Proving space is semi-locally simply connected

algebraic-topologycovering-spacesgeneral-topology

Let $X$ be connected and locally path connected. Suppose $X$ has a covering map $p:\tilde{X} \rightarrow X$ such that $\pi_1(\tilde{X};\tilde{x_0})$ is the trivial group for some basepoint $\tilde{x_0} \in \tilde{X}$.

I would like to show that this implies that $X$ is semi-locally simply connected.

So I need to show that for every $x \in X$ and any neighbourhood $V$ of $x$ there exists an open subset $U \subset X$ such that $x \in U$ and the homomorphism $i_*:\pi_1(U;x) \rightarrow \pi_1(X;x)$ induced by the inclusion map $i:U \rightarrow X$ is trivial.

I am not really sure where to start. Do I need to use the lifting criterion?

Best Answer

Assuming $\tilde X$ is connected. So your condition shows that $ \pi_1(\tilde X,\tilde x)=0 \ \forall \ \tilde x\in \tilde X$.

I shall show if $x_0\in X$ and $\tilde x_0$ lies in the fiber over $x_0$ of $\tilde X\xrightarrow{p}X$, then $ \pi_1(\tilde X,\tilde x_0)=0\implies X$ is semi-locally simply connected at $x_0$.

Choose an open path-connected neighbourhood $U\ni x_0$ and let $\tilde U\ni \tilde x_0$ such that $p:\tilde U\rightarrow U$ is an isomorphism. Then we have the commutative diagram

$$\require{AMScd} \begin{CD} \tilde U @>\tilde i>> \tilde X\\ @Vp|_{\tilde U}VV @VpVV \\ U @>i>> X \end{CD} $$ Applying $\pi_1$ we get the following commutative diagram $$\require{AMScd} \begin{CD} \pi_1(\tilde U,\tilde x_0) @>\tilde i_*>> \pi_1(\tilde X,\tilde x_0)\\ @V(p|_{\tilde U})_*VV @Vp_*VV \\ \pi_1(U,x_0) @>i_*>> \pi_1(X,x_0) \end{CD} $$

Thus we get from the commutativity $i_*(p|_{\tilde U})_*=p_*\tilde i_*=0$ since $\pi_1(\tilde X,\tilde x_0)=0$

Since $(p|_{\tilde U})_*$ is an isomorphism, we get $i_*=0$

Applying this argument to other points completes the proof.

Edit: For the general case, let $\tilde X=\bigsqcup_i \tilde X_i$ be the connected components of $\tilde X$. Say $\tilde x_0\in \tilde X_{i_0}$ Then $p|_{\tilde X_{i_0}}:\tilde X_{i_0}\rightarrow X$ is a covering map with $\tilde X_{i_0} $ connected and you are back to the previous case.