Difference between revisions of "Algebra and Algebraic Geometry Seminar Spring 2018"

The seminar meets on Fridays at 2:25 pm in room B113.

Here is the schedule for the previous semester.

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Spring 2018 Schedule

date speaker title host(s)
January 26 Tasos Moulinos (UIC) Derived Azumaya Algebras and Twisted K-theory Michael
February 2 Daniel Erman (Wisconsin) TBA Local
February 8 2:30-3:30 in VV B113 Roman Fedorov (University of Pittsburgh) A conjecture of Grothendieck and Serre on principal bundles in mixed characteristic Dima
February 9 Juliette Bruce (Wisconsin) Asymptotic Syzygies in the Semi-Ample Setting Local
February 16 Andrei Caldararu (Wisconsin) Computing a categorical Gromov-Witten invariant Local
February 23 Aron Heleodoro (Northwestern) Normally ordered tensor product of Tate objects and decomposition of higher adeles Dima
March 2 Moisés Herradón Cueto (Wisconsin) Local type of difference equations Local
March 9 Eva Elduque (Wisconsin) On the signed Euler characteristic property for subvarieties of Abelian varieties Local
March 16 Harrison Chen (Berkeley) Equivariant localization for periodic cyclic homology and derived loop spaces Andrei
March 23 Phil Tosteson (Michigan) TBA Steven
April 6 Wei Ho TBA Daniel/Wanlin
April 13 Reserved Daniel
April 20 Alena Pirutka (NYU) TBA Jordan
April 27 Alexander Yom Din (Caltech) Drinfeld-Gaitsgory functor and contragradient duality for (g,K)-modules Dima
May 4 John Lesieutre (UIC) TBA Daniel

Abstracts

Tasos Moulinos

Derived Azumaya Algebras and Twisted K-theory

Topological K-theory of dg-categories is a localizing invariant of dg-categories over $\mathbb{C}$ taking values in the $\infty$-category of $KU$-modules. In this talk I describe a relative version of this construction; namely for $X$ a quasi-compact, quasi-separated $\mathbb{C}$-scheme I construct a functor valued in the $\infty$-category of sheaves of spectra on $X(\mathbb{C})$, the complex points of $X$. For inputs of the form $\operatorname{Perf}(X, A)$ where $A$ is an Azumaya algebra over $X$, I characterize the values of this functor in terms of the twisted topological K-theory of $X(\mathbb{C})$. From this I deduce a certain decomposition, for $X$ a finite CW-complex equipped with a bundle $P$ of projective spaces over $X$, of $KU(P)$ in terms of the twisted topological K-theory of $X$ ; this is a topological analogue of a result of Quillen’s on the algebraic K-theory of Severi-Brauer schemes.

Roman Fedorov

A conjecture of Grothendieck and Serre on principal bundles in mixed characteristic

Let G be a reductive group scheme over a regular local ring R. An old conjecture of Grothendieck and Serre predicts that such a principal bundle is trivial, if it is trivial over the fraction field of R. The conjecture has recently been proved in the "geometric" case, that is, when R contains a field. In the remaining case, the difficulty comes from the fact, that the situation is more rigid, so that a certain general position argument does not go through. I will discuss this difficulty and a way to circumvent it to obtain some partial results.

Juliette Bruce

Asymptotic Syzygies in the Semi-Ample Setting

In recent years numerous conjectures have been made describing the asymptotic Betti numbers of a projective variety as the embedding line bundle becomes more ample. I will discuss recent work attempting to generalize these conjectures to the case when the embedding line bundle becomes more semi-ample. (Recall a line bundle is semi-ample if a sufficiently large multiple is base point free.) In particular, I will discuss how the monomial methods of Ein, Erman, and Lazarsfeld used to prove non-vanishing results on projective space can be extended to prove non-vanishing results for products of projective space.

Andrei Caldararu

Computing a categorical Gromov-Witten invariant

In his 2005 paper "The Gromov-Witten potential associated to a TCFT" Kevin Costello described a procedure for recovering an analogue of the Gromov-Witten potential directly out of a cyclic A-inifinity algebra or category. Applying his construction to the derived category of sheaves of a complex projective variety provides a definition of higher genus B-model Gromov-Witten invariants, independent of the BCOV formalism. This has several advantages. Due to the categorical invariance of these invariants, categorical mirror symmetry automatically implies classical mirror symmetry to all genera. Also, the construction can be applied to other categories like categories of matrix factorization, giving a direct definition of FJRW invariants, for example.

In my talk I shall describe the details of the computation (joint with Junwu Tu) of the invariant, at g=1, n=1, for elliptic curves. The result agrees with the predictions of mirror symmetry, matching classical calculations of Dijkgraaf. It is the first non-trivial computation of a categorical Gromov-Witten invariant.

Aron Heleodoro

Normally ordered tensor product of Tate objects and decomposition of higher adeles

In this talk I will introduce the different tensor products that exist on Tate objects over vector spaces (or more generally coherent sheaves on a given scheme). As an application, I will explain how these can be used to describe higher adeles on an n-dimensional smooth scheme. Both Tate objects and higher adeles would be introduced in the talk. (This is based on joint work with Braunling, Groechenig and Wolfson.)

Local type of difference equations

The theory of algebraic differential equations on the affine line is very well-understood. In particular, there is a well-defined notion of restricting a D-module to a formal neighborhood of a point, and these restrictions are completely described by two vector spaces, called vanishing cycles and nearby cycles, and some maps between them. We give an analogous notion of "restriction to a formal disk" for difference equations that satisfies several desirable properties: first of all, a difference module can be recovered uniquely from its restriction to the complement of a point and its restriction to a formal disk around this point. Secondly, it gives rise to a local Mellin transform, which relates vanishing cycles of a difference module to nearby cycles of its Mellin transform. Since the Mellin transform of a difference module is a D-module, the Mellin transform brings us back to the familiar world of D-modules.

Eva Elduque

On the signed Euler characteristic property for subvarieties of Abelian varieties

Franecki and Kapranov proved that the Euler characteristic of a perverse sheaf on a semi-abelian variety is non-negative. This result has several purely topological consequences regarding the sign of the (topological and intersection homology) Euler characteristic of a subvariety of an abelian variety, and it is natural to attempt to justify them by more elementary methods. In this talk, we'll explore the geometric tools used recently in the proof of the signed Euler characteristic property. Joint work with Christian Geske and Laurentiu Maxim.

Harrison Chen

Equivariant localization for periodic cyclic homology and derived loop spaces

There is a close relationship between derived loop spaces, a geometric object, and (periodic) cyclic homology, a categorical invariant. In this talk we will discuss this relationship and how it leads to an equivariant localization result, which has an intuitive interpretation using the language of derived loop spaces. We discuss ongoing generalizations and potential applications in computing the periodic cyclic homology of categories of equivariant (coherent) sheaves on algebraic varieties.

Alexander Yom Din

Drinfeld-Gaitsgory functor and contragradient duality for (g,K)-modules

Drinfeld suggested the definition of a certain endo-functor, called the pseudo-identity functor (or the Drinfeld-Gaitsgory functor), on the category of D-modules on an algebraic stack. We extend this definition to an arbitrary DG category, and show that if certain finiteness conditions are satisfied, this functor is the inverse of the Serre functor. We show that the pseudo-identity functor for $(\mathfrak{g},K)$-modules is isomorphic to the composition of cohomological and contragredient dualities, which is parallel to an analogous assertion for p-adic groups.

In this talk I will try to discuss some of these results and around them. This is joint work with Dennis Gaitsgory.