Non-equilibrium steady states
The mathematical physics of mechanical systems in thermal equilibrium is a well studied, and relatively easy, subject, because the Gibbs distribution is in general an adequate guess for the equilibrium state. On the other hand, the mathematical physics of {\em non-equilibrium} systems, such as that of a chain of masses connected with springs to two (infinite) heat reservoirs is more difficult, precisely because no such {\em a priori} guess exists. Recent work has, however, revealed that under quite general conditions, such states can not only be shown to exist, but are {\em unique}, using the H\"ormander conditions and controllability. Furthermore, interesting properties, such as energy flux, exponentially fast convergence to the unique state, and fluctuations of that state have been successfully studied. Finally, the ideas used in these studies can be extended to certain stochastic PDE's using Malliavin calculus to prove regularity of the process.