Calendar

will speak on:

The goal of this talk is to precisely describe how certain operator properties of continuum QFT (e.g. operator product expansions, current algebras, vertex operator algebras) emerge from an underlying lattice theory.  The main lesson will be that a “continuum limit” must always involve two or more cutoffs being taken to zero in a specific order.  In other words, the naive statement that continuum theories are obtained from lattice ones by letting a “lattice spacing” go to zero is never sufficient to describe the lattice-continuum correspondence.  Using these insights, I will show in detail how the Kac-Moody algebra arises from a nonperturbatively well defined, fully regularized model of free fermions, and I will comment on generalizations and applications to bosonization.  Time permitting, I will describe more intricate examples involving scalar fields, and I will discuss several open questions.

via Zoom Video Conferencing:  https://harvard.zoom.us/j/837429475

via Zoom Video Conferencing:  https://harvard.zoom.us/j/779283357

We study the nonlinear Schroedinger equation (NLS) with bounded initial data which
does not vanish at infinity. Examples include periodic, quasi-periodic and random initial data.
On the lattice we prove that solutions are polynomially bounded in time for any bounded data.
In the continuum, local existence is proved for real analytic data by a Newton iteration scheme.
Global existence for NLS with a regularized nonlinearity follows by analyzing a local energy norm.

This is joint work with B. Dodson and A. Soffer.

via Zoom Video Conferencing: link TBA

The anisotropic Calderon inverse problem consists in recovering the metric of a compact connected Riemannian manifold with boundary from the knowledge of the Dirichlet-to-Neumann map at fixed energy. A fundamental result due to Lee and Uhlmann states that there is uniqueness in the analytic case. We shall present counterexamples to uniqueness in cases when: 1) The metric smooth in the interior of the manifold, but only Holder continuous on one connected component of the boundary, with the Dirichlet and Neumann data being measured on the same proper subset of the boundary. 2) The metric is smooth everywhere and Dirichlet and Neumann data are measured on disjoint subsets of the boundary. This is joint work with Thierry Daude (Cergy-Pontoise) and Francois Nicoleau (Nantes).

I will talk about some ideas that are essential to build a general framework for topological phases in arbitrary dimensions. I will also discuss how these ideas are applied when global symmetry or higher symmetry is present, and how to understand and classify such higher SET/SPT orders.

via Zoom Video Conferencing:  https://harvard.zoom.us/j/977347126

via Zoom Video Conferencing: https://harvard.zoom.us/j/972495373

Strata of abelian differentials have long been of interest for their dynamical and algebro-geometric properties, but relatively little is understood about their topology. I will describe a project aimed at understanding the (orbifold) fundamental groups of non-hyperelliptic stratum components. The centerpiece of this is the monodromy representation valued in the mapping class group of the surface relative to the zeroes of the differential. For g \ge 5, we give a complete description of this as the stabilizer of the framing of the (punctured) surface arising from the flat structure associated to the differential. This is joint work with Aaron Calderon.

will speak on:

Spacetimes with compact directions, which have special holonomy such as Calabi-Yau spaces, play an important role in supergravity and string theory. In this talk I will discuss the global, non-linear stability for the vacuum Einstein equations on a spacetime which is a cartesian product of a high dimensional Minkowski space with a compact Ricci flat internal space with special holonomy. I will start by giving a brief overview of related stability problems which have received a lot of attention recently, including the black hole stability problem. This is based on joint work with Pieter Blue, Zoe Wyatt and Shing-Tung Yau.

*via Zoom Video Conferencing: https://harvard.zoom.us/j/952543678

via Zoom Video Conferencing: https://harvard.zoom.us/j/977347126

Quantum spin systems are many-body models which are of wide interest in modern physics and at the same time amenable to rigorous mathematical analysis. A central question about a quantum spin system is whether its Hamiltonian exhibits a spectral gap above the ground state. The existence of such a spectral gap has far-reaching consequences, e.g., for the ground state complexity. In this talk, we survey recent progress regarding spectral gaps for frustration-free quantum spin systems in dimensions greater than 1 such as the antiferromagnetic models of Affleck-Kennedy-Lieb-Tasaki (AKLT).