Blaschke selection theorem

The Blaschke selection theorem is a result in topology and convex geometry about sequences of convex sets. Specifically, given a sequence ${\displaystyle \{K_{n}\}}$ of convex sets contained in a bounded set, the theorem guarantees the existence of a subsequence ${\displaystyle \{K_{n_{m}}\}}$ and a convex set ${\displaystyle K}$ such that ${\displaystyle K_{n_{m}}}$ converges to ${\displaystyle K}$ in the Hausdorff metric. The theorem is named for Wilhelm Blaschke.

• A succinct statement of the theorem is that the metric space of convex bodies is locally compact.
• Using the Hausdorff metric on sets, every infinite collection of compact subsets of the unit ball has a limit point (and that limit point is itself a compact set).

As an example of its use, the isoperimetric problem can be shown to have a solution.¹ That is, there exists a curve of fixed length that encloses the maximum area possible. Other problems likewise can be shown to have a solution:

• Lebesgue's universal covering problem for a convex universal cover of minimal size for the collection of all sets in the plane of unit diameter,¹
• the maximum inclusion problem,¹
• and the Moser's worm problem for a convex universal cover of minimal size for the collection of planar curves of unit length.²