Mathematics
Compactness in Metric Spaces and Its Consequences
Quick fact
In a metric space, compactness is equivalent to the property that every infinite set has a limit point — a far-reaching generalization of the Bolzano-Weierstrass theorem.
Why this is interesting
You know that every continuous function on a closed interval attains a maximum and a minimum. But why does that work, and what happens when we leave the familiar real line?