Path Homotopy is Equivalence Relation
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Theorem
Let $X$ be a topological space.
Let $p, q \in X$.
Then path homotopy on the set of all paths in $X$ from $p$ to $q$ is an equivalence relation.
Proof
Suppose $X$ is a topological space.
Let $p, q \in X$.
Let $\sim$ denote the path homotopy on the set of all paths in $X$ from $p$ to $q$.
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Checking in turn each of the criteria for equivalence:
Reflexivity
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Thus $\sim$ is seen to be reflexive.
$\Box$
Symmetric
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Thus $\sim$ is seen to be symmetric.
$\Box$
Transitive
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Thus $\sim$ is seen to be transitive.
$\Box$
$\sim$ has been shown to be reflexive, symmetric and transitive.
Hence the result.
$\blacksquare$
Sources
- 2011: John M. Lee: Introduction to Topological Manifolds (2nd ed.) ... (previous) ... (next): $\S 7$: Homotopy and the Fundamental Group. Homotopy