# Past Junior Geometry and Topology Seminar

24 October 2013
16:00
to
17:30
Tom Hawes
Abstract
Consider a smooth, complex projective variety X inside P^n and an action of a reductive linear algebraic group G inside GL(n+1,C). On the one hand, we can view this as an algebra-geometric set-up and use geometric invariant theory (GIT) to construct a quotient variety X // G, which parameterises `most' of the closed orbits of X. On the other hand, X is naturally a symplectic manifold, and since G is reductive we can take a maximal real compact Lie subgroup K of G and consider the symplectic reduction of X by K with respect to an appropriate moment map. The Kempf-Ness theorem then says that the results of these two constructions are homeomorphic. In this talk I will define GIT and symplectic reduction and try to sketch the proof of the Kempf-Ness theorem.
• Junior Geometry and Topology Seminar
17 October 2013
16:00
to
17:30
Jakob Blaavand
Abstract
In the talk we will discuss Quillen's construction of a determinant line bundle associated to a family of Cauchy-Riemann operators. I will first of all try to convince you why this is a cool thing and mention some of the many different applications. The bulk of the talk will be focused on constructing the line bundle, its hermitian metric and calculating the curvature. Hopefully a talk accessible to many.
• Junior Geometry and Topology Seminar
13 June 2013
15:00
Cancelled
Abstract
• Junior Geometry and Topology Seminar
6 June 2013
15:00
Hemanth Saratchandran
Abstract
I will show how to associate a quaternion algebra to a hyperbolic 3-manifold. I will then go through some examples and applications of this theory
• Junior Geometry and Topology Seminar
30 May 2013
12:00
Vittoria Bussi
Abstract
This talk is not a detailed and precise exposition on DAG, but it is conceived more as a kind of advertisement on this theory and some of its interesting new features one should contemplate and try to understand, as they might reveal interesting new insights also on classical objects. We select some of the several motivations for introducing it (non-representability of moduli problem and non-naturality of the obstruction theory), and then we will go through the homotopy theory of simplicial commutative algebras and their cotangent complex. We will introduce the category of derived schemes and we will describe their relation with classical schemes. A good amount of time will be dedicated to examples.
• Junior Geometry and Topology Seminar
23 May 2013
15:00
Rafael Torres
Abstract
"Among the first successes of the h-cobordism theorem was the classification of simply connected closed 5-manifolds. Dimension five is sufficiently large to be able to implement the tools of surgery theory, yet low enough to allow an explicit classification of the manifolds. These traits make dimension five interesting in terms of existence results of geometric structures, like Riemannian metrics of positive Ricci/nonnegative sectional/positive sectional curvature, Einstein metrics, contact structures, Sasakian structures, among others. The talk will be a limited survey of the five-dimensional symbiosis between topology and geometry"
• Junior Geometry and Topology Seminar
16 May 2013
15:00
Robert Kropholler
Abstract
I will be taking us on a journey through low dimensional topology, starting in 2 dimensions motivating handles decompositions in a dimension that we can visualize, moving onto to a brief of note of what this means in 3 dimensions and then moving onto the wild world of 4 manifolds. I will be showing a way in which we can actually try and view a 4 manifold before moving onto a way of manipulating these diagrams to give diffeomorphic 4 manifolds. Hopefully, I will have time to go into some ways in which Kirby calculus has been used to show that certain potential exotic 4 spheres are not exotic and some results on stable diffeomorphims of 4 manifolds.
• Junior Geometry and Topology Seminar
9 May 2013
15:00
Alejandro Betancourt
Abstract
Based on ideas from Eells and Sampson, the Ricci flow was introduced by R. Hamilton in 1982 to try to prove Thurston's Geometrization Conjecture (a path which turned out to be successful). In this talk we will introduce the Ricci flow equation and view it as a modified heat flow. Using this we will prove the basic results on existence and uniqueness, and gain some insight into the evolution of various geometric quantities under Ricci flow. With this results we will proceed to define Perelman's $\mathcal{F}$ and $\mathcal{W}$ entropy functionals to view the Ricci flow as a gradient flow. If time permits we will briefly sketch some results from Cheeger and Gromov's compactness theory, which, along with the entropy functionals, alow us to blow up singularities.This is meant to be an introductory talk so I will try to develop as much geometric intuition as possible and stay away from technical calculations.
• Junior Geometry and Topology Seminar
2 May 2013
15:00
Subhojoy Gupta
Abstract
We shall introduce complex projective structures on a surface, and discuss a new result that relates grafting, which are certain geometric deformations of these structures, to the Teichmuller geodesic flow in the moduli space of Riemann surfaces. A consequence is that for any Fuchsian representation of a surface-group, the set of projective structures with that as holonomy, is dense in moduli space.
• Junior Geometry and Topology Seminar
7 March 2013
15:00
Benjamin Volk
Abstract
This talk will give a quick and dirty introduction to orbifold bordism. We will start by briefly recalling some basic properties and definitions of orbifolds and sketch (very roughly) how orbifolds can be defined in the language of $C^\infty$-stacks due to Joyce (after introducing these). We will then review classical bordism theory for manifolds (in some nonstandard way) and discuss which definitions and results generalize to the orbifold case. A word of warning: this talk is intended to be an introduction and wants to give an overview over the subject, so it is likely that we will be sloppy here and there.
• Junior Geometry and Topology Seminar