# cobordism

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

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

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## Slice-ribbon problem ★★★★

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

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