In mathematics, the Harish-Chandra isomorphism, introduced by Harish-Chandra (1951), is an isomorphism of commutative rings constructed in the theory of Lie algebras. The isomorphism maps the center of the universal enveloping algebra of a reductive Lie algebra to the elements of the symmetric algebra of a Cartan subalgebra that are invariant under the Weyl group .

Introduction and setting

Let be a semisimple Lie algebra, its Cartan subalgebra and be two elements of the weight space (where is the dual of ) and assume that a set of positive roots have been fixed. Let and be highest weight modules with highest weights and respectively.

Central characters

The -modules and are representations of the universal enveloping algebra and its center acts on the modules by scalar multiplication (this follows from the fact that the modules are generated by a highest weight vector). So, for and ,

and similarly for , where the functions are homomorphisms from to scalars called central characters.

Statement of Harish-Chandra theorem

For any , the characters if and only if and are on the same orbit of the Weyl group of , where is the half-sum of the positive roots, sometimes known as the Weyl vector.[1]

Another closely related formulation is that the Harish-Chandra homomorphism from the center of the universal enveloping algebra to (the elements of the symmetric algebra of the Cartan subalgebra fixed by the Weyl group) is an isomorphism.

Explicit isomorphism

More explicitly, the isomorphism can be constructed as the composition of two maps, one from to and another from to itself.

The first is a projection . For a choice of positive roots , defining

as the corresponding positive nilpotent subalgebra and negative nilpotent subalgebra respectively, due to the Poincaré–Birkhoff–Witt theorem there is a decomposition

If is central, then in fact

The restriction of the projection to the centre is , and is a homomorphism of algebras. This is related to the central characters by

The second map is the twist map . On viewed as a subspace of it is defined with the Weyl vector.

Then is the isomorphism. The reason this twist is introduced is that is not actually Weyl-invariant, but it can be proven that the twisted character is.

Applications

The theorem has been used to obtain a simple Lie algebraic proof of Weyl's character formula for finite-dimensional irreducible representations.[2] The proof has been further simplified by Victor Kac, so that only the quadratic Casimir operator is required; there is a corresponding streamlined treatment proof of the character formula in the second edition of Humphreys (1978, pp. 143–144).

Further, it is a necessary condition for the existence of a non-zero homomorphism of some highest weight modules (a homomorphism of such modules preserves central character). A simple consequence is that for Verma modules or generalized Verma modules with highest weight , there exist only finitely many weights for which a non-zero homomorphism exists.

Fundamental invariants

For a simple Lie algebra, let be its rank, that is, the dimension of any Cartan subalgebra of . H. S. M. Coxeter observed that is isomorphic to a polynomial algebra in variables (see Chevalley–Shephard–Todd theorem for a more general statement). Therefore, the center of the universal enveloping algebra of a simple Lie algebra is isomorphic to a polynomial algebra. The degrees of the generators of the algebra are the degrees of the fundamental invariants given in the following table.

Lie algebraCoxeter number hDual Coxeter numberDegrees of fundamental invariants
R001
Ann + 1n + 12, 3, 4, ..., n + 1
Bn2n2n  12, 4, 6, ..., 2n
Cn2nn + 12, 4, 6, ..., 2n
Dn2n  22n  2n; 2, 4, 6, ..., 2n  2
E612122, 5, 6, 8, 9, 12
E718182, 6, 8, 10, 12, 14, 18
E830302, 8, 12, 14, 18, 20, 24, 30
F41292, 6, 8, 12
G2642, 6

The number of the fundamental invariants of a Lie group is equal to its rank. Fundamental invariants are also related to the cohomology ring of a Lie group. In particular, if the fundamental invariants have degrees , then the generators of the cohomology ring have degrees . Due to this, the degrees of the fundamental invariants can be calculated from the Betti numbers of the Lie group and vice versa. In another direction, fundamental invariants are related to cohomology of the classifying space. The cohomology ring is isomorphic to a polynomial algebra on generators with degrees .[3]

