Classification of Discrete Series by Minimal K type

Transcription

1 Classification of Discrete Series by Minimal K type Rajagopalan Parthasarathy Abstract. Following the proof by Hecht and Schmid of Blattner s conjecture for K multiplicities of representations belonging to the discrete series it turned out that some results which were earlier known with some hypothesis on the Harish Chandra parameter of the discrete series representation could be extended removing those hypotheses. For example this was so for the geometric realization problem. Occasionally a few other results followed by first proving them for Harish Chandra parameters which are sufficiently regular and then using Zuckerman translation functors, wall crossing methods etc.. Recently, Hongyu He raised the question (private communication) whether the characterization of a discrete series representation by its lowest K-type which was proved by this author and R. Hotta with some hypothesis on the Harish Chandra parameter of the discrete series representations can be extended to all discrete series representations excluding none, using a combination of these powerful techniques. In this article we will answer this question using Dirac operator methods and a result of Susana Salamanca-Riba. 1. Preliminaries and Statement of the main Theorem. Let G e be a connected linear real semisimple Lie group whose complexification is simply connected. We assume that G e admits discrete series (which implies that G e has a compact Cartan subgroup). In Sections 1,2 and 3 of this paper we write G instead of G e ; in Section 4 G will not be assumed to be connected and G e will denote the identity component. Let T be a compact Cartan subgroup of G. Let K be a maximal compact subgroup of G containing T. Denote by t, g, k 2010 M SC Primary 22E46, Secondary 22D30. Key words and phrases: Dirac operator inequality, Dirac cohomology, Harish- Chandra class group, Mackey s criterion, A q,λ. This research was supported by Raja Ramanna Fellowship from DAE. The author thanks the referee for suggestions to improve the article by addressing the case of general groups of Harish Chandra class. His comments on the initial proof of Theorem 1.1 in Section 3 have greatly helped in adding considerable clarity to the original submission. 1

2 2 RAJAGOPALAN PARTHASARATHY the corresponding Lie subalgebras and t C, g C, k C their complexifications. Let g C = k C + p C be the Cartan decomposition. Here p is the orthogonal complement of k in g. Fix a positive system P in the set of roots (g C, t C ). The set of roots (k C, t C ) is a subset of (g C, t C ). Intersecting P with (k C, t C ) we get a positive system P k of (k C, t C ). The complement of P k in P is denoted P n and is called the set of noncompact positive roots. The root spaces g α for α P n Pn span p C. Let λ t R = Hom ( 1t, R) be a P -dominant integral linear form. Denote by ρ half the sum of the roots in P, by ρ k half the sum of the roots in P k and by ρ n half the sum of the roots in P n. Then λ + ρ is P -dominant regular integral. ρ n is P k -dominant integral. Let Λ = λ + ρ. Denote by ϖ(λ) an irreducible unitary representation of G belonging to the discrete series representation with Harish Chandra parameter Λ. The restriction ϖ(λ) K of ϖ(λ) to K decomposes as an orthogonal sum of finite dimensional irreducible modules for K. In this restriction the finite-dimensional K representation τ λ+2ρn with P k - highest weight λ+2ρ n occurs and τ µ does not occur if µ is P k -dominant and µ = λ + 2ρ n α where α P n. Our main result is the following Theorem. Let π be an irreducible unitary representation of G. Suppose that (i) the finite-dimensional K-representation τ λ+2ρn with P k - highest weight λ + 2ρ n occurs in π K ; and (ii) τ µ does not occur in π K if µ is P k - dominant and µ = λ + 2ρ n α where α P n. Then π is unitarily equivalent to the discrete series representation ϖ(λ). It may be worthwhile to rephrase the above conclusion slightly differently (leaving no room to doubt whether a certain assumption is vacuously satisfied) companion to Theorm 1.1 Let π be an irreducible unitary representation of G. Suppose that the finite-dimensional K-representation τ λ+2ρn with P k - highest weight λ + 2ρ n occurs in π K ; then one of the following cases occurs: Case (i) There is no α P n such that λ + 2ρ n α is P k -dominant. In this case π ϖ(λ). Case (ii) There is an α P n such that λ + 2ρ n α is P k -dominant. But no P k -highest weight of an irreducible constituent of of π k is of this form. In this case π ϖ(λ). Case(iii) There is an α P n such that λ + 2ρ n α is P k -dominant and is the highest weight of an irreducible k-constituent of π k. In this case π ϖ(λ). After reviewing some preliminary material in the next section we will begin the proof. We remark at the outset that our proof arrives at

3 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 3 the conclusion of the theorem 1.1 (and its companion) under a hypothesis slightly weaker than 1.1(ii). More precisely, still under hypothesis 1.1(i) 1.1.2(i) π G ϖ(λ) if τ µ as in Theorem 1.1(ii) above does not occur in the k-span of π(g)(w λ+2ρn ) where W λ+2ρn is an irreducible k submodule of π K given by 1.1(i) above (ii) If π ϖ(λ) then there is an α P n such that λ + 2ρ n α is P k -dominant and is the highest weight of an irreducible k-constituent of π(g)(w λ+2ρn ). In section 4 we discuss to what extent Theorem 0.1 above can be generalized to more general reductive groups G of Harish Chandra class. This is done from the point of view of inducing representations from an open subgroup of G and using Mackey s irreducibility criterion. The above Theorem 1.1 will be used for G e, the identity component of G. So, G e will be assumed to be linear. Proposition 4.7 and 4.9 describe a convenient way to classify all the discrete series for G and the canonical K type in a discrete series representation of G. The analogue of Theorem 1.1 is Theorem In Section 5 we extend Theorem 1.1 to A q (λ). 2. The Spin Represention and the Dirac Operator. The restriction of the Killing form of g to p is positive definite. Note that p has even dimension because of the equal rank hypothesis. Let so(p) be the orthogonal Lie algebra of p. Let Cliff(p) be the Clifford algebra of p. As p has even dimension the complexified Clifford algebra Cliff(p C ) is isomorphic to a matrix algebra over C. In other words, we have an isomorphism Cliff(p C ) End(L) into the algebra of endomorphisms of a unique simple module L the spin representation of the complex Clifford algebra. Both p and so(p) are embedded in Cliff(p) as a vector subspace and as a Lie subalgebra respectively. The action of the Clifford algebra on the spinors L gives rise to the Clifford multiplication ɛ : p L L, ɛ(x s) = ɛ(x)s and the spin representation σ : so(p) End(L) and the two half spin representations σ ± : so(p) End(L ± ). It should be observed that ɛ(p L ± ) L. The weights of the adjoint representation ad : k so(p) are {α α P n Pn }. Denote σ ad, σ ± ad by χ, χ ± respectively. For a subset A P, A denotes α A α. If γ t R, γ is a weight of χ if and only if there exists A P n such that γ = ρ n A. Then the multiplicity m χ (γ) of γ in χ equals # {A A P n, γ = ρ n A }. We fix notation so that γ is a weight of χ + (resp.χ ) if and only if there exists A P n of even(resp.odd) cardinality such that γ = ρ n A. Evidently, τ ρn χ +. We use the notation χ, χ ± for the set of

