Editing Young tableau (section)

==Definitions==

''Note: this article uses the English convention for displaying Young diagrams and tableaux''.

=== Diagrams <!-- [[Young diagram]] currently redirects to this section]]--> ===
[[Image:Young diagram for 541 partition.svg|thumb|right|150px|Young diagram of shape (5, 4, 1), English notation]]
[[Image:Young diagram for 541 partition-French.svg|thumb|right|150px|Young diagram of shape (5, 4, 1), French notation]]

A '''Young diagram''' (also called a [[Ferrers diagram]], particularly when represented using dots) is a finite collection of boxes, or cells, arranged in left-justified rows, with the row lengths in non-increasing order. Listing the number of boxes in each row gives a [[integer partition|partition]] {{mvar|''λ''}} of a non-negative integer {{mvar|''n''}}, the total number of boxes of the diagram. The Young diagram is said to be of shape {{mvar|''λ''}}, and it carries the same information as that partition. Containment of one Young diagram in another defines a [[partial ordering]] on the set of all partitions, which is in fact a [[lattice (order)|lattice]] structure, known as [[Young's lattice]]. Listing the number of boxes of a Young diagram in each column gives another partition, the '''conjugate''' or ''transpose'' partition of {{mvar|''λ''}}; one obtains a Young diagram of that shape by reflecting the original diagram along its main diagonal.

There is almost universal agreement that in labeling boxes of Young diagrams by pairs of integers, the first index selects the row of the diagram, and the second index selects the box within the row. Nevertheless, two distinct conventions exist to display these diagrams, and consequently tableaux: the first places each row below the previous one, the second stacks each row on top of the previous one. Since the former convention is mainly used by [[English-speaking world|Anglophones]] while the latter is often preferred by [[Francophone]]s, it is customary to refer to these conventions respectively as the ''English notation'' and the ''French notation''; for instance, in his book on [[symmetric function]]s, [[Ian G. Macdonald|Macdonald]] advises readers preferring the French convention to "read this book upside down in a mirror" (Macdonald 1979, p.&nbsp;2). This nomenclature probably started out as jocular. The English notation corresponds to the one universally used for matrices, while the French notation is closer to the convention of [[Cartesian coordinates]]; however, French notation differs from that convention by placing the vertical coordinate first. The figure on the right shows, using the English notation, the Young diagram corresponding to the partition (5, 4, 1) of the number 10. The conjugate partition, measuring the column lengths, is (3, 2, 2, 2, 1).

==== Arm and leg length ====
In many applications, for example when defining [[Jack function]]s, it is convenient to define the '''arm length''' ''a''<sub>λ</sub>(''s'') of a box ''s'' as the number of boxes to the right of ''s'' in the diagram λ in English notation. Similarly, the '''leg length''' ''l''<sub>λ</sub>(''s'') is the number of boxes below ''s''. The '''hook length''' of a box ''s'' is the number of boxes to the right of ''s'' or below ''s'' in English notation, including the box ''s'' itself; in other words, the hook length is ''a''<sub>λ</sub>(''s'') + ''l''<sub>λ</sub>(''s'') + 1.

=== Tableaux ===
[[Image:Young tableaux for 541 partition.svg|thumb|right|150px|A standard Young tableau of shape (5, 4, 1): the numbers 1-10 in the boxes increase in every row and every column.]]

A '''Young tableau''' is obtained by filling in the boxes of the Young diagram with symbols taken from some ''alphabet'', which is usually required to be a [[totally ordered set]]. Originally that alphabet was a set of indexed variables {{mvar|''x''<sub>1</sub>}}, {{mvar|''x''<sub>2</sub>}}, {{mvar|''x''<sub>3</sub>}}..., but now one usually uses a set of numbers for brevity. In their original application to [[representations of the symmetric group]], Young tableaux have {{mvar|''n''}} distinct entries, arbitrarily assigned to boxes of the diagram.  A tableau is called '''standard''' if the entries in each row and each column are increasing. The number of distinct standard Young tableaux on {{mvar|''n''}} entries is given by the [[involution number]]s
:1, 1, 2, 4, 10, 26, 76, 232, 764, 2620, 9496, ... {{OEIS|A000085}}.[[File:Standard Young Tableaux.png|thumb|All standard Young tableaux with at most 5 boxes]]

In other applications, it is natural to allow the same number to appear more than once (or not at all) in a tableau.  A tableau is called '''semistandard''', or ''column strict'', if the entries weakly increase along each row and strictly increase down each column.  Recording the number of times each number appears in a tableau gives a sequence known as the '''weight''' of the tableau.   Thus the standard Young tableaux are precisely the semistandard tableaux of weight (1,1,...,1), which requires every integer up to {{mvar|''n''}} to occur exactly once.

