The n-cohomology of limits of discrete series

Let G be a semisimple Lie group which has a compact Cartan subgroup H, let K be a maximal compact subgroup of G containing H, and let n be the sum of the negative root spaces of G corresponding to an arbitrary choice of a positive root system of (G, H). We compute the n-cohomology of the K-finite vectors in a limit of a discrete series representation π of G. In the special case when π is a discrete series representation our result reduces to the result of W. Schmid. In the special case when π is a holomorphic limit we interpret its multiplicity in L²(ΓβG) cohomologically, where Γ is a discrete subgroup of G. In the general case we present a conjecture for this multiplicity.     

The Faber Operator and its Boundedness

Let G be a domain bounded by a Jordan curve Γ, and let A(G) be the Banach space of functions continuous on G and holomorphic in G. The Faber operator T is a linear mapping from A( ) to A(G) mapping wⁿ onto the nth Faber polynomial F_n(z) (n=0, 1, 2, …). We show that T<∞ if Γ is piecewise Dini-smooth, and give an example of a quasicircle Γ for which T=∞.     

A Theorem on Discrete,Torsion Free Subgroups of Isom H <Superscript><Emphasis Type="Italic">n</Emphasis></Superscript>

Let H ⁿ be the hyperbolic n-space with n⩾ 2. Suppose that Γ H ⁿ is a discrete, torsion free subgroup and a is a point in the domain of discontinuity Ω (Γ). Let p be the projection map from H ⁿ to the quotient manifold M=H ⁿ/Γ. In this paper we prove that there exists an open neighborhood U of a in H ⁿ∪ Ω (Γ) such that p is an isometry on U∩ H ⁿ.Mathematics Subject Classification (2000). primary: 51M10; secondary: 22E40, 57M50.     

Proximality and Equidistribution on the Furstenberg Boundary

Let G be a connected semisimple Lie group with finite center and without compact factors, P a minimal parabolic subgroup of G, and Γ a lattice in G. We prove that every Γ-orbits in the Furstenberg boundary G/P is equidistributed for the averages over Riemannian balls. The proof is based on the proximality of the action of Γ on G/P.*Partially supported by NSF grant 0400631. Mathematics Subject Classification (2000). 37B05, 22E40     

Upper domination and upper irredundance perfect graphs

Let β(G), Γ(G) and IR(G) be the independence number, the upper domination number and the upper irredundance number, respectively. A graph G is called Γ-perfect if β(H) = Γ(H), for every induced subgraph H of G. A graph G is called IR-perfect if Γ(H) =IR(H), for every induced subgraph H of G. In this paper, we present a characterization of Γ-perfect graphs in terms of a family of forbidden induced subgraphs, and show that the class of Γ-perfect graphs is a subclass of IR-perfect graphs and that the class of absorbantly perfect graphs is a subclass of Γ-perfect graphs. These results imply a number of known theorems on Γ-perfect graphs and IR-perfect graphs. Moreover, we prove a sufficient condition for a graph to be Γ-perfect and IR-perfect which improves a known analogous result.     

Limit formulas of period integrals for a certain symmetric pair

Let G be the unitary group of a non-degenerate Hermitian space and H the stabilizer of a one-dimensional positive definite subspace of the Hermitian space. For a uniform lattice Γ in G such that Γ∩H is a uniform lattice of H, we introduce the (averaged) H-period integrals of automorphic forms on Γ G; we study their behavior as Γ shrinks to the identity along a tower of lattices in G and prove a limit formula of the H-period integrals.     

