Let $X$ be a finite dimensional (possibly compact) Alexandrov space with curvature $\geq K$. **Is it true that its boundary is again Alexandrov space with curvature bounded from below? If yes, is the curvature at least $K$?**

## 1 Answer

This is an open problem.

It is a special case of the following question:

Is it true that every extremal subset is again an Alexandrov space?

The answer to this question is "No". Petrunin has constructed a counterexample in codimension three, here.