The LaTeX command for floor is \lfloor x \rfloor (floor function).
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Write \lfloor x \rfloor 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 pseudocode or running text, wrap \lfloor x \rfloor in $...$ so it keeps math spacing and fonts.
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Use \lfloor x \rfloor in math mode; it needs no extra package. For example, $\lfloor x \rfloor$.
None: \lfloor x \rfloor is built into LaTeX, so no extra package is required.