Dmitry Treschev (Steklov Mathematical Institute of RAS, Lomonosov Moscow State University)
  Periscope theorem.

Let F: D₁→ D₂ be a diffeomorphism of two domains on a plane. Consider a parallel bundle of light rays through D₁ perpendicular to the plane. We consider the following problem.

Find a system of mirrors which transforms this bundle to a parallel bundle through D₂ perpendicular to the plane such that any ray through x∈ D₁ is transformed to the ray through F(x)∈D₂.