young tableau if, in addition to (2) and (3), it satisfies the following equivalent conditions:

• every element of $\big\{1, \cdots, n\}$ appears exactly once;

• the following two conditions hold:

1. not just the columns but also the rows are strictly increasing,

2. the last label is $T_{rows(\lambda), \lambda_{rows(\lambda)}} = n$ (1)

(Which implies that the first label of a standard Young tableau must be $T_{1,1} = 1$.)

## Examples

The following is a semistandard Young tableau that does happen to start with 1 but is not a standard Young tableau:

$\array{ 1 & 1 & 1 & 3 \\ 2 & 3 \\ 4 }$

## Properties

Given a partition $\lambda \in Part(n)$, we write

(4)$ssYT_\lambda \;in\; Set$

for the set of semistandard Young tableaux (Def. ) whose underlying Young diagram (i.e. forgetting its labels) is given by $\lambda$.

Given moreover a natural number $N \in \mathbb{N}$ we write

(5)$ssYT_\lambda(N) \;\subset\; ssYT_\lambda$

for the subset on those semistandard Young tableau $T$ whose labels are bounded by $N$ in that $\underset{i,j}{\forall}\; T_{i, j } \;\leq\; N$.

### Relation to Schur polynomials

Given a semistandard Young tableau $T$, we write $X^T$ for the monomial which contains one factor of the variable $x_k$ for each occurrence of $k$ in the Young tableau:

(6)$x^T \;\coloneqq\; x_1^{\#1s} x_1^{\#2s} \cdots \,.$

###### Proposition

For $n \in \mathbb{N}$ and $\lambda$ a partition of $n$, the corresponding Schur polynomial $s_\lambda$ is equal to the sum over the monomials (6) associated with all semistandard Young tableau of shape $\lambda$ (4):

$s_\lambda(x_1, x_2, \cdots) \;\;=\;\; \underset{ {T \in ssYT_\lambda}, }{\sum} x^T \,.$

(Sagan 01, Def. 4.4.1, review in Sagan Enc., p. 1)

This means that Schur polynomials in a finite set $\{x_1, \cdots, x_N\}$ of variables count semistandard Young tableau (of fixed shape and) with bounded labels (5):

(7)$s_\lambda(x_1, \cdots, x_N) \;\;=\;\; \underset{ {T \in ssYT_\lambda(N)}, }{\sum} x^T \,.$

### Hook-length formula

See at hook length formula.

### Hook-content formula

The hook-content formula expresses the number of semistandard Young-Tableau $T$ with

1. shape $\lambda = (\lambda_1 \geq \cdots \geq \lambda_i)$ (a partition),

2. labels in $\{1, \cdots, N\}$,

3. sum of labels $\sum_{i,j} T_{i,j}$

through the following identification of the generating function for numbers of semistandard Young tableaux (ssYT) as a polynomial in a variable $q$:

$\underset{ T \in ssYT_\lambda(N) }{\sum} q^{ \sum_{i,j} T_{i,j} } \;\;=\;\; q^{ \sum_i i \cdot \lambda_i } \underset{ { i \in \{1, \cdots, rows(\lambda)\} } \atop {j \in \{1, \cdots, \lambda_j\}} }{\prod} \frac{ 1 - q^{N + content(i,j)} }{ 1 - q^{\ell hook(i,j)} } \,,$

where

$content(i,j) \;\coloneqq\; j - 1 \,, {\phantom{AAAA}} \ell hook(i,j) \;=\; \text{length of hook at (i,j)th box} \,.$

See at hook-content formula for more.

### Number of sYT with bounded number of rows

We discuss formulas for the number

$\left\vert sYT_n(N) \right\vert \;=\; \underset{ { \lambda \in Part(n) } \atop { rows(\lambda) \leq N } }{ \sum } \left\vert sYT_n \right\vert$

of standard Young tableaux with $n$ boxes and $\leq N$ rows.

#### Asymptotic formulas for large $n$

Asymptotically for large $n$ this is (Regev 81, (F.4.5.1)):

(8)\begin{aligned} \left\vert sYT_n(N) \right\vert & \; \overset{ n \to \infty }{\sim} \; \left( \frac{N}{n} \right)^{ \tfrac{1}{4} N(N-1) } \cdot \frac {N^n} {N!} \cdot \big( \underset{ \sqrt{\pi}/2 }{ \underbrace{ \Gamma(3/2) } } \big)^{- N} \cdot \underoverset {j=1} {N} {\prod} \Gamma \left( 1 + \tfrac{1}{2}j \right) \end{aligned}

Later this appears again as a conjecture in Kotěšovec 13:

