The LaTeX command for floor brackets is \lfloor \rfloor (floor).
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Write \lfloor \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.
Write \lfloor \rfloor; for delimiters that grow with their content wrap them in \left and \right, or pick a fixed size with \bigl and \bigr, often the better choice.
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\left( \right)\left[ \right]\left\{ \right\}\langle \rangle\lceil \rceil\| x \|| x |\lvert x \rvert\lVert x \rVert\left| \right|Delimiters in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \lfloor \rfloor in math mode; it needs no extra package. For example, $\lfloor \rfloor$.
None: \lfloor \rfloor is built into LaTeX, so no extra package is required.