# June 17, 2010

## From Pseudo-geometry to geometry

Posted in Uncategorized at 10:01 pm by noncommutativeag

The general idea is that pseudo-geometric “space” have canonical spectral theories, and a choice of a spectral theory implies a geometric realization of “spaces”, which associates with every “space” a stack whose base is a topological space(the spectrum of the “space”) and fibers at points are local(in an appropriate sense)”space”. Thus, if “spaces” are represented by categories of a certain type(say abelian, exact or triangulated), then their stalks at points are local categories of the same type.

An important realization is the spectral theory of “spaces” represented by svetle abelian categories [R,Ch3],which was started in the middle of the eighties. Below follows a brief outline of some of its basic notions and facts.

4.1. Topologizing subcategories and the spectrum $Spec(-)$.

A full subcategory of an abelian category $C_X$ is called topologizing if it is closed under finite coproducts and subquotients. For an object $M$, we denote by $[M]$ the smallest topologizing subcategory of $C_X$ containing $M$. One can show that objects of $[M]$ are subquotient of finite coproducts of copies of $M$. The spectrum $Spec(X)$ of the “space” $X$ consists of all non zero $M$ such that $[L]=[M]$ for any nonzero suboject $L$ of $M$. We endow $Spec(X)$ with the preorder $\supseteq$ which is called the specialization preorder.

If $M$ is a simple object, then the objects of $[M]$ are isomorphic to finite direct sums of copies of $M$ and $[M]$ is a minimal element of $(Spec(X), \supseteq)$, if $C_X$ is the category of modules over a commutative unital ring $R$, then the map: $p|\rightarrow [R/p]$ is an isomorphism between the prime spectrum of $R$ with specialization preorder and $(Spec(X),\supseteq)$.

4.2. Local “space”

An abelian category $C_Y$ is called local if it has the smallest nonzero topologizing suncategory. It follows that this subcategory coincides with

## Pseudo Geometry III

Posted in Uncategorized at 9:45 pm by noncommutativeag

Copied from A.Rosenberg’s book

Geometry of noncommutative “spaces” and schemes

3.7. Theorem

(a): A pretopology $\tau$ on $Aff_k$ is subcanonical iff $Qcoh(X,\tau)=Qcoh(X)$ for any presheaf of sets on $Aff_k$(in other words,”descent” pretopologies on $Aff_k$ are precisely subcanonical pretopologies). In this case, $Qcoh(X)=Qcoh(X,\tau)\rightarrow Qcoh(X^{\tau})=Qcoh(X^{\tau},\tau)$,where $X^{\tau}$ is the sheaf on $(Aff_k,\tau)$ associated with the presheaf $X$ and $\rightarrow$ is natural full embedding.

(b). If a pretoplogy $\tau$ is of effective descent, then the above embedding becomes a categorical equivalence.

This theorem says that, roughly speaking, the category $Qcoh(X)$ of quasi coherent presheaves knows itself which pretopologies to choose. It also indicates where one should look for a correct noncommutative version of the category $Esp$ (of sheaves of sets on the fpqc site of commutative affine schemes): this should be the category $NEsp_{\tau}$ of sheaves of sets on the presite $(Aff_k,\tau)$,where $\tau$ is a pretopology of effective descent. From the minimalistic point of view, the best choice would be the finest pretopology of effective descent. But there is a more important consideration. The main role of a pretopology is that it is used for gluing new “spaces”. The pretopology that seems to be the most relevant for Grassmannians(in particular, for noncommutative projective space) and a number of other smooth noncommutative spaces constructed in [KR5] is the smooth topology introduced in [KR2].

The theorem is quite useful on a pragmatical level. Namely, if  $\mathfrak{X}$ is a sheaf of sets on $(Aff_k, \tau)$ for an appropriate pretopology of effective descent and $X$ is a presheaf of sets on $Aff_k$ such that its associated sheaf is isomorphic to $\mathfrak{X}$, and $\mathfrak{R}\Rightarrow \mathfrak{U}\rightarrow X$ is an exact sequence of presheaves with $\mathfrak{R}$ and $\mathfrak{U}$ representable, then the category $Qcoh(X)$(hence the category $Qcoh(\mathfrak{X})$) is constructively described (unique up to equivalence) via pair of k-algebra $A\Rightarrow R$ representing $\mathfrak{R}\Rightarrow \mathfrak{U}$. This consideration is used to describe the categories of quasi coherent sheaves on noncommutative “spaces”

3.8. Noncommutative stacks.

There is one more important observation in connection with this theorem: categories which appear in noncommutative algebraic geometry are categories of quasi coherent sheaves on noncommutative stacks.

