nLab Young diagram

Contents

Contents

The idea

Young diagrams (also called Ferrers diagrams) are used to describe various kinds of objects in algebra, combinatorics and representation theory, including:

  • integer partitions. For example, the integer partition

    17=5+4+4+2+1+1 17 = 5 + 4 + 4 + 2 + 1 + 1

    is drawn as the Young diagram

Young diagram (5,4,4,2,1,1) Layer 1

Young diagram

A Young diagram F λF^\lambda, also called Ferrers diagram, is a graphical representation of an unordered integer partition λ=(λ 1λ 2λ l\lambda = (\lambda_1\ge\lambda_2\ge\ldots\ge\lambda_l). If λn\lambda\vdash n is a partition of nn then the Young diagram has nn boxes. A partition can be addressed as a multiset over \mathbb{N}.

There are two widely used such representations. The English one uses matrix-like indices, and the French one uses Cartesian coordinate-like indices for the boxes x i,jx_{i,j} in the diagram F λF^\lambda.

In the English representation the boxes are adjusted to the north-west in the 4th quadrant of a 2-dimensional Cartesian coordinate system, with the ‘y’-axis being downward oriented. For instance the diagram F (5,4,4,2,1,1)F^{(5,4,4,2,1,1)} representing the partition (5,4,4,2,1,1)(5,4,4,2,1,1) of 1717 is given in the English representation as:

Young diagram (5,4,4,2,1,1) Layer 1

Let 𝕐\mathbb{Y} be the set of Young diagrams. Important functions on Young diagrams include:

  • conjugation: denoted by a prime :𝕐𝕐\prime : \mathbb{Y} \rightarrow \mathbb{Y} reflects the Young diagram along its main diagonal (north-west to south-east). In the above example the conjugated partition would be λ =(6,4,3,3,1)\lambda^\prime=(6,4,3,3,1).
  • weight: wt:𝕐wt \colon \mathbb{Y} \rightarrow \mathbb{N} provides the number of boxes.
  • length: :𝕐\ell \colon \mathbb{Y} \rightarrow \mathbb{N} provides the number of rows or equivalently the number of positive parts of the partition λ\lambda. The length of the conjugated diagram gives the number of columns.
  • plus: +:𝕐×𝕐𝕐::(μ,ν)μ+ν=(μ 1+ν 1,,μ l+ν l)+ \colon \mathbb{Y}\times \mathbb{Y} \rightarrow \mathbb{Y} :: (\mu,\nu) \mapsto \mu + \nu = (\mu_1+\nu_1,\ldots,\mu_l+\nu_l)
  • times: ×:𝕐×𝕐𝕐::(μ,ν)(μν) \times \colon \mathbb{Y}\times \mathbb{Y} \rightarrow \mathbb{Y} :: (\mu,\nu) \mapsto (\mu \cup \nu)_{\ge} the unordered union of the multisets. It follows that μ×ν=(μ +ν ) \mu\times \nu =(\mu^\prime + \nu^\prime)^\prime.

A filling of a Young diagram with elements from a set SS is called a Young tableau.

Skew Young diagram

A generalization of a Young diagram is a skew Young diagram. Let μ,ν\mu,\nu be two partitions, and let νμ\nu \le \mu be defined as i:ν iμ i\forall i : \nu_i\le \mu_i (possibly adding trailing zeros). The skew Young diagram F μ/νF^{\mu/\nu} is given by the Young diagram F μF^\mu with all boxes belonging to F νF^\nu when superimposed removed. If μ=(5,4,4,2,1,1)\mu=(5,4,4,2,1,1) and ν=(3,3,2,1)\nu=(3,3,2,1) then F μ/νF^{\mu/\nu} looks like:

Skew Young diagram (5,4,4,2,1,1)/(3,3,2,1) Layer 1
  • A skew diagram is called connected if all boxes share an edge.
  • A skew diagram is called a horizontal strip if every column contains at most one box.
  • A skew diagram is called a vertical strip if every row contains at most one box.
  • conjugation, weight, length extend to skew diagrams accordingly.

Young tableau

References

Quick introduction

  • Alexander Yong: What is… a Young Tableau, Notices of the American Mathematical Society 54 (2007) 240-241 [pdf]

Textbook accounts are in any book on representation theory in general and on the representation theory of the symmetric group in particular; such as:

  • William Fulton, Joe Harris, Representation Theory: a First Course, Springer (1991) [pdf]

  • Bruce E. Sagan: The Symmetric Group: Representations, Combinatorial Algorithms, and Symmetric Functions, Springer (2001)

More details:

  • Kazuhiko Koike, Itaru Terada: Young-diagrammatic methods for the representation theory of the classical groups of type B nB_n, C nC_n, D nD_n, Journal of Algebra, Volume 107, Issue 2, May 1987, Pages 466-511

  • William Fulton, Young Tableaux, with Applications to Representation Theory and Geometry, Cambridge U. Press, 1997 (doi:10.1017/CBO9780511626241)

  • Ron M. Adin, Yuval Roichman, Enumeration of Standard Young Tableaux, Chapter 14 in: Miklós Bóna, Handbook of Enumerative Combinatorics, CRC Press 2015 (arXiv:1408.4497, ISBN:9781482220858)

Relation to algebraic geometry:

With an eye towards application to (the standard model of) particle physics:

category: combinatorics

Last revised on February 2, 2025 at 13:17:30. See the history of this page for a list of all contributions to it.