# "Well-known fact" that every irreducible 3-manifold with non-empty boundary has an incompressible surface

I have seen in several sources that this results holds, however none of them included the proof. Does anyone know where I can find one?

Also, it would be great if someone could provide me with a counterexample, where irreducibility of the manifold matters.

Thank you.

The proof might be too long for this fact. However, here is one reference

Algorithmic Topology and Classification of 3-Manifolds by Sergei Matveev in the series Algorithms and computations in Mathematics, Volume 9, 2003, Springer-Verlag.

You may start reading from page 167.

Counterexample: the 3-ball $$B^3$$. It is irreducible because it is a compact submanifold of $$\mathbb{R}^3$$ with connected boundary. It has non-empty boundary and $$\partial B$$ is simply connected (and incompressible surfaces with boundary must send their boundary to curves which do not bound disks in $$\partial B$$). Hence if there is an incompressible surface it must be closed. Non orientable closed surfaces do not embed in $$B^3$$. Moreover, $$B^3$$ is simply connected and recall that by Dehn's lemma an orientable, closed surface with $$\chi(S)\leq 0$$ is incompressible if the inclusion is injective at the level of fundamental group.

However the following is true:

If $$M$$ is compact with non-empty boundary, oriented , irreducible, and $$\partial$$-irreducible* then either $$M=B^3$$ or $$M$$ contains an incompressible and $$\partial$$-incompressible surface.

This result relies on the fact that (under the above assumptions), given a class in $$H_2(M,\partial M;\mathbb{Z})$$ you can represent it by disjoint union of incompressible and and $$\partial$$-incompressible surfaces. *

You can find all the details for example in Bruno Martelli. An Introduction to Geometric Topology. https://arxiv.org/pdf/1610.02592.pdf Proposition 9.4.3 and Corollary 9.4.5.

*Meaning that there are no essential disks, in other words $$M$$ is not obtained by joining two 3-manifolds with a 1-handle.

• @Haldot The definition of incompressible surface I use (also used in the book mentioned above) concerns surfaces with non-positive Euler characteristic. This rules out 2-disks. In the case of 2-disks the interesting notion is the one of essential disk, which are disks that are not boundary parallel (i.e. are not obtained by pushing the interior or a disk in the boundary $\partial M$ into the interior of $M$). Jun 27, 2020 at 18:35
• This is the "correct" notion of essential disk (as an essential sphere is one that does not bound a B^3 and an essential surface with $\chi\leq 0$ in necessarily incompressible) because as in the case of spheres finding an essential sphere allows you to decompose the manifold wrt connected sum, fiinding an essential 2-disk allows you to decompose the 3-manifold with respect to boundary connected sum) Jun 27, 2020 at 18:35
• Oh, sorry, I deleted my comment before I saw your answer. Thanks! Jun 27, 2020 at 18:43