In Connected Smooth Manifold Any Two Points can be Joined by Admissible Curve
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Theorem
Let $M$ be a connected smooth manifold with or without a boundary.
Let $p, q \in M$ be points.
Let $\gamma : \closedint a b \to M$ be an admissible curve.
Then:
- $\forall p, q \in M : \exists \gamma \subset M : \paren {\map \gamma a = p} \land \paren {\map \gamma b = q}$
Proof
For $p, q \in M$, we write:
- $p \sim q$
if and only if there exists an admissible curve $\gamma : \closedint a b \to M$ such that $\map \gamma a = p$ and $\map \gamma b = q$
Lemma 1
The $\sim$ is a equivalent relation on $M$.
Proof of Lemma 1
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$\Box$
For each $p \in M$, we define:
- $N_p := \set {q \in M : p \sim q}$
Now we need to show that:
- $\forall p \in M : M = N_p$
Lemma 2
For each $p \in N$, $N_p$ is a non-empty open set.
Proof of Lemma 2
Let $p \in M$.
Let $q \in N_p$.
Let $\struct {U, \phi}$ be a chart such that $q \in U$.
As $U$ is open, there is an open ball such that:
- $\map {B_\epsilon} {\map \phi q} \subseteq \phi \sqbrk U$
Let:
- $V := {\phi ^{-1} } \sqbrk {\map {B_\epsilon} {\map \phi q} }$
We shall show that:
- $V \subseteq N_p$
Let $r \in V \setminus \set q$.
Define a regular curve segment:
- $\ell_r : \closedint 0 1 \to M$
by:
- $\ds \map {\ell_r} t := \map {\phi^{-1} } { \map \phi q + t \paren {\map \phi r - \map \phi q} }$
In particular, $\ell_r$ is an admissible curve such that:
- $\map {\ell_r} a = q$
- $\map {\ell_r} b = r$
Therefore:
- $q \sim r$
As $p \sim q$, we have by Lemma 2:
- $p \sim r$
That is:
- $r \in N_p$
$\Box$
Lemma 3
If $N_p \cap N_q \ne \O$, then $N_p = N_q$.
Proof of Lemma 3
Let $r \in N_p \cap N_q$.
That is, $r \sim p$ and $r \sim q$.
By Lemma 1, then $p \sim q$.
$\Box$
Let $p \in M$.
Recall $N_p$ is an non-empty open set.
Observe:
- $\ds M \setminus N_p = \bigcup_{q \mathop \in M \setminus N_p} N_q$
is also an open set.
Thus $N_p$ is an non-empty clopen set.
Since $M$ is connected, we have:
- $M = N_p$
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Sources
- 2018: John M. Lee: Introduction to Riemannian Manifolds (2nd ed.) ... (previous) ... (next): $\S 2$: Riemannian Metrics. Lengths and Distances