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?