# Mathematics Colloquium

#### Arnold Diffusion Via Invariant Cylinders and Mather Variational Method

**Speaker:**
Vadim Kaloshin, University of Maryland

**Location:**
Warren Weaver Hall 1302

**Date:**
Monday, April 16, 2012, 3:45 p.m.

**Synopsis:**

The famous ergodic hypothesis claims that a typical Hamiltonian dynamics on a typical energy surface is ergodic. However, KAM theory disproves this. It establishes a persistent set of positive measure of invariant KAM tori. The (weaker) quasi-ergodic hypothesis, proposed by Ehrenfest and Birkhoff, says that a typical Hamiltonian dynamics on a typical energy surface has a dense orbit. This question is wide open.

In early 60th Arnold constructed an example of instabilities for a nearly integrable Hamiltonian of dimension \(n>2\) and conjectured that this is a generic phenomenon, nowadays, called Arnold diffusion. In the last two decades a variety of powerful techniques to attack this problem were developed. In particular, Mather discovered a large class of invariant sets and a delicate variational technique to shadow them. In a series of preprints: one joint with P. Bernard, K. Zhang and two with K. Zhang we prove Arnold's conjecture in dimension \(n=3\).