Let \((X,d)\) be a metric space and let \(A\subseteq X\) be nonempty. The diameter of \(A\) is \[ \diam(A) \coloneqq \sup_{x, y\in A} d(x, y) \in [0,\infty]. \]
What is the diameter of a set $A$ in a metric space?
Diameter of a subset of a metric space
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