Groups of hyperbolic crystallography

Let H be a subgroup of Γ, the modular group. Let h be the number of orbits of under the action of H. In each orbit, the stabilizers are H-conjugate. Let U be the mapping z↦z + 1. Each stabilizer is Γ-conjugate to 〈Uc〉 for some non-negative integer c. The integer c is the cusp-width of the orbit. Let h0 be the number of orbits with non-trivial stabilizer, i.e. with c>0. The sequence (c(1), …, c(h0)) of non-zero cuspwidths is the cusp-split of H. Clearly, h0<h, and h∞ = h−h0 is the number of orbits with trivial stabilizer.

Groups of class 2n in which all proper subgroups have class at most n

Remarks on a theorem of Swarup on ends of pairs of groups

Infinite index subgroups and finiteness properties of intersections of geometrically finite groups

Let G be an arbitrary topological group. We prove that the cellularity of G is equal to the cellularity of the quotient space G / K {G/K} for every compact subgroup K of G.

In this paper, we give a characterization of the Lefschetz elements in a coinvariant algebra of an arbitrary complex reflection group which contains a binary polyhedral group as a subgroup of index 2. In addition, we examine the relation between the set of the non-strong Lefschetz elements and polyhedra.

Non-supersymmetric multi-wall configurations are generically unstable. It is proposed that the stabilization in compact space can be achieved by introducing a winding number into the model. A BPS-like bound is studied for the energy of configuration with nonvanishing winding number. Winding number is implemented in an ${\cal N}=1$ supersymmetric nonlinear sigma model with two chiral scalar fields and a bound states of BPS and anti-BPS walls is found to exist in noncompact spaces. Even in compactified space $S^1$, this nontrivial bound state persists above a critical radius of the compact dimension.

The covering dimension invariants

Quantum locally compact metric spaces

It is known by the Conley's theorem that the chain recurrent set CR(ϕ) of a deterministic flow ϕ on a compact metric space is the complement of the union of sets B(A) − A, where A varies over the collection of attractors and B(A) is the basin of attraction of A. It has recently been shown that a similar decomposition result holds for random dynamical systems (RDSs) on non-compact separable complete metric spaces, but under a so-called absorbing condition. In the present article, the authors introduce a notion of random chain recurrent sets for RDSs, and then prove the random Conley's theorem on non-compact separable complete metric spaces without the absorbing condition.

Every compact symmetric space $M$ admits a dual noncompact symmetric space $\check{M}$. When $M$ is a generalized Grassmannian, we can view $\check{M}$ as a open submanifold of it consisting of space-like subspaces \cite{HL}. Motivated from this, we study the embeddings from noncompact symmetric spaces to their compact duals, including space-like embedding for generalized Grassmannians, Borel embedding for Hermitian symmetric spaces and the generalized embedding for symmetric R-spaces. We will compare these embeddings and describe their images using cut loci.

Spectral decomposition for topologically Anosov homeomorphisms on noncompact and non-metrizable spaces

Let Γ < SL n ( $$\mathbb{Z}$$ ) be a subgroup of finite index, where n ≥ 5. Suppose Γ acts continuously on a manifold M, where π1(M) = $$\mathbb{Z}$$ n , preserving a measure that is positive on open sets. Further assume that the induced Γ action on H 1(M) is non-trivial. We show there exists a finite index subgroup Γ′ < Γ and a Γ′ equivariant continuous map ψ : M → $$\mathbb{T}$$ n that induces an isomorphism on fundamental group. We prove more general results providing continuous quotients in cases where π1(M) surjects onto a finitely generated torsion free nilpotent group. We also give some new examples of manifolds with Γ actions.

On questions of strict contractibility

A general combinatorial theorem for countably compact, noncompact spaces is given under the Proper Forcing Axiom. It follows that compact Hausdorff spaces of countable tightness are sequential under PFA, solving the Moore-Mrowka Problem. Other applications are also given.