# Representation of the classical groups

*in tensors*

Linear representations (cf. Linear representation) of the groups , , , , , where is an -dimensional vector space over a field and is a non-degenerate symmetric or alternating bilinear form on , in invariant subspaces of tensor powers of . If the characteristic of is zero, then all irreducible polynomial linear representations of these groups can be realized by means of tensors.

In the case the groups above are complex Lie groups. For all groups, except , all (differentiable) linear representations are polynomial; every linear representation of has the form , where and is a polynomial linear representation. The classical compact Lie groups , , , , and have the same complex linear representations and the same invariant subspaces in tensor spaces as their complex envelopes , , , , and . Therefore, results of the theory of linear representations obtained for the classical complex Lie groups can be carried over to the corresponding compact groups and vice versa (Weyl's "unitary trick" ). In particular, using integration on a compact group one can prove that linear representations of the classical complex Lie groups are completely reducible.

The natural linear representation of in is given by the formula

In the same space a linear representation of the symmetric group is defined by

The operators of these two representations commute, so that a linear representation of is defined in . If , the space can be decomposed into a direct sum of minimal -invariant subspaces:

The summation is over all partitions of containing at most summands, is the space of the absolutely-irreducible representation of corresponding to (cf. Representation of the symmetric groups) and is the space of an absolutely-irreducible representation of . A partition can be conveniently represented by a tuple of non-negative integers satisfying and .

The subspace splits in a sum of minimal -invariant subspaces, in each of which a representation can be realized. These subspaces can be explicitly obtained by using Young symmetrizers (cf. Young symmetrizer) connected with . E.g. for (respectively, for ) one has and is the minimal -invariant subspace consisting of all symmetric (respectively, skew-symmetric) tensors.

The representation is characterized by the following properties. Let be the subgroup of all linear operators that, in some basis of , can be written as upper-triangular matrices. Then the operators , , have a unique (up to a numerical factor) common eigenvector , which is called the highest weight vector of . The corresponding eigenvalue (the highest weight of ) is equal to , where is the -th diagonal element of the matrix of in the basis . Representations corresponding to distinct partitions are inequivalent. The character of can be found from Weyl's formula

where are the roots of the characteristic polynomial of the operator , is the generalized Vandermonde determinant corresponding to (cf. Frobenius formula) and is the ordinary Vandermonde determinant. The dimension of is equal to

where .

The restriction of to the unimodular group is irreducible. The restrictions to of two representations and are equivalent if and only if (where is independent of ). The restriction of a representation of to the subgroup can be found by the rule:

where runs through all tuples satisfying

For every Young diagram , corresponding to a partition , the tensor (for notations see Representation of the symmetric groups) is the result of alternating the tensor over the columns of , where is the number of the row of in which the number is located. The tensors thus constructed with respect to all standard diagrams form a basis of the minimal -invariant subspace of in which the representation of is realized.

A linear representation of the orthogonal group in has the following structure. There is a decomposition into a direct sum of two -invariant subspaces:

where consists of traceless tensors, i.e. tensors whose convolution with over any two indices vanishes, and

The space , in turn, decomposes into a direct sum of -invariant subspaces:

where . Moreover, if and only if the sum of the heights of the first two columns of the Young tableau corresponding to does not exceed , and in this case is the space of an absolutely-irreducible representation of . Representations corresponding to distinct partitions are inequivalent. If satisfies the condition , then after replacing the first column of its Young tableau by a column of height one obtains the Young tableau of a partition which also satisfies this condition. The corresponding representations of are related by (in particular, they have equal dimension).

The restriction of to the subgroup is absolutely irreducible, except in the case even and (i.e. the number of terms of is equal to ). In the latter case it splits over the field or a quadratic extension of it into a sum of two inequivalent absolutely irreducible-representations of equal dimension.

In computing the dimension of one can assume that (otherwise replace by ). Let . Then for odd one has

while for even and one has

For the latter formula gives half the dimension of , i.e. the dimension of each of the absolutely-irreducible representations of corresponding to it.

The decomposition of with respect to the symplectic group is analogous to the decomposition with respect to the orthogonal group, with the difference that if and only if . The dimension of can in this case be found from

where .

#### References

[1] | H. Weyl, "The classical groups, their invariants and representations" , Princeton Univ. Press (1946) |

[2] | D.P. Zhelobenko, "Compact Lie groups and their representations" , Amer. Math. Soc. (1973) (Translated from Russian) |

[3] | M. Hamermesh, "Group theory and its application to physical problems" , Addison-Wesley (1962) |

#### Comments

This article describes the classical theory. The contemporary period in this old field of algebra begun with [a1]. It can be described by two words: "characteristic-free representation theorycharacteristic free" . A different approach to the polynomial representations of and was undertaken in [a2]. Further, both classical and characteristic free theories can be found in [a3].

#### References

[a1] | R.W. Carter, G. Lustig, "On the modular representations of the general linear and symmetric groups" Math. Z. , 136 (1974) pp. 193–242 |

[a2] | J.A. Green, "Polynomial representations of " , Lect. notes in math. , 830 , Springer (1980) |

[a3] | G. James, A. Kerber, "The representation theory of the symmetric group" , Addison-Wesley (1981) |

[a4] | W. Feit, "The representation theory of finite groups" , North-Holland (1982) |

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Representation of the classical groups.

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