title: Alexandrov theorem
text: # Alexandrov theorem
  
  In mathematical analysis, the Alexandrov theorem, named after Aleksandr Danilovich Aleksandrov, states that if U is an open subset of $\mathbb {R} ^{n}$ and $f\colon U\to \mathbb {R} ^{m}$ is a convex function, then $f$ has a second derivative almost everywhere.
  
  In this context, having a second derivative at a point means having a second-order Taylor expansion at that point with a local error smaller than any quadratic.
  
  The result is closely related to Rademacher's theorem.
  
  