4 4 RAJAGOPALAN PARTHASARATHY weights of χ, χ ±, respectively. χ is the disjoint union of χ + and χ. Theorem 1.1 was proved by R. Hotta and the author [1] under the hypothesis that (i) (λ, α) 0 for α P n. (ii) (λ, α) (ρ n A, α) for A P n and α P k. Here (, ) is the usual scalar product in t R using the Killing form of g. W. Schmid ([21], Lemma 9) had also proved likewise under the hypothesis that λ +ρ is sufficiently non-singular, i.e., for a suitably chosen positive constant c, (λ + ρ, α) > c, for all positive roots α P. Schmid also observed ([22], Theorem 3) that if the k-type of highest weight λ + 2ρ n occurs in π and no k-type with highest weight λ + 2ρ n A, A P n, A occurs in π, then π ϖ(λ). We give a short proof of Theorem 1.1 which uses the formal Dirac operator for an irreducible representation, a result of J.-S. Huang and P. Pandžić about unitary modules with Dirac cohomology ([2], Theorem 6.1) and a result of Susana Salamanca-Riba [11] on the classification of irreducible unitary representations with the same infinitesimal character as an irreducible finite dimensional representation. For an irreducible unitary representation π of G on a Hilbert space H let H π be the space of K-finite vectors in H. We use the same symbol π when referring to H π considered as a (g, K) module. The Dirac operator D π : H π L H π L is defined as i π(x i) ɛ(x i ) where {X 1, X 2,, X n } is an orthonormal basis of p. It is independent of the choice of the orthonormal basis. Let W be a finite dimensional subspace of H π stable under K. Let p C W be the subspace of H π spanned by {π(x)v X p, v W}. Remark We have an obvious K-module surjection p C W p C W. Remark Clearly D π (W L) (p C W) L Lemma. Let π be an irreducible unitary representation of G. Let W λ+2ρn be a K-stable subspace of H π such that the K-module W λ+2ρn is irreducible and is isomorphic to the finite-dimensional K representation τ λ+2ρn with P k - highest weight λ + 2ρ n. Then, (i) a k- stable subspace W λ+ρn of W λ+2ρn L which is an irreducible k- module with P k -highest weight λ + ρ n. (ii) In Case (i) and Case (ii) of if µ is the P k -highest weight of an irreducible k- submodule of D π (W λ+2ρn L), µ λ + ρ n. Remark For the proof of the Lemma above and Corollary below we need only the weaker hypothesis τ µ of (ii) in Theorem 1.1 does not occur in p C W λ+2ρn Corollary. Let W λ+ρn be as in Lemma 2.2. Then in Case (i) and Case (ii) of D π (W λ+ρn ) = 0.

5 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 5 The corollary is an immediate consequence of 2.2 (ii). Proof of Lemma 2.2. τ λ+2ρn k C τ λ+ρn τ ρn. Also τ ρn k C χ and χ is self-dual. Thus τ λ+ρn k C τ λ+2ρn χ. This proves part (i) of the Lemma. Let W λ+2ρn be as in Lemma 2.2. Any irreducible component of p C W λ+2ρn is of the form τ µ for some µ = λ + 2ρ n ± α where α P n. In Case (i) and Case (ii) of for α P n, λ + 2ρ n α cannot be the highest weight µ of an irreducible component of p C W λ+2ρn. Hence if τ µ p C W λ+2ρn then µ = λ + 2ρ n + α for some α P n. Hence if ξ is the P k -highest weight of an irreducible component τ ξ of (p C W λ+2ρn ) L then ξ = λ + 2ρ n + α + ρ n A for some α P n and A P n. So if A c is the complement of A in P n, ξ = λ + 2ρ n + α + ρ n A = λ + 2ρ n + α + ρ n P n + A c = λ + 2ρ n + α + ρ n 2ρ n + A c = λ + α + ρ n + A c. Hence, ξ + ρ k = λ + ρ + α + A c. In particular since λ + ρ is non-singular and P -dominant (ξ + ρ k, ξ + ρ k ) is strictly greater than (λ + ρ n + ρ k, λ + ρ n + ρ k ). Consequently such a ξ cannot equal λ + ρ n. Since D π (W λ+2ρn L) (p C W λ+2ρn L) it follows that if ξ is the P k -highest weight of an irreducible component τ ξ of D π (W λ+2ρn L) then such a ξ cannot equal λ + ρ n. This proves part(ii) of Lemma The Proof of Theorem 1.1. To apply Lemma 2.2 and Corollary 2.3 for the proof of Theorem 1.1 we need to understand irreducible unitary representations with Dirac cohomology. This is the subject of 6 in the paper [2] of J-S. Huang and Pandžić. We use a theorem of J-S. Huang and Pandžić ([2], Theorem 6.1). The short proof of their Theorem 6.1,[2] relies on their Theorem 2.3,[2] proving a conjecture of D. Vogan : if an irreducible finite dimensional k-module τ ξ with P k -highest weight ξ occurs in the kernel of the Dirac operator D π for an irreducible unitary representation π of G the infinitesimal character of π is given ξ + ρ k. The referee has pointed out that inasmuchas we appeal only to Theorem 6.1,[2] the current paper does not use Vogan s conjecture in an essential way. The referee has further pointed out that for our purposes the techniques of Vogan-Zuckerman ([18], [2], Theorem 5.3) and Kumaresan[20] suffice. Huang and Pandžić also point out below the statement of their Theorem 6.1,[2] that for its proof the same approach as Vogan-Zuckerman s generalization[18] of Kumaresan s result[20] would suffice, but not without some more labor (one or two more pages). Note that to proceed further we have to exhibit the infinitesimal character of the irreducible unitary representation π of Theorem 1.1. In the Dirac cohomology approach of Huang and Pandžić which suggests itself in view of Corollary 2.3, the infinitesimal character can be readily recognized from the above Vogan conjecture which has now been a theorem for long.

