1,720,965 research outputs found
A torsion projective class for a group algebra
No full textWe exhibit a cyclic-by-finite group G and two projective modules P and Q for the rational group algebra of G with the following properties: 1. P+P is isomorphic to Q+Q; 2. P is not stably isomorphic to Q; 3. after tensoring with the complex group algebra, P and Q become isomorphic. The proof that P and Q are not isomorphic is topological and involves the Mobius strip bundle over the circle
Some groups of finite homological type
No full textFor each n greater than or equal to zero we construct a torsion-free group that satisfies K. S. Brown's FHT condition and is type F(n), but is not of type FP(n+1). <br/
Presentations for subgroups of Artin groups
No full textRecently, M. Bestvina and N. Brady have exhibited groups that are of type FP but not finitely presented. We give explicit presentations for groups of the type considered by Bestvina-Brady. This leads to algebraic proofs of some of their results
Artin HNN-extensions virtually embed in Artin groups
No full textWe define an Artin HNN-extension to be an HNN-extension of an Artin group in which the stable letter conjugates a pair of suitably chosen subsets of the standard generating set. We show that some finite index subgroup of any Artin HNN-extension embeds in an Artin group. Similarly, we show that every Coxeter HNN-extension virtually embeds in a Coxeter group
The L-two cohomology of Artin groups
No full textFor each Artin group we compute the reduced ℓ2-cohomology of its 'Salvetti complex'. This is a CW-complex which is conjectured to be a model for the classifying space of the Artin group. When this conjecture is known to hold our calculation describes the ℓ2-cohomology of the Artin group
Some remarks concerning degree zero complete cohomology
No full textWe describe degree zero mod-p complete cohomology modulo its radical in purely group-theoretic terms, for members of a class of groups that includes all groups of finite virtual cohomological dimension. We give examples to show that our description does not apply to all discrete groups. We also give examples of discrete groups to which Quillen's description of the ordinary mod-p cohomology ring (up to F-isomorphism) does not apply
Some examples of discrete group actions on aspherical manifolds
Full text linkWe construct two classes of examples of virtually torsion-free groups G acting properly cocompactly on contractible manifolds X. In the first class of examples, the universal space for proper actions has no model with finitely many orbits of cells (and so the given manifold X cannot have this equivariant homotopy type). The reason is that the centralizers of some finite subgroups of G do not have finite-type classifying spaces. In the second class of examples, X is a CAT(0) manifold upon which G acts by isometries, and hence X is a model for the universal space for proper G actions. In these examples, the fixed-point sets for some finite subgroups of G are not manifolds and the centralizers of these subgroups are not virtual Poincare duality groups. <br/
The spectrum of the Chern subring
No full textFor certain subrings of the mod-p cohomology ring of a compact Lie group, we give a description of the prime ideal spectrum, analogous to Quillen's description of the spectrum of the whole ring. Examples of such subrings include the Chern subring (the subring generated by Chern classes of all unitary representations), and for finite groups the subring generated by Chern classes of representations realizable over any specified field. As a corollary, we deduce that the inclusion of the Chern subring in the cohomology ring is an F-isomorphism for a compact Lie group G if and only if the following condition holds: For any homomorphism f between elementary abelian p-subgroups of G such that f(v) is always conjugate to v, there is an element g of G such that f is equal to conjugation by g
Some free-by-cyclic groups
No full textWe exhibit free-by-cyclic groups containing non-free locally-free subgroups, including some word hyperbolic examples. We also show that these groups are not subgroup separable. We use Bestvina-Brady Morse theory in our arguments
Groups possessing extensive hierarchical decompositions
No full textThe class HF is the smallest class of groups that contains all finite groups and is closed under the following operator: whenever G admits a finite-dimensional contractible G-CW-complex in which all stabilizers are in HF, then G is itself in HF. The class HF admits a natural filtration by the ordinals. For each countable ordinal we show that there is a countable group that is in HF but has not arisen by stage alpha of this filtration. Previously this result was known only for alpha equal to 0, 1, and 2. The groups that we construct contain torsion. We also review the construction of a torsion-free countable group in HF that is not in stage 2 of the filtration. <br/
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