Discrete series representation

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In mathematics, a discrete series representation is an irreducible unitary representation of a topological group G that is a subrepresentation of the left (or equivalently right) regular representation of G on L²(G). In the Plancherel measure, such representations have positive measure. The name comes from the fact that they are exactly the representations that occur discretely in the decomposition of the regular representation.

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[edit] Properties

If G is unimodular, an irreducible unitary representation ρ of G is in the discrete series if and only if one matrix coefficient

\langle \rho(g)\cdot v, w \rangle \,

with v, w non-zero vectors is square-integrable on G, with respect to Haar measure.

[edit] Semisimple groups

A basic result of Harish-Chandra from 1966 classifies the discrete series representations of connected semisimple groups G. In particular, such a group has discrete series representations if and only if it has the same rank as a maximal compact subgroup K. In other words, a maximal torus T in K must be a Cartan subgroup in G. (This result required that the center of G be finite, ruling out groups such as the simply connected cover of SL2(R).) It applies in particular to special linear groups; of these only SL2(R) has a discrete series (for this, see the representation theory of SL2(R)).

Harish-Chandra's classification of the discrete series representations of a semisimple connected Lie group is given as follows. If L is the weight lattice of the maximal torus T, a sublattice of it where t is the Lie algebra of T, then there is a discrete series representation for every vector v of

L + ρ,

where ρ is the Weyl vector of G, that is not orthogonal to any root of G. Every discrete series representation occurs in this way. Two such vectors v correspond to the same discrete series representation if and only if they are conjugate under theWeyl group WK of the maximal compact subgroup K. If we fix a fundamental chamber for the Weyl group of K, then the discrete series representation are in 1:1 correspondence with the vectors of L + ρ in this Weyl chamber that are not orthogonal to any root of G. The infinitesimal character of the highest weight representation is given by v (mod the Weyl group WG of G) under the Harish-Chandra correspondence identifying infinitesimal characters of G with points of

t⊗C/WG.

So for each discrete series representation, there are exactly

|WG|/|WK|

discrete series representations with the same infinitesimal character.

Harish-Chandra went on to prove an analogue for these representations of the Weyl character formula. In the case where G is not compact, the representations have infinite dimension, and the notion of character is therefore more subtle to define since it is a Schwartz distribution (represented by a locally integrable function), with singularities.

The character is given on the maximal torus T by

(-1)^{(\dim(G)-\dim(K))/2} {\sum_{w\in W_K}\det(w)e^{w(v)}\over \prod_{(v,\alpha)>0}(e^{\alpha/2}-e^{-\alpha/2})}

When G is compact this reduces to the Weyl character formula, with v=λ+ρ for λ the highest weight of the irreducible representation. (where the product is over roots α having positive inner product with the vector v).

[edit] Limit of discrete series representations

Points v in the coset L + ρ orthogonal to roots of G do not correspond to discrete series representations, but those not orthogonal to roots of K are related to certain irreducible representations called limit of discrete series representations. There is such a representation for every pair (v,C) where v is a vector of L + ρ orthogonal to some root of G but not orthogonal to any root of K corresponding to a wall of C, and C is a Weyl chamber of G containing v. (In the case of discrete series representations there is only one Weyl chamber containing v so it is not necessary to include it explicitly.) Two pairs (v,C) give the same limit of discrete series representation if and only if they are conjugate under the Weyl group of K. Just as for discrete series representations v gives the infinitesimal character. There are at most |WG|/|WK| limit of discrete series representations with any given infinitesimal character.

Limit of discrete series representations are tempered representations, which means roughly that they only just fail to be discrete series representations

[edit] Further work

Much subsequent work in the area of semisimple groups has revisited Harish-Chandra's foundational results. New methods have been found, in particular algebraic, with the use of homological algebra, and geometric.

An application of the index theorem, to construct all the discrete series representations in spaces of harmonic spinors has been given (Michael Atiyah and Wilfried Schmid, A geometric construction of the discrete series for semisimple Lie groups, 1977). Before the Atiyah-Schmid work, Robert Langlands had conjectured, and several authors proved, a geometric analogue of the Borel-Bott-Weil theorem, for the discrete series, using L2 cohomology instead of coherent sheaf cohomology. The difference in attacks was that the Atiyah-Schmid did not assume Harish-Chandra's main results, while the earlier work of Narasimhan, Okamoto, Parasarathy and Schmid did.

Discrete series representations can also be constructed by cohomological parabolic induction using Zuckerman functors.

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