Lacunary sets on transformation groups

Lacunary hyperbolic groups

We call a finitely generated group lacunary hyperbolic if one of its asymptotic cones is an R-tree. We characterize lacunary hyperbolic groups as direct limits of Gromov hyperbolic groups satisfying certain restrictions on the hyperbolicity constants and injectivity radii. Using central extensions of lacunary hyperbolic groups, we solve a problem of Gromov by constructing a group whose asymptot...

Lacunary Fourier Series on Noncommutative Groups

. FA ] 9 D ec 1 99 2 MULTIPLIERS AND LACUNARY SETS IN NON - AMENABLE GROUPS

C is called a completely bounded multiplier (= Herz-Schur multiplier) if the transformation defined on the linear span K(G) of {λ(x), x ∈ G} by x∈G f (x)λ(x) → x∈G f (x)ϕ(x)λ(x) is completely bounded (in short c.b.) on the C *-algebra C * λ (G) which is generated by λ (C * λ (G) is the closure of K(G) in B(ℓ 2 (G), ℓ 2 (G)).) One of our main results (stated below as Theorem 0.1) gives a simple ...

2 00 9 Lacunary hyperbolic groups

We call a finitely generated group lacunary hyperbolic if one of its asymptotic cones is an R-tree. We characterize lacunary hyperbolic groups as direct limits of Gromov hyperbolic groups satisfying certain restrictions on the hyperbolicity constants and injectivity radii. Using central extensions of lacunary hyperbolic groups, we solve a problem of Gromov by constructing a group whose asymptot...

On the Universal Theory of Torsion and Lacunary Hyperbolic Groups

We show that the universal theory of torsion groups is strongly contained in the universal theory of finite groups. This answers a question of Dyson. We also prove that the universal theory of some natural classes of torsion groups is undecidable. Finally we observe that the universal theory of the class of hyperbolic groups is undecidable and use this observation to construct a lacunary hyperb...