Upper bound of set

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Is it necessary only least upper bound may or may not belongs to the set and not any other upper bound???? Does always upper bound belongs to outside of the set ??

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If $S$ is a set and $a$ is an upper bound for $S$, but not the least upper bound, then there exists a smaller upper bound $a'$. So for every $x\in S$, we have $x\le a'<a$ and cannot have $a\in S$.