In a standard Young tableau, the integer <math>k</math> is a '''descent''' if <math>k+1</math> appears in a row strictly below <math>k</math>. The sum of the descents is called the '''major index''' of the tableau.<ref name="ste89"/>

=== Variations ===

There are several variations of this definition: for example, in a row-strict tableau the entries strictly increase along the rows and weakly increase down the columns. Also, tableaux with ''decreasing'' entries have been considered, notably, in the theory of [[plane partition]]s. There are also generalizations such as domino tableaux or ribbon tableaux, in which several boxes may be grouped together before assigning entries to them.

=== Skew tableaux ===
[[Image:Skew tableau 5422-21.svg|thumb|right|150px|Skew tableau of shape (5, 4, 2, 2) / (2, 1), English notation]]

A '''skew shape''' is a pair of partitions ({{math|''λ''}}, {{math|''μ''}}) such that the Young diagram of {{math|''λ''}} contains the Young diagram of {{math|''μ''}}; it is denoted by {{math|''λ''/''μ''}}.  If {{math|''λ'' {{=}} (''λ''<sub>1</sub>, ''λ''<sub>2</sub>, ...)}} and {{math|''μ'' {{=}} (''μ''<sub>1</sub>, ''μ''<sub>2</sub>, ...)}}, then the containment of diagrams means that {{math|''μ''<sub>''i''</sub>&nbsp;≤&nbsp;''λ''<sub>''i''</sub>}} for all {{mvar|i}}. The '''skew diagram''' of a skew shape {{math|''λ''/''μ''}} is the set-theoretic difference of the Young diagrams of {{mvar|''λ''}} and {{mvar|''μ''}}: the set of squares that belong to the diagram of {{mvar|''λ''}} but not to that of {{mvar|''μ''}}. A '''skew tableau''' of shape {{math|''λ''/''μ''}} is obtained by filling the squares of the corresponding skew diagram; such a tableau is semistandard if entries increase weakly along each row, and increase strictly down each column, and it is standard if moreover all numbers from 1 to the number of squares of the skew diagram occur exactly once. While the map from partitions to their Young diagrams is injective, this is not the case for the map from skew shapes to skew diagrams;<ref>For instance the skew diagram consisting of a single square at position (2,4) can be obtained by removing the diagram of {{math|''μ''&nbsp;{{=}}&nbsp;(5,3,2,1)}} from the one of {{math|''λ'' {{=}} (5,4,2,1)}}, but also in (infinitely) many other ways. In general any skew diagram whose set of non-empty rows (or of non-empty columns) is not contiguous or does not contain the first row (respectively column) will be associated to more than one skew shape.</ref> therefore the shape of a skew diagram cannot always be determined from the set of filled squares only. Although many properties of skew tableaux only depend on the filled squares, some operations defined on them do require explicit knowledge of {{mvar|''λ''}} and {{mvar|''μ''}}, so it is important that skew tableaux do record this information: two distinct skew tableaux may differ only in their shape, while they occupy the same set of squares, each filled with the same entries.<ref>A somewhat similar situation arises for matrices: the 3-by-0 matrix {{mvar|''A''}} must be distinguished from the 0-by-3 matrix {{mvar|''B''}}, since {{math|''AB''}} is a 3-by-3 (zero) matrix while {{math|''BA''}} is the 0-by-0 matrix, but both {{mvar|''A''}} and {{mvar|''B''}} have the same (empty) set of entries; for skew tableaux however such distinction is necessary even in cases where the set of entries is not empty.</ref> Young tableaux can be identified with skew tableaux in which {{mvar|''μ''}} is the empty partition (0) (the unique partition of 0).

Any skew semistandard tableau {{mvar|''T''}} of shape {{math|''λ''/''μ''}} with positive integer entries gives rise to a sequence of partitions (or Young diagrams), by starting with {{mvar|''μ''}}, and taking for the partition {{mvar|''i''}} places further in the sequence the one whose diagram is obtained from that of {{mvar|''μ''}} by adding all the boxes that contain a value&nbsp; ≤&nbsp;{{mvar|''i''}} in {{mvar|''T''}}; this partition eventually becomes equal to&nbsp;{{mvar|''λ''}}. Any pair of successive shapes in such a sequence is a skew shape whose diagram contains at most one box in each column; such shapes are called '''horizontal strips'''. This sequence of partitions completely determines {{mvar|''T''}}, and it is in fact possible to define (skew) semistandard tableaux as such sequences, as is done by Macdonald (Macdonald 1979, p.&nbsp;4). This definition incorporates the partitions {{mvar|''λ''}} and {{mvar|''μ''}} in the data comprising the skew tableau.