The LaTeX command for sup is \sup (supremum).
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Write \sup inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
In display math \sup takes its full size; inline it shrinks to fit the line, and \displaystyle forces the large form.
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Use \sup in math mode; it needs no extra package. For example, $\sup f(x)$.
None: \sup is built into LaTeX, so no extra package is required.