## Pseudo Geometry II

Posted in Uncategorized at 5:20 pm by noncommutativeag

Copied from A.Rosenberg’s book

Geometry of noncommutative “spaces” and schemes

3.4 Theorem

The category $Aff_S$ is equivalent to the category $Ass_S$ whose objects are continuous monads on the category $C_S$ and morphisms are conjugacy classes of monads morphisms.

If $C_S=Z-mod$, then the category $Ass_S$ is equivalent to the category whose objects are associative unital rings and morphisms are conjugacy classes of ring morphisms. If $C_S=Sets$, then $Ass_S$ is equivalent to the category whose objects are monoids and morphisms are conjugacy classes of monoids morphism. This shows that the choice of base “space” $S$ influences drastically the rest of the story.

3.5. Locally affine relative “space”. Locally affine $S$-“space” are defined in an obvious way, once a notion of a cover(a quasi-pretopology)is fixed. We introduce several canonical quasi-pretopologies on the category $|Cat|^o$. Their common feature is the following: if a set of morphisms to $X$ is a cover, then the set of their inverse image functors is conservative and all inverse image functors are exact in a certain mild way. If, in addition, morphisms of covers are continuous, $X$ has a finite affine cover, and the category $C_S$ has finite limits, then this requirement suffices to recover the object $X$ from the covering date uniquely up to isomorphism(i.e the category $C_X$ is reovered uniquely up to equivalence)via “flat descent”

3.6 “Spaces” determined by presheaves of sets on $Aff_k$.

By definition, the category $Aff_k$ of noncommutative affine k-schemes is the category opposite to the category $Alg_k$ of associative unital k-algebras; so that presheaves of sets on $Aff_k$ are functors from $Alg_k$ to Sets. The presheaves of sets on $Aff_k$ appeared in our work with Maxim Kontsevich, for the first time in order to introduce noncommutative projective spaces. It was an attempt to imitate the standard commutative approach realizing schemes(and more general spaces) as sheaves of sets on the category of affine schemes endowed with an appropriate Grothendieck  pretopology. It turned out that it is not clear a priori what an appropriate pretopology in the noncommutative case is: Zariski pretopology is irrelevant, because the noncommutative projective space is not a scheme- it does not have an affine Zariski cover. Flat affine covers seemed to be a as a natural choice, but, they do not form a pretoplogy-invariance under base change fails. Similar story with Grassmannians and other analogs of commutative constructions. The elucidation of this problem is as follows. Consider the fibered category $\widetilde{Aff_k}$  with the base $Aff_k$ whose fibers are categories of left modules over corresponding algebras. For every presheaf of sets $X$ on $Aff_k$, we have the fibered category $\widetilde{Aff_k}/X$ induced by $\widetilde{Aff_k}$ along the forgetful functor $\widetilde{Aff_k}/X\rightarrow Aff_k$. The category $Qcoh(X)$ of quasi coherent sheaves on the presheaf $X$ is defined as the category opposite to the category of cartesian sections of the fibered category $\widetilde{Aff_k}$. For a pretopology $\tau$ on $Aff_k/X$, we define the subcategory $Qcoh(X,\tau)$ of quasi-coherent sheaves on $(Aff_k/X,\tau)$.

## Pseudo Geometry I

Posted in Uncategorized at 2:51 pm by noncommutativeag

Copied from A.Rosenberg’s book

Geometry of noncommutative “spaces” and schemes

3. Pseudo-geometric start.

The pseudo-geometric noncommutative landscape sketched above is a natural point of departure, by simple reason that it includes most examples of interest. Instead of trying to impose, from the very beginning, general notions of spaces and morphism of spaces, which absorb all the known case, we approach these notions by studying algebraic geometry in certain key pseudo-geometric settings, which are simple enough to not to get lost and, at the same time, sufficient to obtain a rich theory and to see what one should expect or look for in more sophisticated pseudo-geometries.

3.1. “Spaces” represented by categories. In the very first, in a sense the simplest, setting of this kind, “spaces”are represented by svetle(equivalent to small)categories and morphisms of “spaces” $X\rightarrow Y$ are isomorphism classes of(inverse image)functors $C_Y\rightarrow C_X$ between the corresponding categories. This defines the category $|Cat|^o$ of “spaces”. A morphism of “space” is called continuous if its inverse image functor has a right adjoint (called a direct image functor), and it is called flat, if ,in addition, the inverse image functor is left exact(i.e. preserves finite limits). A continuous morphism is called affine if its direct image functor is conservative(i.e. reflects isomorphism) and has a right adjoint. These notions(introduced in [R])unveil unexpectedly rich algebraic geometry,more precisely, geometries, living inside of $|Cat|^o$. They appear as follows.

