Representations of complex hyperbolic lattices into rank 2 classical Lie groups of Hermitian type

Let Г be a torsion-free uniform lattice of SU(m, 1), m > 1. Let G be either SU(p, 2) with p ≥ 2, ${{\rm Sp}(2,\mathbb {R})}Let Г be a torsion-free uniform lattice of SU(m, 1), m > 1. Let G be either SU(p, 2) with p ≥ 2, or SO(p, 2) with p ≥ 3. The symmetric spaces associated to these G’s are the classical bounded symmetric domains of rank 2, with the exceptions of SO*(8)/U(4) and SO*(10)/U(5). Using the correspondence between representations of fundamental groups of K?hler manifolds and Higgs bundles we study representations of the lattice Г into G. We prove that the Toledo invariant associated to such a representation satisfies a Milnor-Wood type inequality and that in case of equality necessarily G = SU(p, 2) with p ≥ 2m and the representation is reductive, faithful, discrete, and stabilizes a copy of complex hyperbolic space (of maximal possible induced holomorphic sectional curvature) holomorphically and totally geodesically embedded in the Hermitian symmetric space SU(p, 2)/S(U(p) × U(2)), on which it acts cocompactly.     

Symplectic geometry of semisimple orbits

Let G be a complex semisimple group, T G a maximal torus and B a Borel subgroup of G containing T. Let Ω be the Kostant-Kirillov holomorphic symplectic structure on the adjoint orbit O = Ad(G)c G/Z(c), where c Lie(T), and Z(c) is the centralizer of c in G. We prove that the real symplectic form Re Ω (respectively, Im Ω) on O is exact if and only if all the eigenvalues ad(c) are real (respectively, purely imaginary). Furthermore, each of these real symplectic manifolds is symplectomorphic to the cotangent bundle of the partial flag manifold G/Z(cc)B, equipped with the Liouville symplectic form. The latter result is generalized to hyperbolic adjoint orbits in a real semisimple Lie algebra.     

Bounds on smooth matrix coefficients on L 2-spaces

Let G be a semisimple Lie group with a finite number of connected components and a finite center. Let K be a maximal compact subgroup. Let X be a smooth G-space equipped with a G-invariant measure. In this paper, we give upper bounds for K-finite and ${\mathfrak k}Let G be a semisimple Lie group with a finite number of connected components and a finite center. Let K be a maximal compact subgroup. Let X be a smooth G-space equipped with a G-invariant measure. In this paper, we give upper bounds for K-finite and \mathfrak k{\mathfrak k}-smooth matrix coefficients of the regular representation L ²(X) under an assumption about supp(L²(X)) ?[^(G)]_K{{\rm supp}(L^2(X)) \cap \hat G_K}. Furthermore, we show that this bound holds for unitary representations that are weakly contained in L ²(X). Our result generalizes a result of Cowling–Haagerup–Howe (J Reine Angew Math 387:97–110, 1988). As an example, we discuss the matrix coefficients of the O(p, q) representation L²(\mathbbR^p+q){L^2(\mathbb{R}^{p+q})}.     

On the complexity of the circular chromatic number

Circular chromatic number, χ_c is a natural generalization of chromatic number. It is known that it is NP ‐hard to determine whether or not an arbitrary graph G satisfies χ(G)=χ_c(G). In this paper we prove that this problem is NP ‐hard even if the chromatic number of the graph is known. This answers a question of Xuding Zhu. Also we prove that for all positive integers k ≥ 2 and n ≥ 3, for a given graph G with χ(G) = n, it is NP ‐complete to verify if . © 2004 Wiley Periodicals, Inc. J Graph Theory 47: 226–230, 2004     

On a conjecture of Thomassen and Toft

This article is motivated by a conjecture of Thomassen and Toft on the number s₂(G) of separating vertex sets of cardinality 2 and the number υ₂(G) of vertices of degree 2 in a graph G belonging to the class 𝒢 of all 2-connected graphs without nonseparating induced cycles. Let ‖G‖ denote the number of edges of the graph G. Thomassen and Toft conjectured in [C. Thomassen & B. Toft, J. Combin. Theory B 31 (1981), 199–224] the existence of a positive constant c satisfying s₂(G) + υ₂(G) > c · ‖G‖ for all G ∈ 𝒢. We shall see that this is not true in general. Restricting ourselves to planar graphs, we obtain s₂(G) + υ₂(G) > · ‖G‖ for all planar G ∈ 𝒢, where is best-possible. © 1999 John Wiley & Sons, Inc. J Graph Theory 32: 118–122, 1999     

