Continuous Image of Compact Space is Compact/Corollary 3/Proof 2
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Corollary to Continuous Image of Compact Space is Compact
Let $S$ be a compact topological space.
Let $f: S \to \R$ be a continuous real-valued function.
Then $f$ attains its bounds on $S$.
Proof
By Continuous Image of Compact Space is Compact, $f \sqbrk S$ is compact.
From Compact Metric Space is Complete and Compact Metric Space is Totally Bounded, $f \sqbrk S$ is complete and totally bounded.
A Totally Bounded Metric Space is Bounded.
Hence both the supremum and the infimum of $f \sqbrk S$ exist in $\R$.
Because $f \sqbrk S$ is complete:
- $\sup f \sqbrk S \in f \sqbrk S$
and:
- $\inf f \sqbrk S \in f \sqbrk S$
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$\blacksquare$