Examples

  • If is the Lie algebra , then the center of the universal enveloping algebra is generated by the Casimir invariant of degree 2, and the Weyl group acts on the Cartan subalgebra, which is isomorphic to , by negation, so the invariant of the Weyl group is the square of the generator of the Cartan subalgebra, which is also of degree 2.
  • For , the Harish-Chandra isomorphism says is isomorphic to a polynomial algebra of Weyl-invariant polynomials in two variables (since the Cartan subalgebra is two-dimensional). For , the Weyl group is which acts on the CSA in the standard representation. Since the Weyl group acts by reflections, they are isometries and so the degree 2 polynomial is Weyl-invariant. The contours of the degree 3 Weyl-invariant polynomial (for a particular choice of standard representation where one of the reflections is across the x-axis) are shown below. These two polynomials generate the polynomial algebra, and are the fundamental invariants for .
  • For all the Lie algebras in the classification, there is a fundamental invariant of degree 2, the quadratic Casimir. In the isomorphism, these correspond to a degree 2 polynomial on the CSA. Since the Weyl group acts by reflections on the CSA, they are isometries, so the degree 2 invariant polynomial is where is the dimension of the CSA , also known as the rank of the Lie algebra.
  • For , the Cartan subalgebra is one-dimensional, and the Harish-Chandra isomorphism says is isomorphic to the algebra of Weyl-invariant polynomials in a single variable . The Weyl group is acting as reflection, with non-trivial element acting on polynomials by . The subalgebra of Weyl-invariant polynomials in the full polynomial algebra is therefore only the even polynomials, generated by .
Invariant cubic
Weyl-invariant cubic for A2, corresponding to the degree 3 fundamental invariant
  • For , the Weyl group is , acting on two coordinates , and is generated (non-minimally) by four reflections, which act on coordinates as . Any invariant quartic must be even in both and , and invariance under exchange of coordinates means any invariant quartic can be written Despite this being a two-dimensional vector space, this contributes only one new fundamental invariant as lies in the space. In this case, there is no unique choice of quartic invariant as any polynomial with (and not both zero) suffices.

Generalization to affine Lie algebras

The above result holds for reductive, and in particular semisimple Lie algebras. There is a generalization to affine Lie algebras shown by Feigin and Frenkel showing that an algebra known as the Feigin–Frenkel center is isomorphic to a W-algebra associated to the Langlands dual Lie algebra .[4][5]

The Feigin–Frenkel center of an affine Lie algebra is not exactly the center of the universal enveloping algebra . They are elements of the vacuum affine vertex algebra at critical level , where is the dual Coxeter number for which are annihilated by the positive loop algebra part of , that is,

where is the affine vertex algebra at the critical level. Elements of this center are also known as singular vectors or Segal–Sugawara vectors.

The isomorphism in this case is an isomorphism between the Feigin–Frenkel center and the W-algebra constructed associated to the Langlands dual Lie algebra by Drinfeld–Sokolov reduction:

There is also a description of as a polynomial algebra in a finite number of countably infinite families of generators, , where have degrees and is the (negative of) the natural derivative operator on the loop algebra.

See also

Notes

  1. Humphreys 1978, p. 130.
  2. Humphreys 1978, pp. 135–141.
  3. Borel, Armand (Apr 1954). "Sur la cohomologie des espaces homogenes des groupes de Lie compacts". American Journal of Mathematics. 76 (2): 273–342.
  4. Molev, Alexander (19 January 2021). "On Segal–Sugawara vectors and Casimir elements for classical Lie algebras". Letters in Mathematical Physics. 111 (8). arXiv:2008.05256. doi:10.1007/s11005-020-01344-3. S2CID 254795180.
  5. Feigin, Boris; Frenkel, Edward; Reshetikhin, Nikolai (3 Apr 1994). "Gaudin Model, Bethe Ansatz and Critical Level". Commun. Math. Phys. 166: 27–62. arXiv:hep-th/9402022. doi:10.1007/BF02099300. S2CID 17099900.

External resources

Notes on the Harish-Chandra isomorphism

References

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