Characterisation of plus-constructive fibrations

For the numerous applications of the plus-construction, a key question concerns when a fibration F → E → B induces another F⁺ → E⁺ → B⁺. A complete solution (with proof) is given, together with a more easily verifiable simplification in special cases.     

Configurations, braids, and homotopy groups

The main results of this article are certain connections between braid groups and the homotopy groups of the -sphere. The connections are given in terms of Brunnian braids over the disk and over the -sphere. The techniques arise from the natural structure of simplicial and -structures on fundamental groups of configuration spaces.

     

The Homotopy Fixed Point Theorem and the Quillen–Lichtenbaum conjecture in Hermitian K-theory

We settle two conjectures for computing higher Grothendieck–Witt groups (also known as Hermitian K-groups) of noetherian schemes X, under some mild conditions. It is shown that the comparison map from the Hermitian K-theory of X to the homotopy fixed points of K  -theory under the natural Z/2$\mathbb{Z}/2$-action is a 2-adic equivalence. We also prove that the mod 2^ν$2^{\nu }$ comparison map between the Hermitian K-theory of X and its étale version is an isomorphism on homotopy groups in the same range as for the Quillen–Lichtenbaum conjecture in K-theory. Applications compute higher Grothendieck–Witt groups of complex algebraic varieties and rings of 2-integers in number fields, and hence values of Dedekind zeta-functions.     

Fibrations with Hopfian properties

This study explores the homotopy-theoretic meeting-point of topics in differential topology, combinatorial group theory and algebraicK-theory. The first two are due to H. Hopf and date from around 1930. The third arose in the author’s characterisation of plus-constructive fibrations. LetF ( _→ ^ί )E →B be a fibration such thati induces an isomorphism of homology with trivial integer coefficients; what is the effect ofi on fundamental groups? In particular, when one passes to hypoabelianisations by factoring out perfect radicals, doesi induce an epimorphism? Numerous conditions are determined which force an affirmative answer. On the other hand, negative examples of a non-finitary nature are also provided. This leaves the question open in the finitely generated case, where it forms a homological version of the dual to Hopf’s original, famous question in group theory.     

Perfect and Acyclic Subgroups of Finitely Presentable Groups

Acyclic groups of low dimension are considered. To indicatethe results simply, let G' be the nontrivial perfect commutatorsubgroup of a finitely presentable group G. Then def(G)1. Whendef(G)=1, G' is acyclic provided that it has no integral homologyin dimensions above 2 (a sufficient condition for this is thatG' be finitely generated); moreover, G/G' is then Z or Z². Naturalexamples are the groups of knots and links with Alexander polynomial1. A further construction is given, based on knots in S²x S¹.In these geometric examples, G' cannot be finitely generated;in general, it cannot be finitely presentable. When G is a 3-manifoldgroup it fails to be acyclic; on the other hand, if G' is finitelygenerated it has finite index in the group of a Q-homology 3-sphere.     

The localization sequence inK-theory

The localizationR of a ringR with respect to a right denominator set of non-zero-divisors is shown to induce inK-theory the exact localization sequence

Hermitian <Emphasis Type="Italic">K</Emphasis>-theory and 2-regularity for totally real number fields

We completely determine the 2-primary torsion subgroups of the hermitian K-groups of rings of 2-integers in totally real 2-regular number fields. The result is almost periodic with period 8. Moreover, the 2-regular case is precisely the class of totally real number fields that have homotopy cartesian “Bökstedt square”, relating the K-theory of the 2-integers to that of the fields of real and complex numbers and finite fields. We also identify the homotopy fibers of the forgetful and hyperbolic maps relating hermitian and algebraic K-theory. The result is then exactly periodic of period 8 in the orthogonal case. In both the orthogonal and symplectic cases, we prove a 2-primary hermitian homotopy limit conjecture for these rings.