Bases of total Witt groups and lax-similitude
We explain how to work with total Witt groups, more specifically, how to circumvent the classical embarrassment of making choices for line bundles up to isomorphisms and up to squares.
math.KT↗
arXiv subjects
Paul Balmer
Publications and source records attributed to Paul Balmer.
The fundamental isomorphism conjecture via non-commutative motives
Given a group, we construct a fundamental additive functor on its orbit category. We prove that any isomorphism conjecture valid for this fundamental isomorphism functor holds for all additive functors, like K-theory, cyclic homology, topological Hochschild homology, etc. Finally, we reduce this fundamental isomorphism conjecture for K-theoretic ones.
math.KT↗
Geometric description of the connecting homomorphism for Witt groups
We give a geometric setup in which the connecting homomorphism in the localization long exact sequence for Witt groups decomposes as the pull-back to the exceptional fiber of a suitable blow-up followed by a push-forward.
math.AG↗
Gluing in tensor triangular geometry
We discuss gluing of objects and gluing of morphisms in tensor triangulated categories. We illustrate the results by producing, among other things, a Mayer-Vietoris exact sequence involving Picard groups.
math.AG↗
The spectrum of prime ideals in tensor triangulated categories
We define the spectrum of a tensor triangulated category $K$ as the set of so-called prime ideals, endowed with a suitable topology. In this very generality, the spectrum is the universal space in which one can define supports for objects of $K$. This construction is functorial with respect to all tensor triangulated functors. Several elementary properties of schemes hold for such spaces, e.g. the existence of generic points and some quasi-compactness. Locally trivial morphisms are proved to be nilpotent. We establish in complete generality a classification of thick tensor-ideal subcategories in terms of arbitrary unions of closed subsets with quasi-compact complements (Thomason's theorem for schemes, mutatis mutandis). We also equip this spectrum with a sheaf of rings, turning it into a locally ringed space. We compute examples and show that our spectrum unifies the schemes of algebraic geometry and the support varieties of modular representation theory.
math.CT↗
Codescent theory I: Foundations
Consider a cofibrantly generated model category $S$, a small category $C$ and a subcategory $D$ of $C$. We endow the category $S^C$ of functors from $C$ to $S$ with a model structure, defining weak equivalences and fibrations objectwise but only on $D$. Our first concern is the effect of moving $C$, $D$ and $S$. The main notion introduced here is the ``$D$-codescent'' property for objects in $S^C$. Our long-term program aims at reformulating as codescent statements the Conjectures of Baum-Connes and Farrell-Jones, and at tackling them with new methods. Here, we set the grounds of a systematic theory of codescent, including pull-backs, push-forwards and various invariance properties.
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Codescent theory II: Cofibrant approximations
We establish a general method to produce cofibrant approximations in the model category $U_S(C,D)$ of $S$-valued $C$-indexed diagrams with $D$-weak equivalences and $D$-fibrations. We also present explicit examples of such approximations. Here, $S$ is an arbitrary cofibrantly generated simplicial model category and $D\subset C$ are small categories. An application to the notion of homotopy colimit is presented.
math.KT↗
Model theoretic reformulation of the Baum-Connes and Farrell-Jones conjectures
The Isomorphism Conjectures are translated into the language of homotopical algebra, where they resemble Thomason's descent theorems.
math.KT↗
Presheaves of triangulated categories and reconstruction of schemes
To any triangulated category with tensor product $(K,\otimes)$, we associate a topological space $Spc(K,\otimes)$, by means of thick subcategories of $K$, a la Hopkins-Neeman-Thomason. Moreover, to each open subset $U$ of $Spc(K,\otimes)$, we associate a triangulated category $K(U)$, producing what could be thought of as a presheaf of triangulated categories. Applying this to the derived category $(K,\otimes):=(D^{perf}(X),\otimes^L)$ of perfect complexes on a noetherian scheme $X$, the topological space $Spc(K,\otimes)$ turns out to be the underlying topological space of $X$; moreover, for each open $U\subset X$, the category $K(U)$ is naturally equivalent to $D^{perf}(U)$. As an application, we give a method to reconstruct any reduced noetherian scheme $X$ from its derived category of perfect complexes $D^{perf}(X)$, considering the latter as a tensor triangulated category with $\otimes^L$.
math.AG↗