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 $\otimes^L$.Comment: 18 pages; minor change

Stacks of group representations

We start with a small paradigm shift about group representations, namely the observation that restriction to a subgroup can be understood as an extension-of-scalars. We deduce that, given a group $G$, the derived and the stable categories of representations of a subgroup $H$ can be constructed out of the corresponding category for $G$ by a purely triangulated-categorical construction, analogous to \'etale extension in algebraic geometry. In the case of finite groups, we then use descent methods to investigate when modular representations of the subgroup $H$ can be extended to $G$. We show that the presheaves of plain, derived and stable representations all form stacks on the category of finite $G$-sets (or the orbit category of $G$), with respect to a suitable Grothendieck topology that we call the sipp topology. When $H$ contains a Sylow subgroup of $G$, we use sipp Cech cohomology to describe the kernel and the image of the homomorphism $T(G)\to T(H)$, where $T(-)$ denotes the group of endotrivial representations.Comment: Slightly revised version of the 2012 June 21 versio

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.Comment: 19 pages, minor details added, reference to published paper adde