# Math Help - Covering Spaces

1. ## Covering Spaces

Hi it would be of great assistance if :

Use Galois correspondence to find all the covering spaces of
S^1 x S^1 x RP^2 (real projective plane is RP^2)

Any help would be great. Thanks

2. Originally Posted by sorrow
Hi it would be of great assistance if :

Use Galois correspondence to find all the covering spaces of
S^1 x S^1 x RP^2 (real projective plane is RP^2)

Any help would be great. Thanks
Let $X = S^1 \times S^1 \times RP^2$.

Since X is semi-locally simply connected, there exists a Galois correspondence between subgroups of the fundamental group $\pi_1(X)$ and path-connected covering spaces of X.

The fundamental group of X is $\pi_1(S^1 \times S^1 \times RP^2) =\pi_1(S^1) \times \pi_1(S^1) \times \pi_1(RP^2) = \mathbb{Z} \times \mathbb{Z} \times \mathbb{Z}/2$.

We shall find all the subgroups of $\pi_1(X)=\mathbb{Z} \times \mathbb{Z} \times \mathbb{Z}/2$ and connect each subgroup to its corresponding path-connected covering space of X.

("A subgroup of $\pi_1(X)$" $\longrightarrow$ "A corresponding path-connected covering space of X")

1. {e} $\longrightarrow \mathbb{Re} \times \mathbb{Re} \times S^2$. This is a universal cover of X.
2. $\{e\} \times \{e\} \times \mathbb{Z}/2 \longrightarrow\mathbb{Re} \times \mathbb{Re} \times RP^2$.
3. $\{e\} \times \mathbb{Z} \times \mathbb{Z}/2 \longrightarrow\mathbb{Re} \times S^1 \times RP^2$.
4. $\mathbb{Z} \times \mathbb{Z} \times \{e\} \rightarrow S^1 \times S^1 \times S^2$.
5. $n\mathbb{Z} \times m\mathbb{Z} \times \{e\} \rightarrow S^1 \times S^1 \times S^2$, where n and m are positive integers greater than 1.
In this case, consider a covering map $p_n:S^1 \rightarrow S^1$ given by $p_n(1, \theta) = (1, n\theta)$, where $(r, \theta)$ is a polar coordinate in the plane $\mathbb{Re}^2$. The map $p_n$, where n is a positive integer, wraps the circle around itself n times. Thus $(S^1, p_n)$ is a covering space of $S^1$, corresponding a fundamental group $n\mathbb{Z}$.

6. $n\mathbb{Z} \times \{e\} \times \mathbb{Z}/2 \rightarrow S^1 \times \mathbb{Re}^1 \times RP^2$, where n is a positive integer greater than 1. It is similar to (5).

7. $\mathbb{Z} \times \mathbb{Z} \times \mathbb{Z}/2 \longrightarrow X$ itself.

This problem becomes much more difficult if $X = S^1 \vee S^1 \vee RP^2$. It is because free product groups are involved in the fundamental group of X, and it is not easy to find all subgroups of a free product group and draw its Cayley graph.