Polaron theory is a model of an electron in a crystal lattice.
It is studied in the framework of nonequilibrium statistic mechanics.
There are two different mathematical models: H. Frohlich proposed a
quantum model in 1937; L. Landau and S. I. Pekar proposed a system of
nonlinear PDEs in 1948. In this talk I will present a proof that these
two models are equivalent to certain orders, and present some other
related works. These are joint works with Rupert Frank.