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A partition of elements labelled is represented as a diagram, with lines connecting elements in the same subset. In the following example, the subset gives rise to the lines , and could equivalently be represented by the lines (for instance).
For and , the partition algebra is defined by a -basis made of partitions, and a multiplication given by diagram concatenation. The concatenated diagram comes with a factor , where is the number of connected components that are disconnected from the top and bottom elements.
Generators and relations
The partition algebra is generated by elements of the type
The symmetric group algebra is the group ring of the symmetric group over . The Motzkin algebra is sometimes called the dilute Temperley–Lieb algebra in the physics literature.[4]
Inclusions from allowing top-top and bottom-bottom lines:
We have the isomorphism:
More subalgebras
In addition to the eight subalgebras described above, other subalgebras have been defined:
The totally propagating partition subalgebra is generated by diagrams whose blocks all propagate, i.e. partitions whose subsets all contain top and bottom elements.[5] These diagrams from the dual symmetric inverse monoid, which is generated by .[6]
The quasi-partition algebra is generated by subsets of size at least two. Its generators are and its dimension is .[7]
The uniform block permutation algebra is generated by subsets with as many top elements as bottom elements. It is generated by .[8]
An algebra with a half-integer index is defined from partitions of elements by requiring that and are in the same subset. For example, is generated by so that , and .[2]
Periodic subalgebras are generated by diagrams that can be drawn on an annulus without line crossings. Such subalgebras include a translation element
such that . The translation element and its powers are the only combinations of that belong to periodic subalgebras.
Representations
Structure
For an integer , let be the set of partitions of elements (bottom) and (top), such that no two top elements are in the same subset, and no top element is alone. Such partitions are represented by diagrams with no top-top lines, with at least one line for each top element. For example, in the case :
Partition diagrams act on from the bottom, while the symmetric group acts from the top. For any Specht module of (with therefore ), we define the representation of
A basis of can be described combinatorially in terms of set-partition tableaux: Young tableaux whose boxes are filled with the blocks of a set partition.[1]
Assuming that is semisimple, the representation is irreducible, and the
set of irreducible finite-dimensional representations of the partition algebra is
Representations of subalgebras
Representations of non-planar subalgebras have similar structures as representations of the partition algebra. For example, the Brauer-Specht modules of the Brauer algebra are built from Specht modules, and certain sets of partitions.
In the case of the planar subalgebras, planarity prevents nontrivial permutations, and Specht modules do not appear. For example, a standard module of the Temperley–Lieb algebra is parametrized by an integer with , and a basis is simply given by a set of partitions.
The following table lists the irreducible representations of the partition algebra and eight subalgebras.[3]
Algebra
Parameter
Conditions
Dimension
The irreducible representations of are indexed by partitions such that and their dimensions are .[5] The irreducible representations of are indexed by partitions such that .[7] The irreducible representations of are indexed by sequences of partitions.[8]
Schur-Weyl duality
Assume .
For a -dimensional vector space with basis , there is a natural action of the partition algebra on the vector space . This action is defined by the matrix elements of a partition in the basis :[2]
This matrix element is one if all indices corresponding to any given partition subset coincide, and zero otherwise. For example, the action of a Temperley–Lieb generator is
Duality between the partition algebra and the symmetric group
Let be integer.
Let us take to be the natural permutation representation of the symmetric group. This -dimensional representation is a sum of two irreducible representations: the standard and trivial representations, .
Then the partition algebra is the centralizer of the action of on the tensor product space ,
Moreover, as a bimodule over , the tensor product space decomposes into irreducible representations as[1]
where is a Young diagram of size built by adding a first row to , and is the corresponding Specht module of .
Dualities involving subalgebras
The duality between the symmetric group and the partition algebra generalizes the original Schur-Weyl duality between the general linear group and the symmetric group. There are other generalizations. In the relevant tensor product spaces, we write for an irreducible -dimensional representation of the first group or algebra:
Tensor product space
Group or algebra
Dual algebra or group
Comments
The duality for the full partition algebra
Case of a partition algebra with a half-integer index[2]
^ abOrellana, Rosa; Saliola, Franco; Schilling, Anne; Zabrocki, Mike (2021-12-27). "Plethysm and the algebra of uniform block permutations". arXiv:2112.13909v1 [math.CO].