Geometric evolution equations like the Ricci flow and the mean curvature flow play a central role in differential geometry. The main problem is to understand singularity formation. In this talk, I will discuss recent results which give a complete picture of all the possible limit flows in 2D mean curvature flow with positive mean curvature, and in 3D Ricci flow.
Godement-Jacquet use the Schwartz space of n-by-n matrices to construct the standard L-function for GL_n. Ben-Zvi, Sakellaridis and Venkatesh conjecture that the local unramified part of this theory can be categorified to an equivalence between an 'analytic' category of constructible sheaves and a 'spectral' category of dg modules. In this talk I will explain the proof of this equivalence and some of its properties. I will also discuss connections to conjectures of Braverman-Kazhdan on constructions of general automorphic L-functions. This is joint work with Tsao-Hsien Chen (in preparation).
Mathematics can be tasty! Its a way of thinking, and not just about numbers. Through unexpectedly connected examples from music, juggling, and baking, I will show that math can be made fun and intriguing for all, through hands-on activities, examples that everyone can relate to, and funny stories. I'll present surprisingly high-level mathematics, including some advanced abstract algebra usually only seen by math majors and graduate students. There will be a distinct emphasis on edible examples.
Data science @ the new york times - what is data science? - where did it come from? - what does it mean at NYT? - what does it mean at Columbia? This is a public lecture targeted at a general audience. Undergraduates are particularly encouraged to come.
I will discuss the relative trace formula approach to the global Gan-Gross-Prasad conjectures for unitary groups. The focus will be on the spectral side. I will present the various terms that appear in the spectral development of the relative trace formula and discuss what is still missing. This is a joint work with Pierre-Henri Chaudouard.
Laplace gave the simplest early statement of reductionism. His Demon, if supplied with the positions and momenta of all the particles in the universe, could, using Newton's laws, calculate the entire future and past of the universe. Add fields, quantum mechanics, and General Relativity and you have, roughly, modern physics. There are four features to Laplace's reductionism: (I) Everything that happens is deterministic, called into question a century later by quantum mechanics and the familiar Copenhagen interpretation and Born rule. (ii) All that is ontologically real are "nothing but" particles in motion. (iii) All that happens in the universe is describable by universal laws. (iv) There exists at least one language able to describe all of reality. Quantum mechanics is evidence against (i). I will argue that biological evolution, the coming into existence in the universe of hearts and humming birds co-evolving with the flowers that feed them and that they pollenate, cannot be deduced or simulated from the basic laws of physics. In Weinberg's phrase, they are not entailed by the laws of physics. I will then claim that at levels above the atom, the universe will never make all possible proteins length 200 amino acids, all possible organisms, or all possible social systems. The universe is indefinitely open upwards in complexity. More, proteins, organisms, and social systems are ontologically real, not just particles in motion. Most radically, I will contest (iii). I will try to show that we cannot pre-state Darwinian pre-adaptations, where a pre-adaptation is a feature of an organism of no use in the current selective environment, but of use in a different environment, hence selected for a novel function. Swim bladders are an example. Let me define the "adjacent possible" of the biosphere. Once there were the lung fish that gave rise to swim bladders, swim bladders were in the adjacent possible of the biosphere. Before there were multi-celled organisms, swim bladders were not in the adjacent possible of the biosphere. What I am claiming is that we cannot pre-state the adjacent possible of the biosphere. How could we pre-state the selective conditions? How could we pre-specify the features of one or several organisms that might become pre-adaptations? How could we know that we had completed the list? The implications are profound, if true. First, we can make no probability statement about pre-adaptations, for we do not know the sample space, so can formulate no probability measure. Most critically, if a natural law is a compact description before hand and afterward of the regularities of a process, then there can be no natural law sufficient to describe the emergence of swim bladders. Thus, the unfolding of the universe is partially lawless! This contradicts our settled convictions since Descartes, Galileo, Newton, Einstein and Schrödinger. It says that (iii) is false. In place of law is a ceaseless creativity, a self consistent self construction of the biosphere, the economy, our cultures, partially beyond law. Were reductionism sufficient, the existence of swim bladders in the universe would be entailed by physical law, hence "explained". But it appears that physics, as stated, is not sufficient in its reductionist version. Then we must explain the existence in the universe of swim bladders and humming birds pollenating flowers that feed them, on some different ground. We need a post-reductionist science. Autocatalytic mutualisms of organisms, the biosphere, and much of the economy, may be part of the explanation we seek. In turn this raises profound questions about how causal systems can coordinate their behaviors, let alone the role of energy, work, power, power efficiency, in the self-consistent construction of a biosphere. There is a lot to think about.
