The LaTeX command for flat (♭) is \flat.
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Write \flat 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.
\flat is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ♭ in a formula gives the same symbol.
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\mathfrak{g}\mathfrak{h}\mathfrak{n}\dagger\ddagger^\circ\sharp\natural\checkmark\maltese\clubsuit\diamondsuit\heartsuit\spadesuitMore math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \flat in math mode; it needs no extra package. For example, $\flat$.
None: \flat is built into LaTeX, so no extra package is required.