Uniform estimates for polynomial approximation in domains with corners

Let be a domain with a Jordan boundary ∂G, consisting of l smooth curves Γ_j, such that {z_j}Γ_j-1∩Γ_j≠, j=1,…,l, where Γ₀Γ_l. Denote by α_jπ, 0<α_j2, the angles at z_j's between the curves Γ_j-1 and Γ_j, exterior with respect to G. Let Φ be a conformal mapping of the exterior of onto the exterior of the unit disk, normed by Φ^′(∞)>0. We assume that there is a neighborhood U of , such that , where

z≠z_j if α_j1. Set g_Gsup{|g(z)|:zG}. Then we prove Theorem. Let and 0βr. If a function f is analytic in G and f^(r)β_G<+∞, then for each nlr there is an algebraic polynomial P_n of degree <n, such that

     

Parabolic and quasiparabolic subgroups of free partially commutative groups

Let Γ be a finite graph and G be the corresponding free partially commutative group. In this paper we study subgroups generated by vertices of the graph Γ, which we call canonical parabolic subgroups. A natural extension of the definition leads to canonical quasiparabolic subgroups. It is shown that the centralisers of subsets of G are the conjugates of canonical quasiparabolic centralisers satisfying certain graph theoretic conditions.     

Correspondance de Jacquet–Langlands explicite II. Le cas de degréégal à la caractéristique résiduelle

Let F be a non-Archimedean locally compact field, and let p be its residual characteristic. Put G=GL _p (F) and let G ^′=D _×, where $D$ is a division algebra with centre F and of degree p ² over F. The Jacquet–Langlands correspondence is a bijection between the discrete series of G and that of G ^′. We describe this explicitly, in terms of Carayol's parametrization of these discrete series. Received: 25 November 1999     

Relative property (T) and related properties of wreath products

Let H and Γ be two discrete groups and X be a set on which Γ acts. We investigate when the wreath product H≀_XΓ has relative property (T) and give a complete characterization of property (T) for such groups. Moreover we determine a lower bound of a Kazhdan constant for H≀_XΓ in terms of those for H and Γ. Property (τ) of H≀_XΓ is also discussed in some instances.Supported by grant 20-65060.01 of the Swiss National Fund for Scientific Research.Supported by FWF (Austrian Science Fund) project P15577.     

Twice Q-polynomial distance-regular graphs are thin

Let Γ be a distance-regular graph of diameter d 3. For each vertex χ of Γ, let T(χ) denote the subconstituent algebra for Γ with respect to χ. An irreducible T(χ)-module W is said to be thin if dim E_i^*(χ) W 1 for 0 i d, where E_i^*(χ) is the projection onto the ith subconstituent for Γ with respect to χ. The graph Γ is said to be thin if, for each vertex χ of Γ, very irreducible T(χ)-module is thin. Our main result is the following Theorem: If Γ has two Q-polynomial structures, then Γ is thin.     

Almost isometric actions,property (<Emphasis Type="Italic">T</Emphasis>), and local rigidity

Let Γ be a discrete group with property (T) of Kazhdan. We prove that any Riemannian isometric action of Γ on a compact manifold X is locally rigid. We also prove a more general foliated version of this result. The foliated result is used in our proof of local rigidity for standard actions of higher rank semisimple Lie groups and their lattices in [FM2].One definition of property (T) is that a group Γ has property (T) if every isometric Γ action on a Hilbert space has a fixed point. We prove a variety of strengthenings of this fixed point properties for groups with property (T). Some of these are used in the proofs of our local rigidity theorems. To Yakov G. Sinai on his 70th birthday     

A simple proof of the uniqueness theorem in impedance tomography

Let·(σ(x)u)= 0 in D R³, where D is a bounded domain with a smooth boundary. Suppose that σ ≥ m> 0, σ H³(D), where H^ℓ is the Sobolev space. Let the set {u, σu_N} be given on Γ for all u H^3/2(Γ), where u_N is the normal derivative of u on Γ.     

