## How to unify various reconstruction theorems (Gabriel-Rosenberg, Tannaka,Balmers)

What I am talking about are reconstruction theorems for commutative scheme and group from category. Let me elaborate a bit. (I am not an expert, if I made mistake, feel free to correct me)

Reconstruction of commutative schemes

Given a quasi compact and quasi separated commutative scheme $(X,O_{X})$ (actually, quasi compact is not necessary), we can reconstruct the scheme from $Qcoh(X)$ as category of quasi coherent sheaves on $(X,O_{X})$. This is Gabriel-Rosenberg theorem. Let me sketch the statement of this theorem which led to the question I want to ask:

The reconstruction can be taken as geometric realization of $Qcoh(X)$ (abelian category). Let $C_X$=$Qcoh(X)$. We define spectrum of $C_X$ and denote it by $Spec(X)$

(For example, if $C_X=R-mod$.where $R$ is commutative ring, then $Spec(X)$ coincides with prime spectrum $Spec(R)$). We can define Zariski topology on $Spec(X)$, we have open sets respect to Zariski topology. Then we have contravariant pseudo functor from category of Zariski open sets of the spectrum $Spec(X)$ to $Cat$,$U\rightarrow C_{X}/S_{U}$,where $U$ is Zariski open set of $Spec(X)$ and $S_U=\bigcap_{Q\in U}^{ }\hat{Q}$

(Note:$Spec(X)$ is a set of subcategories of $C_{X}$ satisfying some conditions, so here,$Q$ is subcategory belongs to open set $U$ and $\hat{Q}$=union of all topologizing subcategories of $C_{X}$ which do not contain $Q$.)

For each embedding:$V\rightarrow U$, we have correspondence localization functor: $C_{X}/S_{U}\rightarrow C_{X}/S_{V}$. Then we have fibered category over the Zariski topology of $Spec(X)$, so we have given a geometric realization of $Qcoh(X)$ as a stack of local category which means the fiber(stalk)at each point $Q$,

$colim_{Q\in U} C_{X}/S_{U}$=$C_{X}/\hat{Q}$ is a local category.

Zariski geometric center

Define a functor $O_{X}$:$Open(Spec(X))\rightarrow CRings$

$U|\rightarrow End(Id_{C_{X}/S_{U}})$

It is easy to show that $O_{X}$ is a presheaf of commutative rings on $Spec(X)$, then Zariski geometric center is defined as $(Spec(X),\hat{O_{X}})$,where $\hat{O_{X}}$ is associated sheaf of $O_{X}$.

Theorem:

Given X=$(\mathfrak{X},O_{\mathfrak{X}})$ a quasi compact and quasi separated commutative scheme. Then the scheme X is isomorphic to Zariski geometric center of $C_X=Qcoh(X)$. If we denote the fibered category mentioned above by $\mathfrak{F}_{X}$.

then center of each fiber (stalk) of $\mathfrak{F}_{X}$ recover the presheaf of commutative ring defined above and hence Zariski geometric center. Moreover, Catersion section of this fibered category is equivalent to $Qcoh(X)$ when $X$ is commutative scheme

## Question

It is well known that for a compact group $G$, we can use Tanaka formalism to reconstruct this group from category of its representation $(Rep(G),\bigotimes _{k},Id)$. Is this reconstruction Morally the same as Gabriel-Rosenberg pattern reconstruction theorem?

From my perspective, I think $Qcoh(X)$ and category of group representations are very similar because $Qcoh(X)$ can be taken as "category of representation of scheme". On the otherhand, group scheme is a scheme compatible with group operations. Therefore, I think it should have united formalism to reconstruct both of them. Then I have following questions:

1 Is there a Tanaka formalism for reconstruction of general scheme$(X,O_{X})$ from $Qcoh(X)$?

2.Is there a Gabriel-Rosenberg pattern reconstruction (geometric realization of category of category of representations of group) to recover a group scheme? I think this formalism is natural (because the geometric realization of a category is just a stack of local categories and the center(defined as endomorphism of identical functor)of the fiber of this stack can recover the original scheme) and has good generality (can be extend to more general settings)

Maybe one can argue that these two reconstruction theorems live in different nature because reconstruction of group schemes require one to reconstruct the group operations which force one to go to monoidal categories (reconstruct co-algebra structure) while reconstruction of non-group scheme doesn't (just need to reconstruct algebra structure).