(9)\begin{aligned} \left\vert sYT_n(N) \right\vert & \;\sim\; \underoverset {j = 1} {N} {\prod} \underset{ \Gamma(1 + j/2) \tfrac{2}{j} }{ \underbrace{ \Gamma(j/2) } } \cdot \frac{N^n}{\pi^{N/2}} \cdot \left( \frac{N}{n} \right)^{ \tfrac{1}{4}(N(N-1)) } \\ & \;=\; \underoverset {j = 1} {N} {\prod} \Gamma \big( 1 + j/2 \big) \cdot \frac{2^N}{N!} \cdot \frac{N^n}{\pi^{N/2}} \cdot \left( \frac{N}{n} \right)^{ \tfrac{1}{4}(N(N-1)) } \,. \end{aligned}

(The first line is the expression conjectured in Kotěšovec 13, under the brace we use the translation formula, and the second line shows agreement with Regev 81 (F.4.5.1) as in (8).)

This may further be re-expressed (Kotěšovec 13, p. 2) in terms of the Barnes G-function $G(-)$ (see there):

(10)\begin{aligned} \left\vert sYT_n(N) \right\vert & \;\underset{n \to \infty}{\sim}\; \underset{ \mathclap{ \frac { G(N/2 + 1) \cdot G(N/2 + 1/2) } { G(1/2) } } }{ \underbrace{ \underoverset {j = 1} {N} {\prod} \Gamma(j/2) } } \cdot \frac{N^n}{\pi^{N/2}} \cdot \left( \frac{N}{n} \right)^{ \tfrac{1}{4}(N(N-1)) } \end{aligned}

#### Asymptotic formulas for large $n$ and large $N$

Using the large-$N$ asymptotic expansion (here) of the Barnes G-function

$G(N/2 + 1) \underset{N \to \infty}{\sim} \exp \left( \tfrac{1}{8} N^2 ln (N) - \left( \tfrac{3}{16} + \tfrac{ln(2)}{8} \right) N^2 + \tfrac{ln(2\pi)}{4} N - \tfrac{1}{12} ln(N) + \big( \zeta^'(-1) - \tfrac{ln(2)}{12} \big) \right)$

inside the large-$n$ asymptotic expansion (10), we obtain the doubly asymptotic expansion of the number of standard Yound tableaux with $n$ boxes and $\leq N$ rows:

(11)\begin{aligned} & ln \big( \left\vert sYT_n(N) \right\vert \big) \\ & \; \underset{ { n \to \infty } \atop { N \to \infty } }{\sim}\; n ln(N) - \tfrac{1}{4}N^2 ln(n) + \tfrac{1}{4}N ln(n) \\ & \phantom{ \underset{ { n \to \infty } \atop { N \to \infty } }{\sim}\; } + \tfrac{1}{2} N^2 ln(N) - \left( \tfrac{3}{8} + \tfrac{ln(2)}{4} \right) N^2 - \tfrac{1}{4} N ln(N) + \tfrac{ln(2)}{2} N - \tfrac{1}{6} ln(N) \\ & \; \phantom{ \underset{ { n \to \infty } \atop { N \to \infty } }{\sim}\; } + \big( 2 \zeta^'(-1) - \tfrac{ln(2)}{6} - ln G(1/2) \big) \end{aligned}

#### Exact formulas for small $N$

Exact formulas for small $N$:

$\left\vert sYT_{2n}(2) \right\vert \;=\; \frac{ (2n)! }{ (n!)^2 } \,, \;\;\;\;\;\; \left\vert sYT_{2n+1}(2) \right\vert \;=\; \frac{ (2n + 1)! }{ n! (n+1)! }$
$\left\vert sYT_{n}(3) \right\vert \;=\; \underoverset {i = 0} { \rfloor n/2 \lfloor } {\sum} \left( n \atop {2i} \right) Catalan(i) \;=\; Motzkin(n) \,,$

where

• $Catalan(k) \;=\; \frac{(2k)!}{k! (k+1)!}$ are the Catalan numbers;

• $Motzkin(-)$ are the Motzkin numbers.

This is attributed in Gouyou-Beauchamps 89 to Regev 81.

For $N = 4,5$:

$\left\vert sYT_{2n-1}(4) \right\vert \;=\; Catalan(n) Catalan(n) \,, \;\;\;\;\;\; \left\vert sYT_{2n}(4) \right\vert \;=\; Catalan(n) Catalan(n+1)$

due to Gouyou-Beauchamps 89.

## References

### General

Textbook account:

Survey in the context of Schur functions:

Specifically for standard Young tableaux: