Annals of Global Analysis and Geometry 18: , Kluwer Academic Publishers. Printed in the Netherlands.

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1 Annals of Global Analysis and Geometry 18: , Kluwer Academic Publishers. Printed in the Netherlands. 331 Flag Duality Dedicated to the memory of Alfred Gray ALAN T. HUCKLEBERRY 1 and JOSEPH A. WOLF 2 1 Institut für Mathematik, Ruhr-Universität Bochum, D Bochum, Germany. ahuck@cplx.ruhr-uni-bochum.de 2 Institut für Mathematik, Ruhr-Universität Bochum, D Bochum, Germany. jawolf@math.berkeley.edu (Received: 26 June 1999) Abstract. We introduce a duality on complex flag manifolds that extends the usual point-hyperplane duality of complex projective spaces. This has consequences for the structure of the linear cycle spaces of flag domains, especially when those flag domains are not measurable. Mathematics Subject Classifications (2000): 14M15, 15A03, 51M35. Key words: complex flag manifolds, duality, flag domains. 1. Introduction If G is a complex semi-simple group, Q a parabolic subgroup and Z = G/Q the associated flag manifold, then any real form G 0 has only finitely many orbits in Z (see [3] for this and other general results). In particular G 0 has open orbits on Z. Let D be one of those open orbits. A maximal compact subgroup K 0 of G 0 has an essentially unique complex orbit in D. Given the standard root theoretic set-up at the neutral point z Z, there is a canonical way of choosing K 0 so that K 0 (z) := Y 0 is a complex submanifold of Z. The moduli space of linear cycles is then defined to be M D := {g(y 0 ) : g G and g(y 0 ) D}. In the measurable case, i.e., that where D possesses a G 0 -invariant pseudo- Kählerian metric, a great deal is known about the complex geometry of M D and associated G 0 -representations on spaces of holomorphic functions on M D (see, e.g., [6]). The goal of the present note is to set up a duality which should facilitate a study of nonmeasurable flag domains D = G 0 (z). This can be used at least in a conceptual way to compare the cycle space M D to that of its measurable model D. Research partially supported by SFB 237 of the Deutsche Forschungsgemeinschaft. Research partially supported by the Alexander von Humboldt-Stiftung. Permanent address: Department of Mathematics, University of California, Berkeley, CA , U.S.A.

2 332 ALAN T. HUCKLEBERRY AND JOSEPH A. WOLF Operating from this point of view we show in the case of the unique open orbit D of G 0 = Sl n+1 (R) acting in the usual way on Z = P n (C) that the connected component of the moduli space M D which contains Y 0 can be identified with that of D. 2. Background and Notation Fix a complex semisimple Lie group G, a parabolic subgroup Q = Q where is a subset of the simple root system relative to a root order and a choice of Cartan subalgebra h g. Express q = q = q r + q n where the nilradical q n = n g α. Then the opposite parabolic is q = q r +q +n where the nilradical q +n = n g +α. Thedual parabolic is the complex conjugate (of g over g 0 ) q = q of the opposite. Similarly the parabolic subgroups Q G opposite to Q and Q G dual to Q are the parabolic subgroups with respective Lie algebras q and q.ourflag duality will be between Z = G/Q and Z = G/Q, (2.1) where Q is the parabolic subgroup of G dual to Q. Fix a real form G 0 G and suppose that Q = Q z where D = G 0 (z) is an open G 0 -orbit in Z. In other words, q + q = g. Sending each root to its negative, and then applying complex conjugation, it follows that q + q = g. Thus q = q z,the isotropy subalgebra of g at a point z Z such that D = G 0 (z ) is an open orbit in Z. D is the dual of D. Fix a Cartan involution θ of G 0 that preserves the Cartan subalgebra h = h used here. Then the corresponding maximal compact subgroup K 0 = G θ 0 has complex orbit Y = K 0 (z) in D, and similarly has complex orbit Y = K 0 (z ) in D. One also knows [5, lemma 2.2] that p := q q is a parabolic subalgebra of g. The reader should be careful here: our q is the r of [5], and our p is the q of [5]. Recall that q and q are G-conjugate if and only if the open G 0 -orbits on Z are measurable. P denotes the parabolic subgroup of G corresponding to p. Now we have three complex flag manifolds, Z = G/Q, Z = G/Q and X = G/P.NoteP = Q Q. We thus have a G-equivariant holomorphic double fibration X = G/P µ µ Z = G/Q Z = G/Q (2.2) The G-equivariance of course implies G 0 -equivariance. Let Q = Q z for the open orbit D = G 0 (z) Z. ThenQ = Q z for the open orbit D = G 0 (z ) Z, so we expect a correspondence of open G 0 -orbits on Z and Z. Before making this precise let us fix the notation clearly.

3 FLAG DUALITY 333 (2.3a) q z = q = q r + q n is the starting parabolic subalgebra of g, Q z = Q = Q r Q n is the corresponding parabolic subgroup of G, andz = G/Q is the associated flag manifold. (2.3b) D = G 0 (z) is an open G 0 -orbit on Z. (2.3c) q z = q = q r + q +n is the starting parabolic subalgebra of g, Q z = Q = Q r Q +n is the corresponding parabolic subgroup of G, andz = G/Q is the associated flag manifold. (2.3d) D = G 0 (z ) is the open G 0 -orbit on Z given by the point z Z whose complex isotropy algebra is q. (2.3e) p = q q is the parabolic subalgebra, P = Q Q parabolic subgroup, X = G/P flag manifold, and µ and µ are the projections of the equivariant holomorphic double fibration above. E = µ 1 (D) and E = (µ ) 1 (D ). (2.3f) x X is the base point, defined by p = p x,and D is the open (see Lemma 2.4 below) orbit G 0 (x). LEMMA 2.4. D = G 0 (x) is open in X, D = µ( D), and D = µ ( D). Proof. The first statement is [5, lemma 2.3]. The second is µ( D) = µ(g 0 (x)) = G 0 (µ(x)) = G 0 (z) = D. The third is µ ( D) = µ (G 0 (x)) = G 0 (µ (x)) = G 0 (z ) = D. LEMMA 2.5. Y := K 0 (z) M D, Y := K 0 (z ) M D, and Ỹ := K 0 (x) M D. Furthermore µ(ỹ) = Y and µ (Ỹ) = Y. Proof. The alignments that the various K 0 ( ) be complex submanifolds are conditions on the Cartan involution θ defining K 0 = G θ 0 : that it preserve h 0.Thatis the same for q, forq,andforp. The further statements follow as in Lemma 2.3 with K 0 in place of G 0. LEMMA 2.6. µ : Ỹ Y and µ : Ỹ Y are biholomorphic diffeomorphisms. Proof. This is [5, lemma 2.5] for Y, and the situation is symmetric for Y. 3. The Duality Automorphism As before g u is the compact real form k s 0 of g where g 0 = k 0 + s 0 under its Cartan involution θ, and of course we extend θ to g and restrict it to g u.the fundamental Cartan subalgebra h 0 = t 0 + a 0 of g 0, the sum of its intersections with k 0 and s 0, defines the Cartan subalgebra h u = t a 0 of g u. PROPOSITION 3.1. There is an involutive automorphism ν of g u that preserves h u, sends every root to its negative, and commutes with θ.

4 334 ALAN T. HUCKLEBERRY AND JOSEPH A. WOLF Proof. We may assume that the symmetric space G u /K 0 is irreducible, for otherwise everything decomposes as a product with that irreducibility. Now we run through some cases. Case 1: θ is inner on G u. By general symmetric space theory, θ = Ad(t) for some t K 0,soK 0 contains a maximal torus of G u.thent is contained in every maximal torus of K 0, in particular in the maximal torus T 0 = exp(t 0 ) of G u.note here that a u = 0. Extend the map ξ ξ from t 0 to an automorphism ν of order 2ofg u.thenν preserves h u = t 0 and sends every root to its negative, but also νθ = ν Ad(t) = Ad(t 1 )ν = θ 1 ν = θν. For the rest of the proof θ is an outer automorphism on g u. As in the inner case we extend the map 1 from h u to an involutive automorphism ν of g u,and ν preserves h u and sends every root to its negative. Now we must show that ν commutes with θ. Case 2: θ is outer on G u,andg u is not simple. Then g u = m u m u for some compact simple Lie algebra m u, h u = r u r u for a Cartan subalgebra r u of m u, and θ is the interchange (ξ 1,ξ 2 ) (ξ 2,ξ 1 ).Alsok 0 = g θ u is the diagonal, and ν has form (ξ 1,ξ 2 ) (φ(ξ 1 ), φ(ξ 2 )) for an involutive automorphism φ of m u that is 1 onr u and thus sends every root of (m u, r u ) to its negative. So νθ(ξ 1,ξ 2 ) = ν(ξ 2,ξ 1 ) = (φ(ξ 2 ), φ(ξ 1 )) = θ(φ(ξ 1 ), φ(ξ 2 )) = θν(ξ 1,ξ 2 ). Case 3: g 0 = sl(n; R). Theng = sl(n; C) and g u is the Lie algebra su(n), of the special unitary group U(n). We use the Cartan subalgebra h u ={diag {ia 1,...,ia n } a j real, a j = 0}. Let A = A 1 denote the antidiagonal, 1 s from the upper right-hand corner to the lower left and 0 s elsewhere, for example A = ( 0 1 ) 1 0 for n = 2. Define ν(ξ) = AξA 1 = AξA and note that ν sends a root ɛ i ɛ j to ɛ j ɛ i. Now compute θν(ξ) = θ(aξa) = t (AξA) = t A( t ξ) t A = A( t ξ)a = νθ(ξ), thatis, θν = νθ. Case 4: θ is outer on G u, G u is simple, and 1 is in the Weyl group of K 0. Here we have a Weyl group element w 0 W(K 0,T 0 ) such that w 0 (ξ) = ξ for all ξ t 0. Represent w 0 = Ad(s) t0 with, of course, s K 0.Thenθ(s) = s, we define ν = Ad(s)θ, and we have ν(ξ) = ξ for all ξ t 0. But we need LEMMA 3.2. ν(ξ) = ξ for all ξ h u. Proof. Let w 1 be the element of the Cartan group of g u the Weyl group extended by adjoining outer automorphisms that sends every ξ h to its negative. Then w 1 = ω h where ω is an outer automorphism of g u that normalizes h u and whose square is inner. Now both θ and ω Ad(s) are outer automorphisms of g u whose squares are inner. A result of de Siebenthal [1] says that their fixed point sets have the same rank. For θ that is of course rank K 0 = dim T 0. But the fixed point set of ω Ad(s) contains T 0, so it cannot be larger, and this forces Ad(s)(ξ) = ξ for

5 FLAG DUALITY 335 every ξ a. Nowν(ξ) = ξ for all ξ a, and this completes the argument that ν(ξ) = ξ for all ξ h u. Continuation of proof of Proposition 3.1. We have θ(s) = s, soθ commutes with w 0 and thus commutes with ν. It remains only to check that ν 2 = 1, in other words that Ad(s) 2 = 1, i.e. that s 2 is central in G u. Note that w 2 0 = 1 in the Weyl group W = W(K 0,T 0 ),sos 2 T 0 and w(s 2 ) = s 2 for all w W. We may assume G centerless; that only impinges on the center of G u. This assumption made, K 0 is centerless because G 0 /K 0 is irreducible and θ is outer. Express s 2 = exp(σ ) for some σ t 0.Hereσ is unique up to a root lattice translation. So we minimize σ by choosing it in a fundamental closed cell C.In this regard, recall that every positive root system + = + (k 0, t 0 ) defines such a cell from its simple root system {ψ j } and its maximal root µ by C ={ξ t 0 µ( iξ) 1 and each ψ j ( iξ) 0}. The important property is that each element of K 0 is conjugate to some exp(ζ ), ζ C, andζ is unique modulo the root lattice and the action of W = W(K 0,T 0 ). Thus w(σ) = σ for all w W. But the action of W on t 0 is a sum of nontrivial irreducible representations because k 0 is semisimple. It follows that σ =0.Now s 2 = 1, so s 2 is central in G u as required. Completion of proof of Proposition 3.1. By classification, if θ is outer and g u is simple then g 0 is one of (a) sl(n; R), Lie algebra of the real special linear group, which has maximal compact subgroup K 0 = SO(n), (b) sl(m; H), Lie algebra of the quaternion special linear group, which has maximal compact subgroup K 0 = Sp(m), (c) so(2u + 1, 2v + 1), Lie algebra of the indefinite orthogonal group that has maximal compact subgroup K 0 = SO(2u + 1) SO(1 + 2v), (d) e 6,c4, Lie algebra of the real group G 0 of type E 6 with maximal compact subgroup K 0 of type C 4, (e) e 6,f4, Lie algebra of the real group G 0 of type E 6 with maximal compact subgroup K 0 of type F 4. Here (a) is Case 3 above, and (b), (c), (d) and (e) are covered by Case 4 above. That completes the proof of Proposition 3.1. Now we use the map ν to start the duality theory. PROPOSITION 3.3. Let ν be as in Proposition 3.1. Thenα: G G by α(g) = ν(g), complex conjugation of G over G 0, has the properties

6 336 ALAN T. HUCKLEBERRY AND JOSEPH A. WOLF (1) α 2 is the identity, (2) α(q) = Q and α(q ) = Q, (3) the induced map φ: Z Z given by gq α(g)q and the other induced map φ 1 : Z Z given by gq α(g)q, are biholomorphic diffeomorphisms, (4) α(g 0 ) = G 0, and φ maps an arbitrary G 0 -orbit G 0 (z) Z onto a G 0 -orbit G 0 (φ(z)) Z, and (5) φ: D D defines a biholomorphic diffeomorphism M φ : M D M D of linear cycle spaces. Proof. The automorphism ν : G u G u of Proposition 3.1 extends to G and commutes with θ, and thus preserves G 0. Now evidently α(g 0 ) = G 0.In fact, if g G 0 then α(g) = ν(g) so α 2 (g) = ν 2 (g) = g. Asα 2 is a holomorphic automorphism of G, α 2 = 1. Compute dα(q) = dν(q) = q = q,soα(q) = Q, and also Q = α 2 (Q) = α(q ). It follows immediately that α induces the maps φ: Z Z and φ 1 : Z Z, as asserted, at the real analytic level. Since α(g 0 ) = G 0 it follows as well that φ and φ 1 map G 0 -orbits to G 0 -orbits. The holomorphic tangent space to Z at the base point z 0, the one that corresponds to q, isgivenbyq n = β n g β. The holomorphic tangent space to Z at the base point z0, the one that corresponds to q,isgivenby(q ) n = q n = dα(q n ). Thus φ: Z Z is holomorphic, and the same argument shows that φ 1 : Z Z is holomorphic. Let Y = K 0 (z 0 ) be the base point in M D. Similarly Y = K 0 (z0 ) is the base point in M D.Noteα(K 0 ) = K 0 so φ(y) = Y. Thus, if g G then φ(gy) = α(g)φ(y) = α(g)y.butφ(d) = D so gy D exactly when α(g)y D.In other words, gy M D if and only if φ(gy) M D. Thus φ defines a real analytic diffeomorphism M φ : M D M D of linear cycle spaces. It is holomorphic because φ is holomorphic. 4. The Case of Complex Projective Space Here we discuss the cycle space of the unique open orbit D of G 0 = Sl n+1 (R) in the complex projective space P n (C). Our goal is to prove M D = M D. Webegin with some notation. The action of G := Sl n+1 (C) on Z = P n (C) is defined by its standard representation on V := C n+1. The dual representation on V defines its action on Z = P(V ). A point in Z (resp. Z ) is a complex line L in V (resp. a hyperplane H ). Let e 0,...,e n be the standard basis for V and choose L 0 = C.(e 0 + ie 1 ) as a base point in Z = P(V ). It follows that the orbit D = G 0 (L 0 ) is open. In fact, its complement is the set of real points Z(R) = P(V (R)) which is also a G 0 -orbit. If

7 FLAG DUALITY 337 the maximal compact subgroup K 0 of G 0 is chosen to be K 0 := SO n+1 (R), then Y 0 := K 0 (L 0 ) is the unique complex K 0 -orbit in D. It is the quadric hypersurface Y 0 ={[z 0 :... : z n ]: zj 2 = 0}. The complex group KC has two orbits in Z, the above quadric and its complement. Let H 0 be the projective tangent hyperplane to Y 0 at the point L 0 regarded as a hyperplane in V.ThenH 0 = {(z 0,...,z n ) : z 0 + iz 1 = 0} = ((e 0 + ie 1,e 2,...,e n )). It follows that K 0 (H 0 ) = K0 C(H 0) =: Y0 P(V ) is the dual quadric of tangent hyperplanes to Y 0. It is likewise the unique complex K 0 -orbit in the unique open G 0 -orbit D in Z. Define Q (resp. Q )tobetheg 0 -isotropy at L 0 (resp. H 0 )andletp = Q Q. Then X = G/P is the flag manifold of lines L contained in hyperplanes H in V. We let (L H) denote a point in X. The projection π: X Z (resp. π : X Z )isdefinedby(l H) L (resp. (L H) H ). Note that the π and π -fibers are (n 1)-dimensional projective spaces THE G 0 -ORBIT STRUCTURE For the sake of completeness we outline the proof of the following elementary PROPOSITION 4.1. The group G 0 has 5 orbits in X. In ascending order of codimension they are (1) The unique open orbit D ={(L H) : L = L, H = H, L H }. (2) The top-dimensional boundary orbit := {(L H) : L = L, H = H, L H }. (3) Two intermediate orbits which are exchanged by flag duality: and M := {(L H) : L = L, H = H } M := {(L H) : L = L, H = H }. (4) The minimal orbit X(R) := {(L H) : L = L, H = H }.

8 338 ALAN T. HUCKLEBERRY AND JOSEPH A. WOLF Remark. We do not consider the case of Z := P 1 (C).ForZ = P 2 (C) the orbits, M and M coincide. Proof. For (1) note that given (L 1 H 1 ) and (L 2 H 2 ) in D, sinceg 0 acts transitively on the complement of the real points in Z, we may assume that L 1 = L 2 =: L. Let E := L L and E j := H j H j, j = 1, 2. It follows that V = E E 1 = E E 2 and, since all of these spaces are defined over R, there exists T G 0 such that T E = id E and T(E 1 ) = E 2.SinceH j = L E j, j = 1, 2, it follows that T maps (L 1 H 1 ) to (L 2 H 2 ). Consequently D is a G 0 -orbit. For (2) let E = L L as above and let Ẽ be a complementary subspace in V whichisdefinedoverr. To prove that is a G 0 -orbit it suffices to remark that there exits T G 0 which fixes E pointwise, stabilizes Ẽ and interchanges the hyperplanes H 1 Ẽ and H 2 Ẽ which are not defined over R in Ẽ. If L 1 = L 2 = L and L = L,thenweletE = L and argue as in (2) to show that M is a G 0 -orbit. The dual argument handles M. The transitivity of the G 0 -action on X(R) can be proved in a similar way. Remark. It is a simple matter to compute the dimensions of all orbits. For this we first note that, since the π-fibers are (n 1)-dimensional, it follows that dim C X = dim C D = 2n 1. Now π and π map onto the open G 0 -orbits in Z and Z respectively. For example, the π -fiber over L can be identified with the complement of the real points in the (n 2)-dimensional projective space of hyperplanes H which contain both L and L. Thus is 2-codimensional (over R) inx. Analogously, since π M maps M surjectively onto the real points in Z and its fiber over a point L is the set of hyperplanes H containing L with H = H,it follows that dim R M = dim R M = n + 2(n 1) = 3n 2. Finally, dim R X(R) = dim C X = 2n TRANSVERSALITY OF CYCLE INTERSECTION WITH INTERMEDIATE ORBITS Let Ỹ 0 be the base cycle in D, g G an arbitrary element of the complex group and Ỹ := g(ỹ 0 ).NowỸ 0 maps to Y 0 and Y0 respectively and, since Y 0 is the dual quadric of tangent hyperplanes to Y 0, it follows that a point (L H) Ỹ 0 consists of L Y 0 and the hyperplane H which corresponds to the projective tangent plane of Y 0 at L. SinceG acts by linear transformations, this holds for (L H) Ỹ as well, i.e., L Y and H corresponds to the tangent hyperplane of Y at L. Weuse this fact to prove the following transversality statement. PROPOSITION 4.2. At any point p of Ỹ M (resp. Ỹ M ) the tangent spaces T p Ỹ and T p M (resp. T p Ỹ and T p M ) are transversal in T p X.

9 FLAG DUALITY 339 Proof. We give the proof for p Ỹ M. Let Y := π(ỹ) be the associated cycle in Z and define B := π 1 (Y ). SinceM is a G 0 -orbit which is mapped surjectively to the open G 0 -orbit in Z and B is π- saturated, it follows that B intersects M transversally at p. Thus dim R T p (B M) = 3n 4. If p is the flag (L H), then, recalling that P(H ) is the projective tangent space of Y at π(p) = P(L), it follows that the pre-image (π B ) 1 (H ) can be identified with P(H ) Y.Thisinan(n 2)-dimensional quadric cone with vertex at p. Its tangent space at p generates the full tangent space T p F of the fiber (π ) 1 (H ). Since F M, it follows that T p F T p (B M). Now suppose that Ỹ is not transversal to M at p. In this case dim(t p Ỹ T p M) > dim R Ỹ + (3n 4) dim R B = n 2. But, since the cycle Ỹ intersects the π -fibers transversally, it follows that T p F (T p Ỹ T p M) T p (B M) which, contrary to the transversality of the intersection B M, implies that dim R T p (B M) > 2(n 1) + (n 2) = 3n CYCLE INTERSECTION WITH THE TOP-DIMENSIONAL BOUNDARY ORBIT Ourgoalhereistoprovethefollowing PROPOSITION 4.3. Let Ỹ t, 0 t 1, be a continuous curve of cycles in X with Ỹ 0 the base cycle in D and let Y t = π(ỹ t ) be the associated curve of cycles in Z.If Ỹ 1 =, then there exists t (0, 1) with Y t Z(R) =. For the proof it is convenient to introduce some notation. Here we deal with projective lines E which are defined over R, i.e., one-dimensional linear subspaces of Z = P n (C) which are invariant with respect to the anti-holomorphic involution τ which is induced from complex conjugation on C n+1. If E is such a line, then E(R) := Fix(τ E ) divides E into two components which are interchanged by τ, i.e., E \ E(R) = E 1 E 2 and τ(e 1 ) = E 2. The basic cycle Y 0 is also defined over R, butfix(τ Y0 ) =. Thus Y 0 E consists of two distinct points z j E j, j = 1, 2. Proof of Proposition 4.3. An intersection point x 1 Ỹ 1 is a flag (L H) with L = L and L H. Recall that π(x 1 ) =: z 1 is a point in the quadric Y 1 with projective tangent plane P(H ). Thus the projective line E := P(L L), which is defined over R, is tangent to Y 1 at z 1. Since E.Y 1 = 2, it follows that E Y 1 ={z 1 }. Without loss of generality we may assume that z 1 E 1 and therefore E 2 Y 1 =.

10 340 ALAN T. HUCKLEBERRY AND JOSEPH A. WOLF On the other hand, for t sufficiently small, Y t E j =, j = 1, 2. By continuity it therefore follows that Y t E(R) = for some intermediate t (0, 1) THE EQUALITY OF CYCLES SPACES As was indicated above, we may regard M D and M D as being identified with subspaces of the full space of linear cycles in X, e.g., M D = {Ỹ = g(ỹ0 ) : π(ỹ) D}. Since π( D) = D, it is clear that in this sense M D M D. Of course both moduli spaces contain the orbit G 0 (Y 0 ) of the base cycle which is connected. Let MD, M D and MD denote the connected components of the respective cycle spaces which contain this orbit. THEOREM 4.4. MD = M D = M D. Proof. It is sufficient to show that M D M D =. For this note first of all that the boundary M D in the full space of linear cycles in X is defined by the condition Ỹ D =.Since D is semi-algebraic, it follows that M D is likewise semi-algebraic. Therefore, at least generically, for Ỹ 1 M it is possible to find D acurveỹ t,0 t 1, beginning at the neutral cycle Ỹ 0 with Ỹ y D for 0 t<1 and Ỹ 1 D =. Now Ỹ 1 M = Ỹ 1 M =, because it is only possible for Ỹ 1 to intersect these orbits transversally (Proposition 4.2). Furthermore, Ỹ 1 =, because otherwise Y t Z(R) = for some t (0, 1) Thus the only possible nonempty intersection is Ỹ 1 X(R) which implies that Y 1 and Y1 are boundary points of M D and M D as well. Since this holds at generic boundary points, the result follows. References 1. De Siebenthal, J.: Sur les groupes de Lie compacts non connexes, Comment. Math. Helv. 31 ( ), Gray, A. and Wolf, J. A.: Homogeneous spaces defined by Lie group automorphisms I, II, J. Differential Geom. 2 (1968), and Wolf, J. A.: The action of a real semisimple group on a complex flag manifold, I: Orbit structure and holomorphic arc components, Bull. Amer. Math. Soc. 75 (1969), Wolf, J. A.: The Stein condition for cycle spaces of open orbits on complex flag manifolds, Ann. of Math. (2) 136 (1992), Wolf, J. A.: Exhaustion functions and cohomology vanishing theorems for open orbits on complex flag manifolds, Math. Res. Lett. 2 (1995), Wolf, J. A. and Zierau, R.: Holomorphic double fibration transforms, in R. Doran and V. S. Varadarajan (eds), The Mathematical Legacy of Harish Chandra, Proc. Sympos. Pure Math., Amer. Math. Soc., Providence, R.I., to appear.