Heine–Cantor theorem

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Not to be confused with Cantor's theorem.

In mathematics, the Heine–Cantor theorem, named after Eduard Heine and Georg Cantor, states that if f : MN is a continuous function between two metric spaces, and M is compact, then f is uniformly continuous. An important special case is that every continuous function from a bounded closed interval to the real numbers is uniformly continuous.

Proof[edit]

Suppose that and are two metric spaces with metrics and , respectively. Suppose further that is continuous, and that is compact. We want to show that is uniformly continuous, that is, for every there exists such that for all points in the domain , implies that .

Fix some positive . Then by continuity, for any point in our domain , there exists a positive real number such that when is within of .

Let be the open -neighborhood of , i.e. the set

Since each point is contained in its own , we find that the collection is an open cover of . Since is compact, this cover has a finite subcover. That subcover must be of the form

for some finite set of points . Each of these open sets has an associated radius . Let us now define , i.e. the minimum radius of these open sets. Since we have a finite number of positive radii, this number is well-defined and positive. We may now show that this works for the definition of uniform continuity.

Suppose that for any two in . Since the sets form an open (sub)cover of our space , we know that must lie within one of them, say . Then we have that . The Triangle Inequality then implies that

implying that and are both at most away from . By definition of , this implies that and are both less than . Applying the Triangle Inequality then yields the desired

For an alternative proof in the case of a closed interval, see the article on non-standard calculus.

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