The LaTeX command for ceiling is \lceil x \rceil (ceiling function).
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Write \lceil x \rceil 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 \lceil x \rceil in $...$ so it keeps math spacing and fonts.
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Use \lceil x \rceil in math mode; it needs no extra package. For example, $\lceil x \rceil$.
None: \lceil x \rceil is built into LaTeX, so no extra package is required.