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# Dmitri Burago (Penn State University), Colloquium

## April 6, 2017 @ 4:00 pm - 5:00 pm

**Tea at 3:30 pm in 330 Phillips Hall**

**Title:** A relaxed discussion of several problems involving Geometry, Dynamics, PDEs, Discretization, Control Theory and such.

**Abstract:** The lecture consists of several mini-talks with just definitions, motivations, some ideas of proofs, and open problems. I will discuss some (hardly all) of the following topics. Or something else:)

1. “A survival guide for feeble fish”. How fish can get from A to B in turbulent waters which maybe much fasted than the locomotive speed of the fish (provided that there is no large-scale drift of the water flow)? This is related to homogenization of G-equation which is believed to govern many combustion processes. Based on a joint work with S. Ivanov and A. Novikov.

2. One of the greatest achievements in Dynamics in the XX century is the KAM Theory. It says that a small perturbation of a non-degenerate completely integrable system still has an overwhelming measure of invariant tori with quasi-periodic dynamics. What happens outside KAM tori has been remaining a great mystery. The main quantitate invariants so far are entropies. It is easy, by modern standards, to show that topological

entropy can be positive. It lives, however, on a zero measure set. We are now able to show that metric entropy can become infinite too, under arbitrarily small C^{infty} perturbations, answering an old-standing problem of Kolmogorov.. Furthermore, a slightly modified construction resolves another long–standing problem of the existence of entropy non-expansive systems. In these modified examples positive positive metric

entropy is generated in arbitrarily small tubular neighborhood of one trajectory. Join with S. Ivanov and Dong. Chen.

3. “What is inside?” Imagine a body with some intrinsic structure, which, as usual, can be thought of as a metric. One knows distances between boundary points (say, by sending waves and measuring how long it takes them to reach specific points on the boundary). One may think of medical imaging or geophysics. This topic is related to minimal fillings and surfaces in normed spaces. Joint work with S. Ivanov.

4. How well can we approximate an (unbounded) space by a metric graph whose parameters (degree of vertices, length of edges, density of vertices etc) are uniformly bounded? We want to control the ADDITIVE error. Some answers (the most difficult one is for $\R^2$) are given using dynamics and Fourier series. Joint with Ivanov.

5.How can one discretize elliptic PDEs without using finite elements, triangulations and such? On manifolds and even reasonably “nice” mm–spaces. A notion of \rho-Laplacian and its stability. Joint with S. Ivanov and Kurylev.

6. A solution of Busemann’s problem on minimality of surface area in normed spaces for 2-D surfaces (including a new formula for the area of a convex polygon). joint with S. Ivanov.