6 6 RAJAGOPALAN PARTHASARATHY Since ξ = λ + ρ n is in the Dirac kernel of our π (from Corollary 2.3) the infinitesimal character of π is given by the orbit of λ + ρ. Let α 1, α 2,, α i be all the positive noncompact roots on which λ vanishes. 3.1 Then using Salamanca-Riba [11], Theorem 6.1 of [2] concludes that there exists a θ-stable parabolic subalgebra q = l + u containing the Borel subalgebra b corresponding to the positive system P such that P n (l C, t C ) {α 1, α 2,, α i } and π A q (λ). These modules are variously referred to as Generalized Enright- Varadarajan modules, Derived Functor modules, Cohomologically induced modules and Modules with nonzero relative Lie algebra cohomology, ([10], [18], [19]). The main properties of these modules can be found in these publications and also ([2], Theorem 5.3). Among the most notable properties the ones relevant to us here are 3.1 (i) The restriction of A q (λ) to k contains the representation of k of the highest weight λ + 2ρ(u p C ). This weight is the same as λ + (u p C ) in earlier notation (third paragraph, 2). 3.1 (ii) If the representation of k of the highest weight δ occurs in A q (λ) then δ = λ + 2ρ(u p C ) + β (u p C ) n β β with n β non-negative integers. Since the representation of k with highest weight λ + 2ρ n occurs in π A q (λ) we note that λ + 2ρ n = λ + 2ρ(u p C ) + β (u p C ) n β β with n β non-negative integers. So, i.e., 2ρ n 2ρ(u p C ) = β (u p C ) n β β, P n (l) = β (u p C ) n β β Evaluating both sides of the last equation on the center of l, while the left side evaluates to zero, the right side would not evaluate to zero unless n β = 0, β (u p C ). Hence λ + 2ρ n = λ + 2ρ(u p C ) = 2ρ n = 2ρ(u p C ) = (u p C ) = P n. Thus, we conclude that π = A q (λ) = A b (λ) ϖ(λ). This completes the proof of Theorem 1.1. (q.e.d) For convenience of later reference (in 4.11 and ) we prove a property of discrete series representations, for which we do not know a reference. (In this section G is connected. In the next section G is disconnected; we will apply the following fact in the next section to the idenity component G e of G.) Let ϖ(λ) be a discrete series representation for G with indicated Harish Chandra parameter. We use other notation already introduced. So Λ is P dominant regular

7 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 7 integral and Λ = λ + ρ. As already noted the irreducible K module τ λ+2ρn with P k highest weight λ+2ρ n occurs in ϖ(λ) K with multiplicity 1. Proposition 3.2 If α P n and λ + 2ρ n α is P k dominant τ λ+2ρn α does not occur in ϖ(λ ) K for any discrete series representation ϖ(λ ) for which Λ Λ. The proof we outline here in fact applies without change to show the following more general assertion. Proposition For a non-empty subset A P n let < A >= α A α. Suppose that λ + 2ρ n < A > is P k dominant. Then τ λ+2ρn <A> does not occur in ϖ(λ ) K for any discrete series representation ϖ(λ ) for which Λ Λ. (Remark: Λ and Λ may be dominant with respect to different positive systems). Proof of 3.2 We will derive it as an application of Dirac inequality for the noncompact symmetric space as well as the dual compact symmetric space ([9] Proposition 2.6) and ([8] Lemma 8.1). The P k highest weight of the PRV component ([7]Lemma 2.26,[15]) and ([16] Theorem 3.5 & 3.2) of τ 1 τ 2 lies in the W K -orbit of the sum highest weight of τ 1 + lowest weight of τ 2. According to V.S. Varadarajan from his excellent reminiscences[15] of the historical evolution of [7] (especialy [15], pages xiv & xv) the PRV component should really have been called PRVV component to underline the important role of S.R.S. Varadhan. For any discrete series representation ϖ(λ 1 ) of G we have the following fact about when the P k highest weight of an irreducible k constituent in H ϖ(λ1 ) L is minimal. Fact Suppose τ µ1 k ϖ(λ 1 ) k, τ µ2 k L and τ ζ k τ µ1 τ µ2. Then ζ + ρ k Λ 1. Equality holds if and only if (a) there exists a positive system P (g C, t C ) containing P k such that (Λ 1, α) > 0, α P, (b) Λ 1 = µ 1 ρ n+ρ k, where ρ n = 1 2 α P α and n ρ = 1 2 α P α (c) the lowest weight of τ µ2 is ρ n, and (d) τ ζ is the PRV component of τ µ1 τ µ2. When equality holds, (e) the sum highest weight of τ µ1 + the lowest weight of τ µ2 is already P k dominant and equals ζ. (f) (c) above implies that µ 2 determines P in (a) :- If γ is the P k highest weight of τµ 2, then P = {α (g C, t C ) (γ + ρ k, α) > 0}. We apply this to prove Proposition 3.2. Suppose τ λ+2ρn α occurs in ϖ(λ ) K. Consider the PRV component in the tensor product τ λ+2ρn α τ ρ n. Denote by ξ the P k dominant linear form in the W K -orbit of λ + ρ n α. Then τ ξ is the PRV component in τ λ+2ρn α τ ρ n. Here τρ n is the representation contragredient to τ ρn. τ ρn has highest weight

8 8 RAJAGOPALAN PARTHASARATHY ρ n, τ ρn k L (the total spin module which is self-dual). So τρ n k L. Also τρ n has lowest weight ρ n. Thus τ ξ occurs in ϖ(λ ) L. Hence we have the Dirac inequality Λ ξ + ρ k. But by hypothesis, Λ Λ. So Λ ξ + ρ k. Denote the irreducible finite dimensional g C module with P highest weight λ by F λ. As λ + ρ n α is a weight of F λ L from ([8] Lemma 8.1, { which can be viewed as Dirac inequality for the compact dual symmetric space}) and ([4] Lemma 5.8), Λ ξ + ρ k. Hence (3.2.2) Λ = ξ + ρ k = Λ. In view of this equality, to draw conclusions using particularly (e) and (f) we use the comparisons below : τ µ1 τ λ+2ρn α τ µ2 τρ n PRVζ PRVξ ρ nis ρ nis lowest weight of τ µ2 lowest weight of τρ n ρ n ρ n ρ ρ P P ϖ(λ 1 ) ϖ(λ ) Λ 1 = µ 1 ρ n + ρ k Λ = (λ + 2ρ n α) ρ n + ρ k Λ 1 is P dominant Λ is P dominant Applying we conclude that (i) the P k dominant linear form in the W (k C, t C ) orbit of λ + ρ n α is λ + ρ n α itself. (ii) ρ n of the discussion in worked out in this context is ρ n, ρ = ρ k + ρ n then becomes ρ k + ρ n, giving rise to P = P, (iii) (Λ, α) > 0, α P which implies (Λ, α) > 0, α P as P = P. But Λ = λ + ρ α = Λ α from the comparison table above. So, (Λ, Λ) = (Λ + α, Λ + α) = (Λ, Λ ) + 2(Λ, α) + (α, α) which implies Λ > Λ which contradicts This completes the proof of Lemma 3.2. (q.e.d) 4. Disconnected G. In this section we discuss briefly to what extent we can generalize Theorem 1.1 to more general real reductive groups G of Harish Chandra class. Let G e be the identity component of G. G satisfies the following axioms. 4.1 (a) G has finitely many connected components. 4.1 (b) There exists a Lie group homomorphism Ψ : G I(g C ) such that for y G, Ad G (y) : g g is given by Ad I(g C )Ψ(y) g. Here I(g C ) is the connected complex adjoint group of inner

