Weyl character formula
In mathematics, the Weyl character formula in representation theory describes the characters of irreducible representations of compact Lie groups in terms of their highest weights. It was proved by Hermann Weyl (1925, 1926a, 1926b).
By definition, the character of a representation r of G is the trace of r(g), as a function of a group element g in G. The irreducible representations in this case are all finite-dimensional (this is part of the Peter–Weyl theorem); so the notion of trace is the usual one from linear algebra. Knowledge of the character χ of r is a good substitute for r itself, and can have algorithmic content. Weyl's formula is a closed formula for the χ, in terms of other objects constructed from G and its Lie algebra. The representations in question here are complex, and so without loss of generality are unitary representations; irreducible therefore means the same as indecomposable, i.e. not a direct sum of two subrepresentations.
Statement of Weyl character formula
- is the Weyl group;
- is the subset of the positive roots of the root system ;
- is the half sum of the positive roots;
- is the highest weight of the irreducible representation ;
- is the determinant of the action of on the Cartan subalgebra . This is equal to , where is the length of the Weyl group element, defined to be the minimal number of reflections with respect to simple roots such that equals the product of those reflections.
Using the Weyl denominator formula, the character formula may be rewritten as
Note that the character is itself a large sum of exponentials. We then multiply the character by an alternating sum of exponentials. The surprising part of the character formula is that when we compute this product, only a small number of terms actually remain. Many more terms than this occur at least once in the product of the character and the Weyl denominator, but most of these terms cancel out to zero. The only terms that survive are the terms that occur only once, namely (which is obtained by taking the highest weight from and the highest weight from the Weyl denominator) and things in the Weyl-group orbit of .
The character of an irreducible representation of a compact connected Lie group is given by
where is the character on with differential on the Lie algebra of the maximal Torus .
If is the differential of a character of , e.g. if is simply connected, this can be reformulated as
Weyl denominator formula
In the special case of the trivial 1-dimensional representation the character is 1, so the Weyl character formula becomes the Weyl denominator formula:
For special unitary groups, this is equivalent to the expression
Weyl dimension formula
By specialization to the trace of the identity element, Weyl's character formula gives the Weyl dimension formula
for the dimension of a finite dimensional representation VΛ with highest weight Λ. (As usual, ρ is the Weyl vector and the products run over positive roots α.) The specialization is not completely trivial, because both the numerator and denominator of the Weyl character formula vanish to high order at the identity element, so it is necessary to take a limit of the trace of an element tending to the identity.
Hans Freudenthal's formula is a recursive formula for the weight multiplicities that is equivalent to the Weyl character formula, but is sometimes easier to use for calculations as there can be far fewer terms to sum. It states
- Λ is a highest weight,
- λ is some other weight,
- mΛ(λ) is the multiplicity of the weight λ in the irreducible representation VΛ
- ρ is the Weyl vector
- The first sum is over all positive roots α.
Weyl–Kac character formula
The Weyl character formula also holds for integrable highest weight representations of Kac–Moody algebras, when it is known as the Weyl–Kac character formula. Similarly there is a denominator identity for Kac–Moody algebras, which in the case of the affine Lie algebras is equivalent to the Macdonald identities. In the simplest case of the affine Lie algebra of type A1 this is the Jacobi triple product identity
The character formula can also be extended to integrable highest weight representations of generalized Kac–Moody algebras, when the character is given by
Here S is a correction term given in terms of the imaginary simple roots by
where the sum runs over all finite subsets I of the imaginary simple roots which are pairwise orthogonal and orthogonal to the highest weight λ, and |I| is the cardinality of I and ΣI is the sum of the elements of I.
The denominator formula for the monster Lie algebra is the product formula
for the elliptic modular function j.
Peterson gave a recursion formula for the multiplicities mult(β) of the roots β of a symmetrizable (generalized) Kac–Moody algebra, which is equivalent to the Weyl–Kac denominator formula, but easier to use for calculations:
where the sum is over positive roots γ, δ, and
Harish-Chandra Character Formula
Harish-Chandra showed that Weyl's character formula admits a generalization to representations of a real, reductive group. Suppose is an irreducible, admissible representation of a real, reductive group G with infinitesimal character . Let be the Harish-Chandra character of ; it is given by integration against an analytic function on the regular set. If H is a Cartan subgroup of G and H' is the set of regular elements in H, then
- W is the complex Weyl group of with respect to
- is the stabilizer of in W
and the rest of the notation is as above.
The coefficients are still not well understood. Results on these coefficients may be found in papers of Herb, Adams, Schmid, and Schmid-Vilonen among others.
- Hall, Brian C. (2015), Lie groups, Lie algebras, and representations: An elementary introduction, Graduate Texts in Mathematics 222 (2nd ed.), Springer
- Infinite dimensional Lie algebras, V. G. Kac, ISBN 0-521-37215-1
- Duncan J. Melville (2001), "Weyl–Kac character formula", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Weyl, Hermann (1925), "Theorie der Darstellung kontinuierlicher halb-einfacher Gruppen durch lineare Transformationen. I", Mathematische Zeitschrift (Springer Berlin / Heidelberg) 23: 271–309, doi:10.1007/BF01506234, ISSN 0025-5874
- Weyl, Hermann (1926a), "Theorie der Darstellung kontinuierlicher halb-einfacher Gruppen durch lineare Transformationen. II", Mathematische Zeitschrift (Springer Berlin / Heidelberg) 24: 328–376, doi:10.1007/BF01216788, ISSN 0025-5874
- Weyl, Hermann (1926b), "Theorie der Darstellung kontinuierlicher halb-einfacher Gruppen durch lineare Transformationen. III", Mathematische Zeitschrift (Springer Berlin / Heidelberg) 24: 377–395, doi:10.1007/BF01216789, ISSN 0025-5874