Indices of Maximal Subgroups of Finite Soluble Groups

We look at the structure of a soluble group G depending on the value of a function m(G)= max m _p G), where m _p(G)=max{log_p|G:M| | M< G, |G:M|=p ^a}, p (G). Theorem 1 states that for a soluble group G, (1) r(G/ (G))= m(G); (2) d(G/ (G)) 1+ (m(G)) 3+m(G); (3) l _p(G) 1+t, where 2^t-1<m _p(G) 2^t. Here, (G) is the Frattini subgroup of G, and r(G), d(G), and l _p(G) are, respectively, the principal rank, the derived length, and the p-length of G. The maximum of derived lengths of completely reducible soluble subgroups of a general linear group GL(n,F) of degree n, where F is a field, is denoted by (n). The function m(G) allows us to establish the existence of a new class of conjugate subgroups in soluble groups. Namely, Theorem 2 maintains that for any natural k, every soluble group G contains a subgroup K possessing the following properties: (1) m(K); k; (2) if T and H are subgroups of G such that K T <_max <_max H G then |H:T|=p ^t for some prime p and for t>k. Moreover, every two subgroups of G enjoying (1) and (2) are mutually conjugate.     

Certain Convolution Operators for Meromorphic Functions

Let (p N) be the class of functions analytic in 0 < |z| < 1. A convolution operator L_p(a, c) on p is introduced. This paper gives some sharp inequalities for f(z) satisfying Re{(1 – )z^pL_p(a, c) f(z) + z^pL_p(a + 1, c) f(z)} > , where 0, < 1, a > 0 and c 0, –1, –2,....AMS Subject Classification (1991) 30C45 30A10     

p-Length and p′-Degree Irreducible Characters Having Values in ℚ p

Let G be a p-solvable group of p-length l, where p is any prime. We show that G has at least 2 ^l irreducible characters of degree coprime to p and having values inside ? _p . This generalizes a previous result for p = 2 [6 Tent , J. ( 2011 ). 2-Length and rational characters of odd degree . Arch. Math. 96 ( 3 ): 201 – 206 .[Crossref], [Web of Science ®] , [Google Scholar]] to arbitrary primes. With the same notation, we prove that if p is odd then G has at least 2 ^l Galois orbits of conjugacy classes of p-elements having values in ? _p .     

On central automorphisms that fix the centre elementwise

Let G be a group and let Aut _c (G) be the group of central automorphisms of G. Let be the set of all central automorphisms of G fixing Z(G) elementwise. In this paper we prove that if G is a finite p-group, then = Inn(G) if and only if G is abelian or G is nilpotent of class 2 and Z(G) is cyclic. This work was supported in part by the Center of Excellence for Mathematics, University of Isfahan, Iran. Received: 30 October 2006     

On random points in the unit disk

Let n be a positive integer and λ > 0 a real number. Let V_n be a set of n points in the unit disk selected uniformly and independently at random. Define G(λ, n) to be the graph with vertex set V_n, in which two vertices are adjacent if and only if their Euclidean distance is at most λ. We call this graph a unit disk random graph. Let and let X be the number of isolated points in G(λ, n). We prove that almost always X ～ n when 0 ≤ c < 1. It is known that if where ?(n) → ∞, then G(λ, n) is connected. By extending a method of Penrose, we show that under the same condition on λ, there exists a constant K such that the diameter of G(λ, n) is bounded above by K · 2/λ. Furthermore, with a new geometric construction, we show that when and c > 2.26164 …, the diameter of G(λ, n) is bounded by (4 + o(1))/λ; and we modify this construction to yield a function c(δ) > 0 such that the diameter is at most 2(1 + δ + o(1))/λ when c > c(δ). © 2005 Wiley Periodicals, Inc. Random Struct. Alg., 2006     

Zero-One Law for Symmetric Convolution Semigroups of Measures on Groups

Let ( _t ) _t>0 be a symmetric weakly continuous semigroup of probability measures on a nonabelien complete separable group G and let v be its Lévy measure. The purpose of this paper is to provide a relatively simple proof of the zero-one law for semigroups with the Lévy measure satisfying either v(H ^c) = or v(H ^c) = 0.     

Linearity of Zp[[t]]-Perfect Groups

Let G be a _p[[t]]-standard group of level 1. Then G is _p[[t]]-perfect if its lower central series is given by powers of the maximal ideal (p, t), i.e. if _n(G) = G((p,t)ⁿ). We prove that a _p[[t]]-perfect group is linear by imitating the proof that a _p[[t]]-standard group is linear.     

