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