Periodic Ordinary Differential Operators

The chapter contains a brief survey of the classical Floquet theory for ordinary differential equations, which is described in many well-known books. Important notions and results are introduced, which are guiding lights for studying the PDE case. It ends with the disappointing conclusion that the direct analog of this approach does not seem to work that well for PDEs. Fortunately, as the rest of the book shows, the theory can be salvaged and comes out of this even more beautiful.