9 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 9 automorphisms of the Lie algebra g C which is a subgroup of GL(g C ). 4.1 (c) If g = c g 1 is the decomposition of g into center and semisimple part the analytic subgroup G 1 of G e with Lie algebra g 1 has finite center. These axioms were introduced by Harish Chandra in [5] and [6]. We will restrict to the case where G 1 is linear and has a compact Cartan subgroup and the center of the identity component G e of G has trivial split component. This implies that G e has compact center. Every X g C gives rise to a left G-invariant vector field l X on G and also a right G-invariant vector field r X. Every u U(g C ) (enveloping algebra) gives rise to a left(right) invariant differential operator on G. 4.2 Since G need not be connected an element z of the center of U(g) may not in general give rise to an Ad invariant differential operator on G. But the hypothesis 4.1 above implies that z indeed gives rise to an Ad invariant differential operator on G. Let I(g C 1 )(resp.i(g 1 )) be the group of inner automorphisms of g C 1 (resp.g 1 ). The following interesting fact was pointed out by B. Kostant during his lectures in TIFR, Bombay in However we will not use this in our arguments below, though we were inspired by Kostant s observation Remark. Let I(g 1 ) θ = {h I(g C 1 ) Ad I(g C 1 )(h)g 1 = g 1 }. Let a 1 be a maximal R-split abelian subalgebra of g 1 ( X a 1, ad g1 (X) : g 1 g 1 is diagonalizable having all eigenvalues real). Let F be the finite subgroup of elements of order utmost 2 in the compact torus in I(g C 1 ) having Lie subalgebra 1a 1. Then I(g 1 ) θ = I(g 1 )F. 4.3 Let C be the center of G. Since G need not be connected in general the center C of G e may be bigger than C G e. However in view of 4.1, C = C G e. Let B = the centralizer of G e in G. For a general group G of Harish Chandra class an element g G may centralize G e without being in the center of G. Note that Ψ(B) = Ψ(e G ). So B = Ψ 1 (Ψ(e G )). 4.4 Both BG e and CG e are normal subgroups of G and each is a union of connected components (G e -cosets) of G. We have G BG e CG e G e. We note B G e = C = C G e. Fix a maximal compact subgroup K 1 G 1 with Lie subalgebra k 1 g 1. Define K = {g G gk 1 g 1 = K 1 }. Put K e = K G e. It is easy to show that K(resp.K e ) is the unique maximal compact subgroup of G(resp.G e ) containing K 1. We note that K e is the identity component of K. Let k be the Lie algebra of K e. We recall a basic fact about I(g C ). If an automorphism fixes pointwise each element of a complex Cartan subalgebra of g C then it is given by the conjugation action of an element of the corresponding complex Cartan subgroup. K e has a compact Cartan subgroup of G e. So we deduce that if y K and y centralizes K e then Ψ(y) belongs to Ψ(G) Z where Z is the complexification of a

10 10 RAJAGOPALAN PARTHASARATHY compact Cartan subgroup of Ψ(K e ). This intersection is that compact Cartan subgroup itself. This is so for every compact Cartan subgroup of Ψ(K e ). The intersection of all compact Cartan subgroups of Ψ(K e ) is Ψ(C K e), where C K e is the center of K e. So Ψ(y) Ψ(C K e) Conclusion The centralizer of K e in K equals Ψ 1 (Ψ(e G ))C K e = BC K e. Note that K BK e K e. The intersection of K and any connected component of G is nonempty using the fact that maximal compact subgroups of G 1 are Ad(G 1 )- conjugate. Denote by θ : g g the Cartan involution corresponding to the Cartan decomposition g = k + p. Since we assumed that G 1 and K 1 have the same rank θ is given by inner conjugation Ad G (θ) : g g by an element θ K 1 (determined uniquely upto group multiplication by an element of C ). 4.5 If Γ, Γ are open subgroups of G, Γ Γ and η : Γ GL(V η ) is a representation of Γ we define as usual the induced representation IΓ Γ (V η). In order to avoid cumbersome notation we use the same symbol IΓ Γ (V η) both for the induced representation as well as the space on which it acts. { So we have IΓ Γ (V V η -valued functions φ : Γ V η satisfying η) = φ(gγ) = η(γ 1 )φ(g), γ Γ, Γ-equivariant property. Group action of g Γ on functions is by left translation l g. When there is no likelihood of confusion we suppress V η in the notation for induced representation. For g, h Γ, (IΓ Γ (g){φ})(h) = (l gφ)(h) = φ(g 1 h). { We have an evaluation at unit element map V η IΓ Γ ev : (V η), φ φ(e). If g Γ, φ IΓ Γ (V η) we have ev(l g φ) = η(g)(ev(φ)). We remark that if W is a nonzero Γ -submodule of IΓ Γ (V η) the evaluation map ev cannot be identically zero on W. If η 1 : Γ GL(V 1 ), η 2 : Γ GL(V 2 ) are two modules and κ : V 1 V 2 is a Γ-module map then it induces a map I(κ) : IΓ Γ (V 1) IΓ Γ (V 2) commuting with the Γ action. Moreover the following diagram is commutative. V 1 ev 1 I Γ Γ (V 1 ) κ V 2 ev 2 I(κ) I Γ Γ (V 2 )

11 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE All the constructions and assertions of 4.5 hold if G is replaced by K and Γ, Γ are open subgroups of K. If E is a finite dimensional Γ -module there is a natural isomorphism µ E : E IΓ Γ (V ) IΓ Γ (E V ), {µ E (x φ)}(k) = (k 1 x) φ(k), (k Γ ) If W is a Γ -submodule of IΓ Γ (V ), then ev(µ E (E W)) = E (ev(w)). If κ : E V V is a Γ-module map and if W IΓ Γ (V ) is a nonzero Γ -submodule then from the commutative diagram E V κ V we conclude that ev (E V ) ev (V ) I Γ Γ (E V ) I(κ ) I Γ Γ (V ) ev (V ) {I(κ) µ E (E W) } = κ (E ev (V ) (W)). Every irreducible finite dimensional (complex) representation τ K of K occurs as an irreducible K-constituent in a representation IK K e(τ K e) of K obtained by inducing from any irreducible K e -constituent τ K e K e (τ K ) K e Let ω G be a discrete series representation of G. In view of 4.2 there exists a homomorphism χ ωg : Z(g) C such that for all z Z(g) and for every K-finite matrix coefficient f : G C of ω G, zf = χ ωg (z)f. (see also [17], proof of Lemma 5.2 and 5.3) From this it can be deduced that (ω G ) G e decomposes into a finite sum of discrete series representations of G e all of them having the same infinitesimal character. Furthermore, if ω1, e ω2 e both occur in ω G G e, then for some g G, ω1 e G e (ω2) e g, where (ω2) e g (h) = ω2(ghg e 1 ) for h G e. Obviously if C acts on the representation space of ω G via scalar multiplication by ξ : C C then on each summand of ω G G e the action of C is given by ξ C. Every irreducible unitary representation π G of G occurs as a G- summand in a representation IG G e(π Ge) of G obtained by inducing from any irreducible G e -summand π G e G e π G G e. We note that π G belongs to the discrete series of G if and only if π G e belongs to the discrete series of G e. In that case each irreducible G-summand of IG G e(π Ge) belongs to discrete series Let ω G e be a discrete series representation of G e. Let ξ : C C be a character of C such that ξ C coincides with the action of ω G e C. Let ν be an irreducible finite dimensional representation of B such that the action of ν C is given by ξ. Then the tensor product ν ω G e is an irreducible representation of B G e. It is in fact a discrete series

12 12 RAJAGOPALAN PARTHASARATHY representation of B G e. Any irreducible unitary representation of B G e is of the form µ π G e where µ is an irreducible finite diensional representation of B and π G e is an irreducible unitary representation of G e. It belongs to discrete series of B G e if and only if π G e belongs to the discrete series of G e. The subgroup {(z, z 1 ) z C } of B G e is in the kernel of ν ω G e. Hence ν ω G e is in fact an irreducible representation of the quotient group BG e. Clearly ν ω G e is a discrete series representation of BG e which occurs as a BG e -summand in IG BGe(ω e G e), the representation of BGe obtained by inducing from the representation ω G e of G e. Conversely every irreducible BG e -summand ω BG e of IG BGe(ω e G e) arises in this way as ν ω Ge for a suitable choice of an irreducible finite dimensional representation ν of B. It can be now deduced that every discrete series reresentation of BG e is of the form ν ω G e for a suitable choice of a finite dimensional irreducible representation ν of B and a discrete series representation ω G e of G e. The induced representation IG G e(ω Ge) can be obtained in two stages: first by inducing ω G e from G e to BG e to get IG BGe(ω e Ge) and then inducing the representation IG BGe(ω e G e) from BGe to G. After the first stage instead of inducing the entire IG BGe(ω e G e) to G from BGe let us pick a BG e -irreducible summand ν ω G e as in and induce only ν ω G e from BG e to G Proposition. The induced representation IBG G e(ν ω Ge) is irreducible. It is a discrete series representation of G. Any discrete series representation of G arises as IBG G e(ν ω Ge) for a suitable discrete series representation ω G e of G e and an irreducible finite dimensional representation ν of B. Proof. We use a simple version of Mackey s irreducibility criterion for an induced representation. For an exposition of Mackey s criterion there are many sources, for example, ([14], 5). Both G e and BG e are open normal subgroups of G of finite index. According to this irreducibility criterion, IBG G e(ν ω Ge) is irreducibe if and only if g G \ BG e {= Latex $G \setminus BG^e$ } the representations ν ω G e and (ν ω G e) g are non-isomorphic. Here, (ν ω G e) g is the representation of BG e on the same Hilbert space as the one on which ν ω G e acts with operators defined by (ν ω G e) g (x) = (ν ω G e)(gxg 1 ), x BG e. For g G, we define a representation ω g G e of G e in the same way: for x G e, ω g G e(x) = ω G e(gxg 1 ). The group B, being the centralizer of G e in G, is itself a normal subgroup of G and we define ν g similarly. Then it is seen that (ν ω G e) g = ν g ω g Ge. We will in fact show that for g G\BG e, ω G e and ω g Ge are non-isomorphic. Observe that the isomorphism class of ω g G depends only on the e Ge -coset of g. Since K e and gk e g 1, both being maximal compact subgroups of G e, are conjugate by an element h G e, replacing g by gh if necessary

13 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 13 we can as well assume that gk e g 1 = K e. By the same argument, in fact we can also assume that (i) gk e g 1 = K e, Ad g C(g)(k) = k, Ad g C(g)(p) = p and (ii) Ad g C(g)b k = b k where b k is a fixed Borel subalgebra of k C. The Borel subgroup of K e,c corresponding to b k intersects K e in a compact Cartan subgroup T e of K e. We note that in addition to (i) and (ii) above we also have (iii) T e = gt e g 1. For later use we note that the above reductions imply Remark : If N K (T e ) is the normalizer of T e in K, then K = N K (T e )K e and G = N K (T e )G e. Let t be the Lie algebra of T e. Let P k (k C, t C ) be the positive system (in the set of compact roots) whose root spaces together with t C span b k. Let W k C = W (K e, T e ) = N K e(t e )/T e be the Weyl group of K e. Here N K e(t e ) is the normalizer of T e in K e. Suppose that ω G e = ϖ(λ) where ϖ(λ) is the discrete series representation of G e corresponding to P k -dominant Harish Chandra parameter Λ which is (g C, t C )-regular. Let P (g C, t C ) be the unique positive system of roots in (g C, t C ) defined by P = {α (g C, t C ) (Λ, α) > 0}. Write ρ = 1 2 α P α. The action of ϖ(λ) C is scalar multiplication by a unitary character with differential (Λ ρ) c. Thus (Λ ρ) c takes values in 2π 1Z on the lattice in c which is the kernel of the exponential map from c to the identity component of C. Furthermore ϖ(λ) G1 is the same as ϖ(λ t g1 ) in the usual Harish Chandra discrete series parametrization for G 1. Let b g be the Borel sualgebra of g C spanned by t C and the root spaces corresponding to roots in P. We write b = Ad g C(g)(b g ). In addition to (i), (ii) and (iii) above we also have (iv) b = Ad g C(g)(b g ) = Ad g C(g)(b g k C ) Ad g C(g)(b g p C ) =b k Ad g C(g)(b g p C ). Let P be the positive system of roots in (g C, t C ) corresponding to the Borel subalgebra b of g. Note that P P k. In the W (g C, t C )- orbit of Λ choose the unique linear form Λ dominant with respect to P. Then ϖ g (Λ) = ϖ(λ ). This implies that ϖ g (Λ) and ϖ(λ) are isomorphic if and only if b = b. Since by (4.1) Ad g C(g) is also the inner conjugation by Ψ(g) I(g C ) (the connected complex adjoint group of g C ) on g C the facts Ad g C(g)(b) = b = b and gt e g 1 = T e imply g Ψ 1 (Ψ(T e )) = T e Ψ 1 (Ψ(e G )) = BT e BG e. This contradicts that we started with g G \ BG e. This completes the proof of Proposition 4.7.

