15:45
Liouville quantum gravity as a mating of trees
Abstract
There is a simple way to “glue together” a coupled pair of continuum random trees to produce a topological sphere. The sphere comes equipped with a measure and a space-filling curve (which describes the “interface” between the trees). We present an explicit and canonical way to embed the sphere into the Riemann sphere. In this embedding, the measure is Liouville quantum gravity with parameter gamma in (0,2), and the curve is space-filling version of SLE with kappa=16/gamma^2. Based on joint work with Bertrand Duplantier and Scott Sheffield
Conjugacy classes and group representations
Abstract
One of the big ideas in linear algebra is {\em eigenvalues}. Most matrices become in some basis {\em diagonal} matrices; so a lot of information about the matrix (which is specified by $n^2$ matrix entries) is encoded by by just $n$ eigenvalues. The fact that lots of different matrices can have the same eigenvalues reflects the fact that matrix multiplication is not commutative.
I'll look at how to make these vague statements (``lots of different matrices...") more precise; how to extend them from matrices to abstract symmetry groups; and how to relate abstract symmetry groups to matrices.
Are resultant methods numerically unstable for multidimensional rootfinding
Abstract
they are competitive practical rootfinders. However, in higher dimensions they are known to be notoriously difficult, if not impossible, to make numerically robust. We will show that the most popular variant based on the Cayley resultant is inherently and spectacularly numerically unstable by a factor that grows exponentially with the dimension. Disastrous. Yet, perhaps, it can be circumnavigated.
17:30
Social Capital and Microfinance
Abstract
Sparse graph limits and scale-free networks
Abstract
We introduce and develop a theory of limits for sequences of sparse graphs based on $L^p$ graphons, which generalizes both the existing $L^\infty$ theory of dense graph limits and its extension by Bollob\'as and Riordan to sparse graphs without dense spots. In doing so, we replace the no dense spots hypothesis with weaker assumptions, which allow us to analyze graphs with power law degree distributions. This gives the first broadly applicable limit theory for sparse graphs with unbounded average degrees.
Joint work with Christian Borgs, Jennifer T. Chayes, and Henry Cohn.
Null singularities in general relativity
Abstract
We consider spacetimes arising from perturbations of the interior of Kerr
black holes. These spacetimes have a null boundary in the future such that
the metric extends continuously beyond. However, the Christoffel symbols
may fail to be square integrable in a neighborhood of any point on the
boundary. This is joint work with M. Dafermos
(HoRSE seminar) Real variation of stabilities and equivariant quantum cohomology II
Abstract
I will describe a version of the definition of stability conditions on a triangulated category to which we were led by the study of quantization of symplectic resolutions of singularities over fields of positive characteristic. Partly motivated by ideas of Tom Bridgeland, we conjectured a relation of this structure to equivariant quantum cohomology; this conjecture has been verified in some classes of examples. The talk is based on joint projects with Anno, Mirkovic, Okounkov and others
(HoRSE seminar) Real variation of stabilities and equivariant quantum cohomology I
Abstract
I will describe a version of the definition of stability conditions on a triangulated category to which we were led by the study of quantization of symplectic resolutions of singularities over fields of positive characteristic. Partly motivated by ideas of Tom Bridgeland, we conjectured a relation of this structure to equivariant quantum cohomology; this conjecture has been verified in some classes of examples. The talk is based on joint projects with Anno, Mirkovic, Okounkov and others