Nansen Petrosyan

Bredon cohomological dimensions for groups acting on CAT(0)-spaces

Let G be a group acting isometrically with discrete orbits on a separable complete CAT(0)-space of bounded topological dimension. Under certain conditions, we give upper bounds for the Bredon cohomological dimension of G for the families of finite and virtually cyclic subgroups. As an application, we prove that the mapping class group of any closed, connected, and orientable surface of genus g greater than 1 admits a (9g-8)-dimensional classifying space with virtually cyclic stabilizers. In addition, our results apply to fundamental groups of graphs of groups and groups acting on Euclidean buildings. In particular, we show that all finitely generated linear groups of positive characteristic have a finite dimensional classifying space for proper actions and a finite dimensional classifying space for the family of virtually cyclic subgroups. We also show that every generalized Baumslag-Solitar group has a 3-dimensional model for the classifying space with virtually cyclic stabilizers.

Commensurators and classifying spaces with virtually cyclic stabilizers

By examining commensurators of virtually cyclic groups, we show that for each natural number n, any locally finite-by-virtually cyclic group of cardinality aleph_n admits a finite dimensional classifying space with virtually cyclic stabilizers of dimension n+3. As a corollary, we prove that every elementary amenable group of finite Hirsch length and cardinality aleph_n admits a finite dimensional classifying space with virtually cyclic stabilizers.

Intermediaries in Bredon (Co)homology and Classifying Spaces

For certain contractible G-CW-complexes and F a family of subgroups of G, we construct a spectral sequence converging to the F-Bredon cohomology of G with E1-terms given by the F-Bredon cohomology of the stabilizer subgroups. As applications, we obtain several corollaries concerning the cohomological and geometric dimensions of the classifying space EFG. We also introduce, for any subgroup closed class of groups F, a hierarchically de ned class of groups and show that if a group G is in this class, then G has finite F ∩ G-Bredon (co)homological dimension if and only if G has jump F ∩ G-Bredon (co)homology.

Spin structures on almost-flat manifolds

We give a necessary and suffcient condition for almost-flat manifolds with cyclic holonomy to admit a Spin structure. Using this condition we find all 4-dimensional orientable almost- flat manifolds with cyclic holonomy that do not admit a Spin structure.

CLASSIFYING SPACES WITH VIRTUALLY CYCLIC STABILIZERS FOR LINEAR GROUPS

We show that every discrete subgroup of GL(n, ℝ) admits a finite-dimensional classifying space with virtually cyclic stabilizers. Applying our methods to SL(3, ℤ), we obtain a four-dimensional classifying space with virtually cyclic stabilizers and a decomposition of the algebraic K-theory of its group ring.

Crystallographic actions on contractible algebraic manifolds

We study properly discontinuous and cocompact actions of a discrete subgroup

F-structures and Bredon–Galois cohomology

\Gamma

Hierarchically cocompact classifying spaces for mapping class groups of surfaces

of an algebraic group

Complete Bredon cohomology and its applications to hierarchically defined groups

G

ON GENERATORS OF CRYSTALLOGRAPHIC GROUPS AND ACTIONS ON FLAT ORBIFOLDS

on a contractible algebraic manifold