14 14 RAJAGOPALAN PARTHASARATHY Q.E.D 4.8 Fix a compact Cartan subgroup T e K e, let t k, t C k C denote the corresponding Lie subalgebras and let b k k C be a Borel subalgebra containing t C. Let b g C be a Borel subalgebra of g C containing b k. Choose positive systems P k (k C, t C ) and P (g C, t C ) containing P k corresponding to b k and b. Let ϖ(λ) be a discrete series representation of G e with Harish Chandra parameter Λ dominant α P with respect to P. Write Λ = λ + ρ where ρ = 1 α. We also 2 write ρ k = 1 2 α P k α, ρ n = 1 2 α P n α, where P n = P \ P k is the set of positive noncompact roots. Let τ λ+2ρn be the finite dimensional irreducible representation of k C with P k highest weight λ + 2ρ n. We use the same symbol τ λ+2ρn to denote the corresponding (group) representations of K e and K e,c. The representation τ λ+2ρn K e ϖ(λ) K e. The group C is the center of G e and C K e. ϖ(λ) C acts by the same unitary character as does τ λ+2ρn C. Let ν be an irreducible finite dimensional representation of B such that ν C and τ λ+2ρn C act by the same character. Then the tensor product ν τ λ+2ρn is an irreducible representation of B K e. The subgroup {(z, z 1 ) z C } of B K e is in the kernel of ν τ λ+2ρn. Hence ν τ λ+2ρn is in fact an irreducible representation of the quotient group BK e. Proposition 4.9 The induced representation I K BK e(ν τ λ+2ρ n ) is irreducible. Proof. The proof is similar to the proof of proposition 4.7. By Mackey s criterion IBK K e(ν τ λ+2ρ n ) is irreducibe if and only if g K \ BK e the representations ν τ λ+2ρn and (ν τ λ+2ρn ) g are nonisomorphic. Replacing if necessary g by gh where h K e we can assume that b g k = b k. Then gt e g 1 = T e. Define the Borel subalgebra b = b g of g C as in the proof of proposition 4.7. Let P k, P n, ρ k, ρ n, ρ denote for P what P k, P n, ρ k, ρ n, ρ represent for P. Since b g k = b k we have P k = P k. We note that τ g λ+2ρ n = τ g Λ+ρ 2ρ = τ k Λ +ρ 2ρ where Λ is k the unique element in the W (g C, t C ) orbit of Λ dominant with respect to P. In order to have τ g λ+2ρ n = τ λ+2ρn we must have Λ + ρ 2ρ k = Λ + ρ 2ρ k. This is possible only if P = P. As in the proof of proposition 4.7 this is possible only if g BK e. This contradicts that we started with g K \ BK e. This completes the proof of Proposition 4.9. Q.E.D 4.10 The main ingredient of the proof of propositions 4.7 and 4.9 has some more important consequences which we explain. Let J B,ν be the finite dimensional Hilbert space on which the representation ν of B acts. With notation as in the proof of 4.7 and 4.9, {IBG G e(ν ϖ(λ))} BG e decomposes as g (ν g ϖ g (Λ)), where the sum runs over coset representatives for G/BG e. The BG e -isomorphism class of ν g

15 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 15 ϖ g (Λ) depends only on the coset gbg e. Each BG e -summand ν g ϖ g (Λ) is a discrete series representation of BG e. IBG G e(ν ϖ(λ)) occurs in IBG G e(νg ϖ g (Λ)) (by Frobenius reciprocity). But the latter is also an irreducible G module, by 4.7, hence both are G-isomorphic. For h G\BG e, ν g ϖ g (Λ) and (ν g ϖ g (Λ)) h are nonisomorphic as BG e - modules as seen in the proof of 4.7. In particular {IBG G e(ν ϖ(λ))} BG e, as a BG e module is multiplicity free. In the same way, {IBK K e(ν τ (λ+2ρ )} n) BK e decomposes as g(ν g τ g (λ+2ρ ), where the sum runs over coset representatives for n) K/BKe. It is multiplicity free. IBK K e(ν τ (λ+2ρ ) is isomorphic to n) IK BK e(νg τ g (λ+2ρ ). It may also be noted that n) ϖg (Λ) = ϖ(λ g ) and τ g (λ+2ρ = n) τ if we write (λ g +2ρ g n) Λg, ρ g for Λ, ρ that we introduced in the proofs above. Finally if IBG G e(ν 1 ϖ 1 ) G IBG G e(ν 2 ϖ 2 ) then in particular IBG G e(ν 1 ϖ 1 ) BG e BG e IBG G e(ν 2 ϖ 2 ) BG e which from the above discussion implies ν 2 ϖ 2 BG e ν g 1 ϖ g 1 for some g G. So, ν 2 B ν g 1 and ϖ 2 G e ϖ g 1 for some g G Lemma If for some non-empty subset A P n, (λ + 2ρ n ) < A > is P k -dominant, then the BK e -module IBG G e(ν ϖ(λ)) BK does e not have a BK e -summand isomorphic to ν τ. (λ+2ρn) <A> Proof Applying proposition this follows from the discussion Q.E.D We continue with notation introduced above. C B and B has trivial adjoint action on g C, in particular on p C. Hence C acts by the same unitary character both on p C W (λ+2ρn) and on W (λ+2ρn). Note that p C W (λ+2ρn) is a module for K e for the tensor product action (Ad g C) p C τ. Let H (λ+2ρn) ϖ,k be the space of e Ke finite vectors in H ϖ, the Hilbert space on which the discrete series representaion ϖ = ϖ(λ) of G e acts. g C acts on H ϖ,k e, on J ν H ϖ,k e (action of g C on the second factor) and on IBG G e(ν ϖ). Choose an irreducible Ke stable subspace of H ϖ,k e isomorphic to τ. We use the same notation W (λ+2ρn) (λ+2ρ n) for such a subspace Theorem Let π G be an irreducible unitary representation of G on a Hilbert space H πg. Let H πg,k be the space of K finite vectors in H πg. Suppose that there exists an irreducible K-submodule W H πg,k which is K isomorphic to an irreducible K module IBK K e(ν τ (λ+2ρ ) n) given by proposition 4.9. We assume one of the following :- Either, (4.12 a) If T is an irreducible K submodule of p C W which does not occur in the discrete series G representation IBG G e(ν ϖ(λ)) K given by proposition 4.7 then it does not occur in π G K.

16 16 RAJAGOPALAN PARTHASARATHY Or, (4.12 b) The irreducible BK e -module ν τ µ does not occur in the BK e -module π G (p C )W if µ is P k -dominant and µ = (λ + 2ρ n ) α where α P n. Then π G G IBG G e(ν ϖ(λ)). Proof. The hypothesis 4.12 (a) is conceptually slightly more elegant than 4.12 (b). Note however that if (λ + 2ρ n ) + α, for some α P n, is the highest weight of an irreducible k-summand of p C W which (λ+2ρn) does not occur in ϖ(λ), 4.12 (a) forbids an occurrence, while 4.12 (b) does not care. For the proof of Theorem 4.12 under the hypothesis 4.12(b) we need only a weaker condition to be satisfied - the BK e - module π G (p C )W does not have a BK e -summand isomorphic to ν τ (λ+2ρn) α for those α P n, such that (λ+2ρ n ) α is P k -dominant and occurs as the highest weight of an irreducible K e -summand in p C W. (λ+2ρn) The proof of Theorem 4.12 under the hypothesis 4.12(a) runs almost parallel; towards the end we use Proposition 3.2. The irreducible unitary representation π G of G occurs as a G- summand in a representation IBG G e(π BGe) of G obtained by inducing from an irreducible BG e -representation π BG e on a Hilbert space H = J πbg e ν H π, where J G e ν is the space on which (B, ν ) acts and H is the Hilbert space on which πg e (Ge, π G e) acts. Let this inclusion be denoted by i : H πg,k IBG G e(π BGe). If v H πg,k, i(v) is a function on G taking values in the space of smooth vectors in H. πbg e For X g, v H πg,k, we note that i(xv) is given by differentiation of the function i(v) by the right invariant vector field r X corresponding to X. Furthermore, ev πbg e(i(xv)) H πbg e,bk e, where H π BG e,bk e is the space of BK e finite vectors in H. We have G = πbg e KBGe. The K module {IBG G e(π BG e)} K is the same as IK BK e((π BG e) BKe) (using the Cartan decomposition G = K exp(p)). By the choice of π BG e, H πg,k K IBK K e((π BG e) BK e). We have an evaluation map (from 4.5) ev π BG : e IBK K e((π BG e) BK e) H π which is BKe BG -equivariant. The evaluation e cannot be identically zero on a nonzero K submodule of the induced module. Hence ev πbg (W) 0, for W as in the statement of the e theorem. The BK e submodules of W are of the form ν g τ g λ+2ρ n, (g K). Under the adjoint action of K, g C is a K module, in particular a BK e module. We have g C = k C p C and p C is a K submodule. We have the following maps: By abuse of notation, we let the variable? in IBK K e(?) stand both for a representation and for the space on which it acts. IBK K e(h π BG e,bk e) IK BK e((π BG e) BKe), (K-inclusion )

