# Non-orientable prime three-manifolds

Recall that a manifold is called prime if whenever it is homeomorphic to a connected sum, one of the two summands is homeomorphic to a sphere.

It follows from the classification of surfaces that the closed prime surfaces are $$S^2$$, $$T^2$$, and $$\mathbb{RP}^2$$. Moreover, every closed surface decomposes as a connected sum of prime manifolds, and in the orientable case, the decomposition is unique up to reordering and $$S^2$$ summands (in the non-orientable case, one can restore uniqueness by prohibiting the use of $$T^2$$).

There is a similar story for closed three-manifolds: they always decompose as a connected sum of prime manifolds, and the decomposition is unique up to reordering and $$S^3$$ summands if the manifold is orientable, and uniqueness can be restored in the non-orientable case by prohibiting the use of $$S^2\times S^1$$. However, there are infinitely many prime three-manifolds. In the orientable case, they fit into three categories:

1. those manifolds covered by $$S^3$$,
2. the manifold $$S^2\times S^1$$, and
3. orientable aspherical manifolds.

These categories can also be characterised via the fundamental group: namely finite, infinite cyclic, and infinite non-cyclic respectively.

Is there a similar categorisation of closed non-orientable prime three-manifolds?

One might suspect that such a categorisation immediately follow from the orientable case by passing to the orientable double cover. However, as Row shows in this paper, there exist closed non-orientable prime three-manifolds whose orientable double cover is not prime.

No, there is no such classification, there are way too many such manifolds. A standard way to construct these is to take a closed aspherical 3-manifold $$L$$ which admits an orientation-reversing involution with nonempty finite fixed-point set $$\tau$$. The quotient $$L/\tau$$ is an oribifold with even number of cone-points. Such orbifolds can be utterly wild, you can realize any finitely-presented group as its topological fundamental group:
Cut out conical neighborhoods of these cone-points, you get a compact 3-manifold $$M$$ with even number of boundary $$RP^2$$'s. Glue them in pairs to get a closed 3-manifold $$N$$. One can show that $$N$$ is prime but it is far from aspherical. More generally, you can take several manifolds $$M$$ like that and glue them along boundary projective planes. This construction is a little dirty secret of 3D topology, which is why people in the field prefer to work with oriented manifolds. It also explains that in order to truly geometrize nonorientable 3-manifolds, one is forced to enlarge the category and work with orbifolds: In this category one modifies the notion of the connected sum by allowing removing not only balls but orbi-balls as well. Accordingly, the notion of a primeness has to be modified as well. This is strangely similar to the MMP in algebraic geometry.