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Definition (Lower Bound, Upper Bound, Bounded, Maximum, Minimum)
Let $S\subseteq \mathbb{R}$.
- 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.
- We say $m$ is a minimum of $S$ if $m$ is a lower bound for $S$ and $m\in S$. We write $\min S$ for the minimum of $S$.
- We say $m$ is a maximum of $S$ if $m$ is an upper bound bound for $S$ and $m\in S$. We write $\max S$ for the maximum of $S$.