Left-exact Localizations of $\infty$-Topoi II: Grothendieck Topologies
Mathieu Anel, Georg Biedermann, Eric Finster, André Joyal
备注：
v2. To be published in JPAA. We change some terminology and notations. We simplified the proof of Proposition 3.3.6 and reorganized Section 2 around that
We revisit the work of Toën--Vezzosi and Lurie on Grothendieck topologies, using the new tools of acyclic classes and congruences. We introduce a notion of extended Grothendieck topology on any $\infty$-topos, and prove that the poset of extended Grothendieck topologies is isomorphic to that of topological localizations, hypercomplete localizations, Lawvere--Tierney topologies, and covering topologies (a variation on the notion of pretopology). It follows that these posets are small and have the structure of a frame. We revisit also the topological--cotopological factorization by introducing the notion of a cotopological morphism. And we revisit the notions of hypercompletion, hyperdescent, hypercoverings and hypersheaves associated to an extended Grothendieck topology. We also introduce the notion of forcing, which is a tool to compute with localizations of $\infty$-topoi.
Solution to Lawvere's first problem: a Grothendieck topos that has proper class many quotient topoi
Graduate School of Mathematical Sciences, The University of Tokyo（东京大学大学院数理科学研究科）；Graduate School of Mathematical Sciences, University of Tokyo（东京大学大学院数理科学研究科）
We provide bicategorical analogs of several aspects of the notion of geometry in the sense of the theory of spectrum. We first introduce a notion of local right biadjoint, and prove it to be equivalent to a notion of bistable pseudofunctor, categorifying an analog 1-categorical result. We also describe further laxness conditions, giving some properties of the already known lax familial pseudofunctors. We also describe 2-dimensional analogs of orthogonality and factorization systems, and use them to construct examples of bistable pseudofunctors through inclusion of left objects and left maps. W…
We are developing tools for working with arbitrary left-exact localizations of $\infty$-topoi. We introduce a notion of higher sheaf with respect to an arbitrary set of maps $Σ$ in an $\infty$-topos $\mathscr{E}$. We show that the full subcategory of higher sheaves $\mathrm{Sh}(\mathscr{E},Σ)$ is an $\infty$-topos, and that the sheaf reflection $\mathscr{E}\to \mathrm{Sh}(\mathscr{E},Σ)$ is the left-exact localization generated by $Σ$. The proof depends on the notion of congruence, which is a substitute for the notion of Grothendieck topology in 1-topos theory.
We investigate Grothendieck topologies (in the sense of sheaf theory) on a poset $¶$ that are generated by some subset of $¶$. We show that such Grothendieck topologies exhaust all possibilities if and only if $¶$ is Artinian. If $¶$ is not Artinian, other families of Grothendieck topologies on $¶$ exist that are not generated by some subset of $¶$, but even those are related to the Grothendieck topologies generated by subsets. Furthermore, we investigate several notions of equivalences of Grothendieck topologies, and using a posetal version of the Comparison Lemma, a sheaf-theoretic result kn…
Lindenhovius has studied Grothendieck topologies on posets and has given a complete classification in the case that the poset is Artinian. We extend his approach to more general posets, by translating known results in locale and domain theory to the study of Grothendieck topologies. In particular, explicit descriptions are given for the family of Grothendieck topologies with enough points and the family of Grothendieck topologies of finite type. As an application, we compute the cardinalities of these families in various examples.
The paper "Grothendieck Topologies on Posets" by A.J. Lindenhovius shows that when $\mathbf{P}$ is an Artinian poset and $\mathbf{E}$ is the topos $\mathbf{Set}^\mathbf{P}$ then there are bijections between the set of subsets of $\mathbf{P}$, the set of Grothendieck topologies on $\mathbf{E}$, and the set of nuclei on the Heyting Algebra $\mathrm{Sub}(1_\mathbf{E})$. It also shows that there are nice formulas for converting between subsets, Grothendieck topologies, and nuclei, but the formula for converting a nucleus to a subset is not spelled out explicitly. These notes fix that gap.
In analogy with the classical theory of topological groups, for finitely complete categories enriched with Grothendieck topologies, we provide the concepts of localized G-topological space, initial Grothendieck topologies and continuous morphisms, in order to obtain the concepts of G-topological monoid and G-topological group objects.
This work presents a way to associate a Grothendieck site structure to a category endowed with a unique factorisation system of its arrows. In particular this recovers the Zariski and Etale topologies and others related to Voevodsky's cd-structures. As unique factorisation systems are also frequent outside algebraic geometry, a construction applies to some new contexts, where it is related with known structures defined otherwise. The paper details algebraic geometrical situations and sketches only the other contexts.
We define a notion on preadditive categories which plays a role similar to the notion of a Grothendieck pretopology on an unenriched category. Each such additive pretopology defines an additive Grothendieck topology and suffices to define the sheaf category. This new notion allows us to study the noetherian and subcanonical nature of topologies, to describe easily the meet of a family of topologies and to identify useful universal properties of the sheaf category. As our main application we derive in which ways one can present a tensor category and show that such presentations lead to remarkab…
We investigate the problem of characterizing the classes of Grothendieck toposes whose internal logic satisfies a given assertion in the theory of Heyting algebras, and introduce natural analogues of the double negation and De Morgan topologies on an elementary topos for a wide class of intermediate logics.
