The LaTeX command for limsup is \limsup (limit superior).
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Write \limsup 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 \limsup takes its full size; inline it shrinks to fit the line, and \displaystyle forces the large form.
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Use \limsup in math mode; it needs no extra package. For example, $\limsup f(x)$.
None: \limsup is built into LaTeX, so no extra package is required.