Landau's constants

Summary

Landau's constants are three numbers that measure how large a disc the image of a normalised holomorphic function on the unit disc must contain. They are the Bloch constant $B$, the Landau constant $L$ and the univalent Landau constant $A$, and they satisfy $B \le L \le A$. They are positive by Bloch's theorem, but their exact values are open problems.