We introduce a new second order parabolic evolution equation where the speed is given by the reciprocal of the null mean curvature. This flow is a generalisation of inverse mean curvature flow and it is motivated by the study of black holes and mass/energy inequalities in general relativity. We present a theory of weak solutions using level-set methods and an appropriate variational principle, and outline a natural application of the flow as a variational approach to constructing marginally outer trapped surfaces (MOTS), which play the role of quasi-local black hole boundaries in general relativity.
The recovery of a data matrix from a sampling of its entries is a problem of considerable practical interest. In partially filled out surveys, for instance, we would like to infer the many missing entries. In the area of recommender systems, users submit ratings on a subset of entries in a database, and the vendor provides recommendations based on the user's preferences. Because users only rate a few items, we would like to infer their preference for unrated items (the famous Netflix problem). Formally, suppose that we observe m entries selected uniformly at random from a matrix. Can we complete the matrix and recover the entries that we have not seen? Surprisingly, one can recover low-rank matrices exactly from what appear to be highly incomplete sets of sampled entries; that is, from a minimally sampled set of entries. Further, perfect recovery is possible by solving a simple convex optimization program, namely, a convenient semi-definite program. We show that our methods are optimal and succeed as soon as recovery is possible by any method whatsoever. Time permitting, we will also present a very efficient algorithm based on iterative singular value thresholding, which can complete matrices with about a billion entries in a matter of minutes on a personal computer.
We show that the distributions occurring in the geometric and spectral side of the twisted Arthur-Selberg trace formula extend to non-compactly supported test functions. The geometric assertion is modulo a hypothesis on root systems proven among other cases, when the group is split. This result extends the work of Finis-Lapid (and Muller, spectral side) in the non-twisted setting. In the end, we will give an application towards residues of Rankin-Selberg L-functions suggested by J. Getz.
We consider the Dirichlet problem for curve shortening flow on surfaces of constant curvature and show long-time existence of the flow when the initial curve is embedded in a convex region. Furthermore, the limit curve of the flow is a geodesic. The proof relies on an adaptation of Huisken's distance comparison estimate for planar curves, a maximum principle of Angenent, and a blow-up analysis of singularities.
I will introduce a product construction in contact topology for Legendrian submanifolds, focusing on products of Legendrian knots. I will then discuss ongoing work to compute a product formula for the Legendrian contact homology invariant and some of the geometric and analytic difficulties involved. In particular, I will describe Ekholm's Morse-theoretic approach to counting holomorphic curves and how to apply it to compute invariants of products of Legendrian knots.
I will present the proof of the random homogenization of general coercive Hamiltonian in 1d with the form as H(p,x,\omega)=H(p)+V(x, \omega). Some interesting and complex phenomena associated with non-convex Hamiltonian will also be discussed. This is a joint work with Scott Armstrong and Hung Tran.
The Mordell-Weil theorem shows that the rational points on an elliptic curve defined over the field of rational numbers is a finitely generated abelian group. The Birch and Swinnerton-Dyer conjecture relates the rank of this group to a number of analytic and algebraic invariants of the curve. (More generally it considers an elliptic curve defined over a number field.) The conjecture is one of the Millennium Prize problems and the Clay Institute is offering a reward of 1 million dollars for a solution. This talk will be an introduction to the conjecture. In following weeks we will have lectures explaining each of the terms in the formula.