3.2. Continuous monads. Fix a “space” S such that the category $C_S$ has cokernels of paris of arrows. We consider $End(C_S)$ of continuous endofunctors of $C_S$. It is a monoidal category with repsect to the composition of functors whose unit object is the identical functor. The monads in this category are called continuous monads on $C_S$. In other words, continuous monads on $C_S$ are pairs $(F,u)$,where $F$ is continuous functor $C_S\rightarrow C_S$ and $u$ is a functor morphism $F^2\rightarrow F$ such that $uFu=uuF,uF\eta=id_F=u\eta F$  for a unique morphism $\eta :Id_{C_S}\rightarrow F$ called the unit of the monad $(F,u)$. A monad morphism is given in a natural way. This defines the category $Mon_c(S)$ of continuous monads on $C_S$.

If $C_S=Z-mod$, then the category $Mon_c(S)$ is naturally equivalent to the category $Rings$ of associative unital rings. If $C_S$ is the category of quasi coherent sheaves on a scheme $(X,O_X)$, then $Mon_c(S)$ is equivalent to the category of quasi coherent sheaves $A$ of rings on $(X,O_X)$ endowed with a morphism $O_X\rightarrow A$ of sheaves of rings. In particular, the sheaf of rings of (twisted)differential operators can be regarded as a monad on $C_S$. If $C_S$ is the category of sets, then the category $Mon_c(S)$ is equivalent to the category of monoids in the usual sense.

3.3. Relative affine “space”. Give a space $S$, we define the category $Aff_S$ of affine $S$-space as the full subcategory of $|Cat|^o/S$ whose objects are pairs $(X,f:X\rightarrow S)$,where $f$ is an affine morphism.

## Historical observation of noncommutative algebraic geometry IV

Posted in Uncategorized at 12:50 pm by noncommutativeag

Copied from A.Rosenberg’s book

Geometry of noncommutative “spaces” and schemes

2.6 “Spaces” defined by presheaves of sets on the category of noncommutative affine schemes.

The category $Aff_k$ of affine noncommutative k-schemes is the category opposite to the category of associative unital k-algebras. Some of the important examples of noncommutative “space”, such as noncommutative Grassmannians, flag varieties and many others [KR1],[KR2],[KR3], are defined in two steps. The first step is a construction of a presheaf of sets on $Aff_k$(i.e. a functor from category of unital associative k-algebras to the category of sets). In commutative algebraic geometry, the second step is taking the associated sheaf with respect to an appropriate(fpqc or Zariski)topology on $Aff_k$. In noncommutative geometry, we assign, instead, to every presheaf of sets on $Aff_k$ a fibered category whose fibers are categories of modules over k-algebras and define the category of quasi-coherent sheaves on this presheaf as the category opposite to the category of cartesian sections of this fibered category [KR4]. The category of quasi coherent presheaves represents the “space” corresponding to the presheaf of sets.

2.7. Commutative “spaces” ,which “live” in symmetric monoidal categories.

After the formalism of Tannakian categories appeared at the end of the sixties-beginning of the seventies [Sa],[DeM], and super-mathematics approximately at the same time, the idea of mathematics(or at least algebra and geometry),which uses general symmetric monoidal categories, instead of the symmetric monoidal category of vector spaces, became familiar. In [De], Deligne presented a sketch of a fragment of commutative projective geometry  in symmetric monoidal k-linear abelian categories as a part of his proof of the characterization of rigid monoidal abelian categories having a fiber functor.

Manin defined the(category of coherent sheaves on the)Proj of a commutative $Z_{+}$-graded algebra in a symmetric monoidal abelian category endowed with a fiber functor [M1]using, once again, the Serre’s description of the category of coherent sheaves on a projective variety as its definition.

2.8. Quantized enveloping algebras and algebraic geometry  in braided monoidal categories.

While working(in 1995) on a quantum analog of Beilinson-Bernstein localization construction, it was discovered  that “spaces” of noncommutative algebraic geometry could be something different from just abelian or Grothendieck categories. In this particular situation, the natural action of the quantized enveloping algebra if a semisimple Lie algebra on its quantum base affine space becomes differential only if the whole picture is put into the monoidal category of $Z^n$-graded modules endowed with a braiding determined by the Cartan matrix of the Lie algebra (see [LR2],[LR3],[LR4]) . This list(which is far from being complete)shows that the range of objects-spaces and morphisms of spaces, of noncommutative algebraic geometry is considerably larger than the range of objects of commutative algebraic geometry.