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Definition (Lower Bound, Upper Bound, Bounded)

Let $S$ be a collection of real numbers (written $S\subseteq \mathbb{R}$, or $S$ is a subset of the real numbers).

  • A lower bound for $S$ is a real number $m$ such that $m\leq x$ for every $x\in S$. We say that $S$ is bounded below if it has a lower bound.
  • An upper bound for $S$ is a real number $m$ such that $m\geq x$ for every $x\in S$. We say that $S$ is bounded above if it has an upper bound.
  • We say that $S$ is bounded if has both a lower and upper bound.