17 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 17 1 p C i : p C W p C IBK K e((π BG e) BKe), (K-equivariant) r : r e : p C W IBK K e((π BG e) BK e), X v r Xv, (K-equivariant) p C H πbg e,bk e, H π BG e,bk e, X v r X v, (BKe -equivariant) I(r e ) : IBK K e(pc H πbg e,bk e) IK BK e(h π BG e,bke), (Kequivariant) ev HπBG : I K e,bke BK e(h π BG e,bk e) H π BG e,bke, (evaluation at e, BK e -equivariant) p C (ev πbg e {i(w)}) pc H πbg e,bk e, ( BKe -equivariant) ev p C H πbg e,bke : IK BK e(pc H πbg e,bk e) pc H πbg e,bk e, (BKe - equivariant) µ p C : p C IBK K e(h π BG e,bk e) IK BK e(pc H πbg e,bke), (K-equivalence, by 4.5) The following diagram is commutative. p C H πbg e,bk e r e H πbg e,bk e ev p C H πbg e,bke ev H πbg e,bke I K BK e(pc H πbg e,bk e) I(r e ) I K BK e(h π BG e,bk e) We also have the following commutative diagram in which both the vertical arrows are isomorphisms; the second one is in fact the identity map. I K BK e(pc H πbg e,bk e) I(r e ) I K BK e(h π BG e,bk e) µ p C 1 I p C I K BK e(h π BG e,bk e) r I K BK e(h π BG e,bk e) The map g C W I K BK e((π BG e) BK e)(= {IG BG e(π BG e)} K ), X v r X (i(v)) is K-equivariant It should be observed that since π G is a G submodule of IBG G e(π BG e), the image of the map g C W IBG G e(π BGe) is a K-submodule of H πg,k. The image of p C W is a K-submodule of the image of g C W. It is the same as (I(r e ) µ p C)(p C W) K H πg,k

18 18 RAJAGOPALAN PARTHASARATHY From our observation below the second commutative square in (4.5) we note that a. r e {p C (ev πbg e {i(w)})} = (ev π BG e I(re )){µ p C(p C W)}. b. From the assumption that I K BK e(ν τ λ+2ρ n ) occurs in (π G ) K we conclude that for some g K, the irreducible BK e module ν g τ λ g +2ρ g n occurs in ev π BG e {i(w)} H π BG e,bk e = J ν H πg e,k e. In particular ν = ν g and τ g λ+2ρ n occurs in π G e K e. As in the discussions of 4.7, 4.8 and 4.9, we will assume (by replacing g in its coset gbk e, if necessary) that Ad G eg fixes the compact Cartan subgroup T e of K e and the Borel subalgebra b k k C containing t Recall from the beginning of the proof of 4.12 that π BG e = ν π G e = ν π G e. Now suppose that π G e G e ϖ g (Λ), where g is as in (b). Applying the companion theorem to π G e and ϖ g (Λ) we conclude that 1. For some noncompact root α P n, (λ g + 2ρ g n) α g is P k - dominant. (This is the same as saying that (λ + 2ρ n ) α is P k -dominant since Ad(g)(P k ) = P k ). 2. If (τ (λ g +2ρ g n) α g, W (λ g +2ρ g n) α g ) is the k-representation with highest weight (λ g +2ρ g n) α g, then the BK e -module J ν g W (λ g +2ρ g n) α g occurs in π BG e(p C )(J ν g W λ g +2ρ g n ) BK e re {p C (ev πbg e {i(w)})} BK e J ν g H πg e,k e Using we now conclude that (ev πbg e I(re )){µ p C(p C W)} has a BK e type ν g τ (λ g +2ρ g n) α g This requires that there exists a K-submodule T of (I(r e )){µ p C(p C W)} such that ν g τ (λ g +2ρ g n) α g occurs in T BK e. Then h K, ν h τ (λ h +2ρ h n) α h, is a BK e -summand of T BK e (b) If we are with hypothesis 4.12(b) this is straight away forbidden (a) Consider the above K e -modules τ (λ. None of these h +2ρ h n) α h K e -modules occur in the G e discrete series representation ϖ(λ y ) for any y G by Lemma 3.2. Consequently, from 4.10 none of the BK e - modules ν h τ (λ occur in {I G h +2ρ h n ) αh BG e(ν ϖ(λ))} BKe, the G-discrete series representation IBG G e(ν ϖ(λ)) regarded as a BKe module. Hence the K module T does not occur in IBG G e(ν ϖ(λ)) K. By hypothesis 4.12 (a) in theorem 4.12, therefore T does not occur in (π G ) K. But this contradicts and

19 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE 19 So, π G e G e ϖ g (Λ), π BG e BG e ν g ϖ g (Λ) are both discrete series representations. The G-representation π G also belongs to the discrete series of G since π G occurs as a G-summand in a representation IBG G e(νg ϖ g (Λ)). The latter is irreducible (Proposition 4.7) and furthermore, from 4.10, IBG G e(νh ϖ h (Λ)) G IBG G e(ν ϖ(λ)), h G. Conclusion: π G G IBG G e{ν ϖ((λ))}. This completes the proof of the Theorem q.e.d 5. A q (λ) For the rest of the article we assume G = G e and is linear and of equal rank with finite center. In 1, 2, 3 we focussed on discrete series representations. We end by discussing briefly how some of the results of 1, 2, 3 can be generalized to the modules A q (λ) ([18], [2] Theorem 5.3) Let q = l + u be a parabolic subalgebra of g C containing b. Let λ be P -dominant integral and vanish on the roots of (l p C ). Let A = α (u) n α α where n α are nonnegative integers. Suppose that λ + 2ρ(u p C ) + A is P k -dominant. Let (τ λ+2ρ(u p C )+A, W λ+2ρ(u p C )+A ) be the k-module with highest weight λ + 2ρ(u p C ) + A. 5.0 Let ξ be the P k highest weight of an irreducible k -summand of W λ+2ρ(u p C )+A L. Then (ξ + ρ k, ξ + ρ k ) (λ + ρ n + ρ k, λ + ρ n + ρ k ). The inequality is strict if A 0. This can be proved in the same way as ([9],(4.8)) whose proof ends with the last line of the paragraph containing ([9], eqn. 4.17). In particular when A = α the inequality is strict. 5.1 For a positive system ˇP P k we denote the PRV component of τ λ+2ρ(u p C ) τ ˇρ n by τ [λ+2ρ(u p C ) ˇρ n ]. The notation [λ + 2ρ(u pc ) ˇρ n ] stands for the P k -dominant weight in the W (k C, t C )-orbit of λ + 2ρ(u p C ) ˇρ n. 5.2 Consider the positive systems P (notation from [9], see third line below 4.11,loc.cit) in (g C, k C ) which contain both P k and (u p C ). The inequality (ξ + ρ k, ξ + ρ k ) (λ + ρ n + ρ k, λ + ρ n + ρ k ) in (5.0) becomes equality precisely when A = 0 and τ ξ is the PRV component τ [λ+2ρ(u p C ) ρ n] in τ λ+2ρ(u p C ) τ ρ. We have ( n ) [λ + 2ρ(u p C ) ρ n] + ρ k, [λ + 2ρ(u p C ) ρ n] + ρ k = (λ + ρ n + ρ k, λ + ρ n + ρ k ). Based on these observations we derive the following analogues of Lemma 2.2, Corollary 2.3 and Theorem 1.1 with almost identical proofs. Lemma 5.3 Let π be an irreducible unitary representation of G. Let W λ+2ρ(u p C ) be a K-stable subspace of H π such that the K-module W λ+2ρ(u p C ) is irreducible and is isomorphic to the finite-dimensional K representation τ λ+2ρ(u p C ) with P k - highest weight λ + 2ρ(u p C ).