However, If we stick to the commutative case. $Qcoh(X)$ has natural symmetric monoidal structure. Then, in this case, I think this argue goes away.

## More Concern

I just look at P.Balmer's paper on reconstruction from derived category, it seems that he also used Gabriel-Rosenberg pattern in triangulated categories: He defined spectrum of triangulated category as direct imitation of prime ideals of commutative ring. Then, he used triangulated version of geometric realization (geometric realization of triangulated category as a stack of local triangulated category) hence, a fibered category arose, then take the center of the fiber at open sets of spectrum of triangulated categories to recove the presheaf (hence sheaf) of commutative rings. This triangulated version of geometric realization is also mentioned in Rosenberg's lecture notes Topics in Noncommutative algebraic geometry,homologial algebra and K-theory page 43-44, it also discuss the relation with Balmer's construction. But in Balmer's consideration, there is tensor structure in his derived category

Therefore, the further question is:

Is there a triangulated version of Tannaka Formalism which can recover P.Balmer's reconstruction theorem. ? I heard from some experts in derived algebraic geometry that Jacob Lurie developed derived version of Tannaka formalism. I know there are many experts in DAG on this site. I wonder whether one can answer this question.

In fact, I still have some concern about Bondal-Orlov reconstruction theorem but I think it seems that I should not ask too many questions at one time.So, I stop here.All the related and unrelated comments are welcome

Reconstruction theorem in nLab reconstruction theorem

EDIT: A nice answer provided by Ben-Zvi for related questions

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Nice question. I don't know enough scheme theory to be able to start on an answer. One comment, however: reconstruction theorems generally require a bit more than just the (abstract) category "of representations". More precisely, the statement of Tannaka-Krein is that given a category C with nice properties and a functor f:C->Vect with nice properties, there is a unique (and constructible, from the data) group G up to isomorphism so that (C,f) is equivalent to (G-modules,forget). You can also relax the properties a bit and replace "group" by "(quasitriangular) (quasi)Hopf algebra", etc. – Theo Johnson-Freyd Feb 24 2010 at 16:29
I thought I'd point out that yes one needs extra structure such as the tensor product in the triangulated case. For instance consider two smooth projective birational Calabi-Yau 3-folds. They are derived equivalent (by Bridgeland) so one needs some extra data or it won't be possible to recover both schemes. I also thought I'd comment that (in my opinion) there should be some good notion of homological fibre functor which comes up when extending Balmer's theory to the compactly generated setting. Knowing these would determine the spectrum, but they should know other interesting things too! – Greg Stevenson Feb 24 2010 at 20:25
There is a lemma in Thomason's "Classification of triangulated subcategories" that says a category is tensor-closed if and only if it closed under tensoring by powers of an ample line bundle. Thus, Bondal-Orlov's reconstruction theorem is a consequence of Balmer's result. A proof of Bondal-Orlov's result along these lines is in Rouquier's notes - see paper 38 on his webpage. One caveat: Bondal-Orlov use only the graded structure of the derived category. With this argument, you need the triangles. – Matthew Ballard Feb 24 2010 at 23:36
If you still have your notes from last semester in the 800 class, Rosenberg gave us his perspective on Tannaka-Krein. – B. Bischof Feb 25 2010 at 0:29
@Shizhuo: In the case of Balmer's approach one will recover the scheme corresponding to the tensor product one picks. If I understand correctly to use Rosenberg's approach one needs an abelian category? In this case one will get the scheme corresponding to the t-structure one picks. Either the choice of t-structure or tensor product fixes the geometry associated to the category. – Greg Stevenson Feb 25 2010 at 20:13

The Tannakian reconstruction for schemes and more generally, geometric stacks from QCoh(X) with its tensor structure (due to Jacob Lurie) is explained in my answer to http://mathoverflow.net/questions/3446/tannakian-formalism/3467#3467 . One can (and one has) try to extend this to the derived setting, where one ought to get a statement much stronger than Balmer's reconstruction theorem. First of all one needs to replace triangulated categories, which are too coarse to work with effectively, with symmetric monoidal $(\infty,1)$-categories (or symm. monoidal dg categories say in characteristic zero).

