Algebraic Geometry Seminar Fall 2010

The seminar meets on Fridays at 2:25 pm in Van Vleck B305.

Fall 2010

date	speaker	title	host(s)
sept 24	Xinyi Yuan (Harvard)	Calabi-Yau theorem and algebraic dynamics	Tonghai
oct 1	Dawei Chen (UIC)	Geometry of Teichmuller curves	Andrei
oct 8	Tom Church (U Chicago)	Representation stability, homological stability, and configurations over finite fields	Jordan
oct 15	Conan Leung (CUHK)	The quantum cohomology of G/P	Andrei
oct 22	Zhiwei Yun (Berkeley)	Springer representation and Hitchin fibration	Shamgar
oct 23-24 (Saturday/Sunday)	Midwest Algebraic Geometry Conference for Graduate Students	[Conference Website http://www.math.wisc.edu/~magc]	UW-Madison Math	=	oct 29	Christian Schnell (UIC)	TBA	Laurentiu
nov 5	Daniel Erman (Stanford)	TBA	Jordan
nov 11	Luca Scala (Chicago)	TBA	Andrei
nov 19	Alina Marian (UIC)	TBA	Andrei
dec 3	Matt Satriano (UMich)	TBA	David Brown
dec 10	Izzet Coskun (UIC)	TBA	Andrei

Abstracts

Xinyi Yuan Calabi-Yau theorem and algebraic dynamics

The uniqueness part of the Calabi-Yau theorem asserts that the Monge-Ampere measure of a (complex) positive hermitian line bundles determines the hermitian metric up to constant. Here we introduce a p-adic analogue of the theorem. Combinning with the equidistribution theory, we obtain the rigidity of preperiodic points on algebraic dynamical systems.

Dawei Chen Geometry of Teichmuller curves

We study Teichmuller curves parameterizing square-tiled surfaces (i.e. covers of elliptic curves with a unique branch point).

The results can be applied to the following problems in algebraic geometry and complex dynamics: (a) construct rigid curves on the moduli space of pointed rational curves; (b) bound the effective cone of the moduli space of genus g curves; (c) verify the invariance of Siegel-Veech constants; (d) calculate the Lyapunov exponents of the Hodge bundle.

Tom Church Representation stability, homological stability, and configurations over finite fields

Homological stability is a remarkable phenomenon where for certain sequences X_n of groups or spaces -- for example SL(n,Z), the braid group B_n, or the moduli space M_n of genus n curves -- it turns out that the homology group H_i(X_n) does not depend on n once n is large enough. However, there are many natural analogous sequences, from pure braid groups to congruence groups to Torelli groups, for which homological stability fails horribly. In these cases the rank of H_i(X_n) blows up to infinity, and in the latter two cases almost nothing is known about H_i(X_n); indeed it's possible there is no nice "closed form" for the answers.

While doing some homology computations for the Torelli group, we found what looked like the shadow of an overarching pattern. In order to explain it and to formulate a specific conjecture, we came up with a notion of "stability of a sequence of representations of groups". This makes it possible to meaningfully talk about "the stable homology of the pure braid group" or "the stable homology of the Torelli group" even though homological stability fails. We have proved that many important sequences are representation-stable in this sense, including the homology of pure braid groups, configuration spaces of manifolds, and certain Malcev Lie algebras, as well as sequences not arising from homology. For other cases, including Torelli groups and congruence subgroups, this notion provides a natural source of analogies and conjectures. In this talk I will explain our broad picture via some of its many instances, focusing on pure braid groups and configuration spaces. This work is joint with Benson Farb. One striking application is a surprisingly strong connection between representation stability for pure braid groups and counting problems for polynomials over finite fields, joint with Jordan Ellenberg and Benson Farb.

Zhiwei Yun Springer representation and Hitchin fibration

Classical Springer representation is the action of the Weyl group on the cohomology of certain subvarieties of the flag variety. I will construct a global analogue of this action, namely, an action of the graded double affine Hecke algebra on the cohomology of parabolic Hitchin fibers. Examples in SL(2) will be described in details. This construction is motivated by Ngo's proof of the fundamental lemma, and has applications to the harmonic analysis on p-adic groups.