20 20 RAJAGOPALAN PARTHASARATHY Suppose that τ µ does not occur in π K if µ is P k -dominant and µ = λ + 2ρ(u p C ) α where α (q p C ). Then, i) for each positive system P containing P k (u) there exists a k-stable subspace W [λ+2ρ(u p C ) ρ n ] of W λ+2ρ(u p C ) L which is the PRV component of τ λ+2ρ(u p C ) τ ρ n. [λ + 2ρ(u p C ) ρ n] + ρ k 2 = (λ + ρ n + ρ k, λ + ρ n + ρ k ). ii) If µ is the P k -highest weight of an irreducible k- submodule of D π (W λ+2ρ(u p C ) L), (µ + ρ k, µ + ρ k ) > (λ + ρ n + ρ k, λ + ρ n + ρ k ). µ [λ + 2ρ(u p C ) ρ n] 5.4. Corollary. For each positive system P containing P k (u) let W [λ+2ρ(u p C ) ρ n ] be as in Lemma 5.3. Then D π (W [λ+2ρ(u p C ) ρ n ] ) = Theorem. Let π be an irreducible unitary representation of G. Suppose that (i) the finite-dimensional K-representation τ λ+2ρ(u p C ) with P k- highest weight λ + 2ρ(u p C ) occurs in π K ; and (ii) τ µ does not occur in π K if µ is P k -dominant and µ = λ + 2ρ(u p C ) α where α (q p C ). Then π is unitarily equivalent to A q (λ). For proving Theorem 5.5 using 5.4 we proceed by starting exactly as in 2, 3. First the infinitesimal character of π is given by the orbit of λ + ρ. We arrive at an interim step as in (3.1) where initially we are able to conclude that there exists a parabolic subalgebra q = l + u containing b k, a Borel subalgebra b q corresponging to a positive system P P k, P -dominant integral linear form λ vanishing on (l p C ) such that π A q (λ ). (See Remark 5.11 at the end). In particular the infinitesimal character of A q (λ ) is given by the orbit of λ + ρ. Hence, λ + ρ and λ + ρ both lie in the same W (g C, t C )- orbit. Since τ λ+2ρ(u p C ) occurs in π k = A q (λ ) k, from 3.1 (ii) λ+2ρ(u p C ) = λ +2ρ(u p C ) + β (u p C ) n ββ. On tensoring with L both τ λ+2ρ(u p C ) and τ λ +2ρ(u p C ) have a summand τ ξ, τ ξ respectively, such that (ξ +ρ k, ξ +ρ k ) = (λ+ρ n +ρ k, λ+ρ n +ρ k ) = (λ +ρ n +ρ k, λ + ρ n + ρ k ) = (ξ + ρ k, ξ + ρ k ). This is possible only if β (u p C ) n β β is zero. So λ+2ρ(u p C ) = λ +2ρ(u p C ). Write τ = τ λ+2ρ(u p C ) = τ λ +2ρ(u p C ) 5.6 The positive systems ˇP P k of (5.1) for which we have equality in (5.2) are

21 CLASSIFICATION OF DISCRETE SERIES BY MINIMAL K TYPE on the one hand those which contain P k (u p C ) if we think of τ as τ λ+2ρ(u p C ), (the ˇP here correspond to Borel subalgebras of q containing b k ) on the other hand those which contain P k (u p C ) if we think of τ as τ λ +2ρ(u p C ). (the ˇP here correspond to Borel subalgebras of q containing b k ) In particular q q contains a Borel subalgebra of g C, hence q q is a parabolic subalgebra of g C. We write ˆq for q q. 5.7 The stabilizer q k k of the line Cv W τ, where v is a nonzero P k -highest weight vector in W τ is a parabolic subalgebra q k = l k + ũ k of k C containing b k. Define a positive system P k (k C, t C ) by P k = (ũ k ) {P k ( l k )}. Define Pn (p C ) by Pn = (u p C ) {P n (l)}. Define P n (p C ) by P n = (u p C ) {P n (l )}. Let ρ n = 1 2 α Pn α and let ρ n = 1 2 α P n α. Both are extreme weights of the spin module L. 5.8 Lemma If α P k and ρ n (α) < 0, then ( λ + 2 (u p C ) ) (α) = 0. If α P k and ρ n (α) < 0, then ( λ + 2 (u p C ) ) (α) = Corollary ρ n is P k -dominant. ρ n is P k -dominant. The idea of the proof of 5.8 is similar to that of ([9], see for example [20], lemma 2.2) 5.10 The observations 5.6, 5.6.1, can also be repeated substituting P k for P k. The set of noncompact roots contained in the intersection of all the positive systems ˇP P k appearing in 5.6 and the positive systems ˇP P k appearing in 5.10 is on the one hand (u p C ) if we think of τ as τ λ+2ρ(u p C ), whereas on the other hand it equals (u p C ), if we think of τ as τ λ +2ρ(u p C ). As a consequence, (u p C ) = (u p C ), (u p C ) = (u p C ) and (q p C ) = (q p C ). Also, 2ρ(u p C ) = 2ρ(u p C ). So λ+2ρ(u p C ) = λ + 2ρ(u p C ) = λ = λ. Define q = q k + q p, where q p = (q p C ) and q k = {X k C [ X, q p ] qp }. Then q is a parabolic subalgebra of g C containing q q. The θ-stable Levi part l of q may have some ideals contained in k C. But, A q (λ) = A q (λ) = A q (λ) = A q (λ ) = π. Remark 5.11 As a matter of fact, in the interim step above, [2],Theorem 6.1 uses [11] to conclude something stronger :- π A q (λ ), where q b and λ = λ; if we had followed the same, some simplifications would have resulted. But we preferred the slightly longer argument here in the hope that perhaps it might simplify the proof of some earlier known results.