The LaTeX command for Re (ℜ) is \Re (real part).
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Write \Re 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.
\Re is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ℜ in a formula gives the same symbol.
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More math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \Re in math mode; it needs no extra package. For example, $\Re$.
None: \Re is built into LaTeX, so no extra package is required.