There's an obvious functor from such objects C to derived stacks Spec C given by Tannakian reconstruction -- it's defined just as in the abelian case (see my answer above). Namely we build Spec C's functor of points, as a functor from derived rings to spaces or simplicial sets: Spec C(R) = the $\infty$-groupoid of tensor functors from C to R-modules. In the case that C is the infinity-categorical form of QCoh (X) for X a scheme this recovers X, giving a result refining Balmer's (this was worked out by Nadler, Francis and myself, and many others I think -- in particular Toen and Lurie understood this long ago). I feel strongly this is a better point of view than trying to define a locally ringed space from a triangulated category, and has more of a chance to extend to stacks. In particular C tautologically sheafifies (as a sheaf of symmetric monoidal $\infty$-categories) over X.

Things get much more interesting though in the case that C=Qcoh X for X a stack (with affine diagonal, or everything fails immediately) -- eg for C=Reps(G)=QCoh (BG), the setting of usual Tannakian formalism. There one quickly learns that the above naive picture is false. Namely take G to be just the multiplicative group $G_m$. Then there are a lot of interesting fiber functors on the derived Rep G = complexes of graded vector spaces that are not given by points of BG_m (ie are not the usual "forgetful" fiber functor) --- namely we can send the defining one-dim rep of $G_m$ not to the one-dim vector space in degree zero, but in any degree we want (ie apply shift) - since this is a generator it determines the rest of the fiber functor. This proves that a naive Tannakian theorem fails in the derived setting, without imposing some "connectivity" hypotheses (ie without having t-structures around). I believe a student of Toen's is writing a thesis on this subject, but I don't know the precise statements (and so won't "out" his results).

Of course it's a little disappointing to need BOTH a tensor structure and a t-structure, since morally either one should be enough for reconstruction (having a t-structure means we can recover the abelian category of quasicoherent sheaves, which by Rosenberg recovers X already in the case of schemes without need for tensor structure). But for general stacks I don't know of a better answer.

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 The recent paper (arxiv.org/abs/1004.3087) by Fukuyama and Iwanari works out the derived ($\infty$-categorical) Tannakian theorem in the case of Deligne-Mumford stacks (thus avoiding the BGm obstruction mentioned above). – David Ben-Zvi Apr 23 2010 at 17:15

I never worked hard on reconstruction and spectra (though I should and plan to). It is an interesting area of deep importance for geometry in general and it is undoubtfully related to Galois theory as well. Tomasz Maszczyk gave a seminar in Warszawa describing how the Galois theory of Grothendieck (and generalization of Joyal-Tierney) and Tannakian formalism, are both special cases of a theorem on base change in certain geometrically motivated setup of bicategories. One should also look for basic mechanisms at the categorical Galois theory of Janelidze and Galois descent:

G. Janelidze, F. Borceux, Galois Theories, Cambridge Studies in Advanced Mathematics 72, Cambridge University Press, 2001

G. Janelidze, Magid’s theorem in categories, Bull. Georgian Acad. Sci. 114, 3, 1984, 497-500 (in Russian)

G. Janelidze, The fundamental theorem of Galois theory, Math. USSR Sbornik 64 (2), 1989, 359-374

G. Janelidze, Precategories and Galois theory, Proc. Como, Springer Lect. Notes in Math. 1488, 1991, 157-173

G. Janelidze, D. Schumacher, R. Street, Galois theory in variable categories, Applied Categorical Structures 1, 1993, 103-110

My advice to myself (as I said I hopew to work more on this) and therefore to you is to look at simpler (like Barr embedding theorem, Mitchell embedding theorem, Giraud's reconstruction theorem) rather than more complicated examples of reconstruction theorems. Yoneda arguments (and sometimes also monadicity) are in basis of all such theorems at some place. A sample usage of Yoneda from hands of Urs is at this nlab page.

To David: I think Rosenberg has looked at the spectra in A-infinity version as well, if my memory does not fail (though I have not seen it written). He sometimes tells his experience about various examples outside of the realm of abelian/triangulated categories (like e.g. spectral reconstructions for smooth manifolds...), but is interested mostly in algebraic examples for his purposes in representation theory which I do not understand enough.

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