Instabilities in Hamiltonian systems and Arnold diffusion(presentation)
               The strongest conjecture concerning instabilities of Hamiltonian 
                systems, the quasi-ergodic hypothesis, is wide open. A less difficult 
                question, called Arnold diffusion, asks whether a typical nearly 
                integrable system exhibit real topological instability. We review 
                some background, and describe a proof of Arnold diffusion in two 
                and half degrees of freedom. Our approach is based on normally 
                hyperbolic cylinders and Mather variational method, based on joint 
                works with P. Bernard and V. Kaloshin.
              
              
              The Back2Fields Colloquium Series celebrates the accomplishments 
              of former postdoctoral fellows of Fields Institute thematic programs. 
              Over the past two decades, these programs attracted the rising stars 
              of their field and often launches very distinguished research careers. 
              As part of the 20th anniversary celebrations, this series of colloquium 
              talks will allow the general mathematical public to become familiar 
              with some of their work.
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