# cobordism

## Which homology 3-spheres bound homology 4-balls? ★★★★

Author(s): Ancient/folklore

**Problem**Is there a complete and computable set of invariants that can determine which (rational) homology -spheres bound (rational) homology -balls?

Keywords: cobordism; homology ball; homology sphere

## Slice-ribbon problem ★★★★

Author(s): Fox

**Conjecture**Given a knot in which is slice, is it a ribbon knot?