Hessenberg varieties are subvarieties of flag varieties invented in the early 1990s by de Mari and Shayman. De Mari and Shayman were motivated by questions in applied linear algebra, but, very quickly, people realized that Hessenberg varieties are very interesting objects of study from the point of view of algebraic groups.
I got interested in Hessenberg varieties because of their connection to questions in combinatorics, in particular, the Stanley-Stembridge conjecture. I'll explain this conjecture, now a theorem due to Hikita, and I will explain some of my work with Tim Chow, which resolved a conjecture of Shareshian and Wachs connecting Hessenberg varieties directly to Stanley-Stembridge. (I'll also try to say a few words about Hikita's work and the very exciting state the field is in now.) Then I'll explain joint work with Escobar, Hong, Lee, Lee, Mellit and Sommers on the moduli of Hessenberg varieties.
The ideas of quantum physics have had a huge impact on the development of mathematics, all its fields have been influenced. Many notions have emerged, such as quantum groups and algebras, quantum calculus, many special functions. Numbers, the most elementary and ancient concept at the heart of mathematics since the Babylonians, should also have their place in the quantum landscape. This talk is an elementary and accessible overview of the emerging theory of quantum numbers, including motivations, first results, and the connection to other parts of mathematics.
Recall that a square matrix P is called a projection matrix if P^2 = P. It makes sense to talk about projection matrices with coefficients in any commutative ring; the image of a projection matrix is called a projective module. This seemingly innocuous notion intercedes in geometric questions in the same spirit as the famous Hodge conjecture because of Serre's dictionary: projective modules are ``vector bundles''. If X is a smooth complex affine variety, we can consider the rings of algebraic or holomorphic functions on X. Which of the holomorphic vector bundles on X admit an algebraic structure? I will discuss recent progress on these questions, using motivic homotopy theory, and based on joint work with Tom Bachmann and Mike Hopkins.
Holomorphic Dynamics (in a narrow sense) is the theory of the iteration of rational maps on the Riemann sphere. It was founded in the classical work by Fatou and Julia around 1918. After about 60 years of stagnation, it was revived in the 1980s, bringing together deep ideas from Conformal and Hyperbolic Geometry, Teichmüller Theory, the Theory of Kleinian Groups, Hyperbolic Dynamics and Ergodic Theory, and Renormalization Theory from physics, illustrated with beautiful computer-generated pictures of fractal sets (such as various Julia sets and the Mandelbrot set). We will highlight some landmarks of this story.
The search for an ultimate axiomatization of mathematics is inevitably incomplete. However, this does not preclude the possibility of strong and natural extensions to the standard axioms of set theory (ZFC) which shed light on many unresolved mathematical questions. Gödel suggested searching for strong axioms of infinity (known as large cardinals) as potential candidates for such extensions -- a pursuit that has continued for over half a century. This talk will survey several central developments in this search, including results concerning finite graphs, questions about Lebesgue measurability of projective sets of reals, the inner model program, and current efforts toward identifying an "ultimate" model of set theory via canonical inner models.
Holonomy is a prime example of mathematical intuition and creativity - it generalises our school knowledge about the sum of angles in a triangle and led to `Berger’s holonomy theorem’ from 1954 which turned out to be a most successful research programme for differential geometry for over 50 years. We are going to tell the story of this development, how holonomy relates to curvature and advanced symmetry concepts, including a small detour to theoretical physics and what Calabi-Yau manifolds have to do with it. We conclude by a small outlook to recent results.
A Riemannian metric is said to be Einstein if it has constant Ricci curvature. Certain peculiar features of 4-dimensional geometry make dimension four into a “Goldilocks zone” for Einstein metrics, with just the right amount of local flexibility managing to coexist with strong global rigidity results. This talk will first describe some aspects of the interplay between Einstein metrics and smooth topology on compact symplectic 4-manifolds without boundary. We will see how ideas from Kähler and conformal geometry allow us to construct Einstein metrics on many such manifolds, while a complimentary tool-box shows that these existence results are optimal in certain specific contexts. The talk will then conclude with a brief discussion of analogous results concerning complete Ricci-flat 4-manifolds.
Classifying real polynomial maps between spheres is a challenging problem in real algebraic geometry. Remarkably, this question has found recent applications in two seemingly unrelated fields:
- in spectral theory, it allowed to solve Kac's celebrated isospectral problem (Can one hear the shape of a drum?) for the connection Laplacian.
- in dynamical systems, it allowed to prove ergodicity for a certain class of partially hyperbolic flows (extensions of the geodesic flow on negatively-curved manifolds).
I will explain these problems and how they all connect together. No prerequisite required -- the talk is intended for a broad audience.