Proof of a conjecture of José L. Rubio de Francia

Given a compact connected abelian group G, its dual group Γ can be ordered (in a non-canonical way) so that it becomes an ordered group. It is known that, for any such ordering on Γ and p in the range 1<p<∞, the characteristic function χ_I of an interval I in Γ is a p—multiplier with a uniform bound (independent of I) on the corresponding operator S_I on L^p(G). In this note it is shown that, for 1<p,q<∞, there is a constant C_p,q, independent of G and the particular ordering on Γ, such that for all sequences {I_j} of intervals in Γ and all sequences {f_j} in L^p(G). Such a result was conjectured by J.L. Rubio de Francia, who noted its validity when The present proof uses a transference argument, an approach which shows that any constant C_p,q for which the inequality holds when G = will serve for every G and every ordering on Γ. An added advantage of this approach is that it adapts to give an extension of the result for functions taking values in a UMD space.The work of the first author was partially supported by a grant from the National Science Foundation (U.S.A.). The second and third authors were partially supported by the HARP network HPRN-CT-2001-00273 of the European Commission and by grant BFM2001-0188 of Ministerio de Ciencia y Tecnologia.     

Squares of characters and finite groups

Let G be a finite group and χ be an irreducible complex character. We study the character χ² in the case that χ(1) is a prime power.     

Linear Optimization Queries

Let Γ₀ be a set of n halfspaces in E^d (where the dimension d is fixed) and let m be a parameter, n ≤ m ≤ n^d/2. We show that Γ₀ can be preprocessed in time and space O(m^1+δ) (for any fixed δ > 0) so that given a vector c E^d and another set Γ_q of additional halfspaces, the function c · x can be optimized over the intersection of the halfspaces of Γ₀ Γ_q in time O((n/m^1/d/2 + |Γ_q|)log^4d+3n). The algorithm uses a multidimensional version of Megiddo′s parametric search technique and recent results on halfspace range reporting. Applications include an improved algorithm for computing the extreme points of an n-point set P in E^d, improved output-sensitive computation of convex hulls and Voronoi diagrams, and a Monte-Carlo algorithm for estimating the volume of a convex polyhedron given by the set of its vertices (in a fixed dimension).     

Involutory Automorphisms of Groups of Odd Order

The following theorem is proved. Let G be a finite group of odd order admitting an involutory automorphism φ. Suppose that G has derived length d and that C_G(φ) is nilpotent of class c. Assume that C_G(φ) is a m-generator. Then [G,φ]^′ is nilpotent of {c,d,m}-bounded class.     

Involutory Automorphisms of Groups of Odd Order

The following theorem is proved. Let G be a finite group of odd order admitting an involutory automorphism φ. Suppose that G has derived length d and that C_G(φ) is nilpotent of class c. Assume that C_G(φ) is a m-generator. Then [G,φ]^′ is nilpotent of {c,d,m}-bounded class.     

The vertex-cover polynomial of a graph

In this paper we define the vertex-cover polynomial Ψ(G,τ) for a graph G. The coefficient of τ^r in this polynomial is the number of vertex covers V′ of G with |V′|=r. We develop a method to calculate Ψ(G,τ). Motivated by a problem in biological systematics, we also consider the mappings f from {1, 2,…,m} into the vertex set V(G) of a graph G, subject to f⁻¹(x)f⁻¹(y)≠ for every edge xy in G. Let F(G,m) be the number of such mappings f. We show that F(G,m) can be determined from Ψ(G,τ).     

On Lp-Generalization of a Theorem of Adamyan, Arov, and Kreın

The paper deals with problems relating to the theory of Hankel operators. Let G be a bounded simple connected domain with the boundary Γ consisting of a closed analytic Jordan curve. Denote by _n,p(G), 1p<∞, the class of all meromorphic functions on G that can be represented in the form h=β/α, where β belongs to the Smirnov class E_p(G), α is a polynomial degree at most n, α0. We obtain estimates of s-numbers of the Hankel operator A_f constructed from fL_p(Γ), 1p<∞, in terms of the best approximation Δ_n,p of f in the space L_p(Γ) by functions belonging to the class _n,p(G).