The LaTeX command for Im (ℑ) is \Im (imaginary part).
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Write \Im 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.
\Im is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ℑ 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 \Im in math mode; it needs no extra package. For example, $\Im$.
None: \Im is built into LaTeX, so no extra package is required.