On J.H.C. Whitehead's aspherical question I.
On the algebraic structure of the unitary group.
We consider the unitary group U of complex, separable, infinite-dimensional Hilbert space as a discrete group. It is proved that, whenever U acts by isometries on a metric space, every orbit is bounded. Equivalently, U is not the union of a countable chain of proper subgroups, and whenever E ⊆ U generates U, it does so by words of a fixed finite length.
On the complexity of braids
We define a measure of “complexity” of a braid which is natural with respect to both an algebraic and a geometric point of view. Algebraically, we modify the standard notion of the length of a braid by introducing generators , which are Garside-like half-twists involving strings through , and by counting powered generators as instead of simply . The geometrical complexity is some natural measure of the amount of distortion of the times punctured disk caused by a homeomorphism. Our main...
On the cut point conjecture.
On the dynamics and the fixed subgroup of a free group automorphism.
On the existence of universal rational structures for groups.
On the group of reduced identities of relatively free solvable groups.
On the linearity problem for mapping class groups.
Optics in Croke-Kleiner Spaces
We explore the interior geometry of the CAT(0) spaces , constructed by Croke and Kleiner [Topology 39 (2000)]. In particular, we describe a diffraction effect experienced by the family of geodesic rays that emanate from a basepoint and pass through a certain singular point called a triple point, and we describe the shadow this family casts on the boundary. This diffraction effect is codified in the Transformation Rules stated in Section 3 of this paper. The Transformation Rules have various applications....
Parabolic isometries of CAT(0) spaces and CAT(0) dimensions.
Platonic triangles of groups.
Plongements quasiisométriques du groupe de Heisenberg dans , d’après Cheeger, Kleiner, Lee, Naor
Bref survol du théorème de non-plongement de J. Cheeger et B. Kleiner pour le groupe d’Heisenberg dans .
Preservation and Distortion of Area in Finitely Presented Groups.
Products of trees, lattices and simple groups.
Pure braid subgroups of braided Thompson's groups.
Quasiflats with holes in reductive groups.
Quasi-isometric maps and Floyd boundaries of relatively hyperbolic groups
We describe the kernel of the canonical map from the Floyd boundary of a relatively hyperbolic group to its Bowditch boundary. Using the Floyd completion we further prove that the property of relative hyperbolicity is invariant under quasi-isometric maps. If a finitely generated group admits a quasi-isometric map into a relatively hyperbolic group then is itself relatively hyperbolic with respect to a system of subgroups whose image under is situated within a uniformly bounded distance...
Regular geodesic languages and the falsification by fellow traveler property.
Regular neighbourhoods and canonical decompositions for groups.
Representations of a free group of rank two by time-varying Mealy automata
In the group theory various representations of free groups are used. A representation of a free group of rank two by the so-calledtime-varying Mealy automata over the changing alphabet is given. Two different constructions of such automata are presented.