The Leading Ideal of a Complete Intersection of Height Two,Part III

Let (S, 𝔫) be an s-dimensional regular local ring with s > 2, and let I = (f, g) be an ideal in S generated by a regular sequence f, g of length two. As in [2 Goto , S. , Heinzer , W. , Kim , M.-K. ( 2006 ). The leading ideal of a complete intersection of height two . J. Algebra 298 : 238 – 247 . [Google Scholar], 3 Goto , S. , Heinzer , W. , Kim , M.-K. ( 2007 ). The leading ideal of a complete intersection of height two, II . J. Algebra 312 : 709 – 732 . [Google Scholar]], we examine the leading form ideal I* of I in the associated graded ring G: = gr_𝔫(S). Let μ _G (I*) = n ≥ 3, and let {ξ₁, ξ₂,…, ξ _n } be a minimal homogeneous system of generators of I* such that ξ₁ = f* and ξ₂ = g*, and c _i : = deg ξ _i  ≤ deg ξ _i+1: = c _i+1 for each i ≤ n ? 1. For m ≤ n, we say that K _m : = (ξ₁,…, ξ _m )G is an ideal generated by part of a minimal homogeneous generating set of I*. Let D _i : = GCD(ξ₁,…, ξ _i ) and d _i  = deg D _i for i with 1 ≤ i ≤ m. Let K _m be perfect with ht _G K _m  = 2. We prove that the following are equivalent: 1. deg ξ _i+1 = deg ξ _i  + (d _i?1 ? d _i ) +1, for all i with 3 ≤ i ≤ m ? 1;

2. deg ξ _i+1 ≤ deg ξ _i  + (d _i?1 ? d _i ) +1, for all i with 3 ≤ i ≤ m ? 1.

Furthermore, if these equivalent conditions hold, then K _m  = I*. Moreover, if e(G/K _m ) = e(G/I*), we prove that K _m  = I*. We illustrate with several examples in the cases where K _m is or is not perfect.     

The Average Sylow Multiplicity Character and the Solvable Residual

Let G be a finite group, and let p₁,…, p_m be the distinct prime divisors of |G|. Given a sequence 𝒫 =P₁,…, P_m, of Sylow p_i-subgroups of G, and g ∈ G, denote by m_𝒫(g) the number of factorizations g = g₁…g_m such that g_i ∈ P_i. The properly normalized average of m_𝒫 over all 𝒫 is a complex character over G whose kernel contains the solvable radical of G [7 Levy , D. ( 2010 ). The average Sylow multiplicity character and solvability of finite groups . Communications in Algebra. 38 : 632 – 644 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]]. The present paper characterizes the solvable residual of G in terms of this character.     

Balloons,cut‐edges,matchings, and total domination in regular graphs of odd degree

A balloon in a graph G is a maximal 2‐edge‐connected subgraph incident to exactly one cut‐edge of G. Let b(G) be the number of balloons, let c(G) be the number of cut‐edges, and let α′(G) be the maximum size of a matching. Let ${\mathcal{F}}_{{{n}},{{r}}}A balloon in a graph G is a maximal 2‐edge‐connected subgraph incident to exactly one cut‐edge of G. Let b(G) be the number of balloons, let c(G) be the number of cut‐edges, and let α′(G) be the maximum size of a matching. Let ${\mathcal{F}}_{{{n}},{{r}}}$ be the family of connected (2r+1)‐regular graphs with n vertices, and let ${{b}}={{max}}\{{{b}}({{G}}): {{G}}\in {\mathcal{F}}_{{{n}},{{r}}}\}$. For ${{G}}\in{\mathcal{F}}_{{{n}},{{r}}}$, we prove the sharp inequalities c(G)?[r(n?2)?2]/(2r²+2r?1)?1 and α′(G)?n/2?rb/(2r+1). Using b?[(2r?1)n+2]/(4r²+4r?2), we obtain a simple proof of the bound proved by Henning and Yeo. For each of these bounds and each r, the approach using balloons allows us to determine the infinite family where equality holds. For the total domination number γ_t(G) of a cubic graph, we prove γ_t(G)?n/2?b(G)/2 (except that γ_t(G) may be n/2?1 when b(G)=3 and the balloons cover all but one vertex). With α′(G)?n/2?b(G)/3 for cubic graphs, this improves the known inequality γ_t(G)?α′(G). © 2009 Wiley Periodicals, Inc. J Graph Theory 64: 116–131, 2010     

Isoperimetric inequalities for lattices in semisimple lie groups of rank 2

LetG be a connected semisimple Lie group with finite center. Let be an irreducible non-uniform lattice inG. We show that if the real rank ofG is 2, then the Dehn (or filling) function of is exponential.