Subcomplexes of aspherical 2-complexes stay aspherical. Open since 1941; LOT complexes from ribbon discs are the test cases.
The Problems
Not schoolwork — the questions that resisted Erdős, Hilbert, and everyone since. The club keeps three shelves: what is still open, what AI recently settled, and what took humanity centuries.
Tagged algebraic-topology — 1 entry. Clear