The LaTeX command for mathscr A (๐) is \mathscr{A} (script A (requires mathrsfs package)).
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Write \mathscr{A} inside math mode, that is between $โฆ$ for inline math or in a \[ โฆ \] or equation block for display math. This symbol needs the mathrsfs package, so add \usepackage{mathrsfs} to your preamble.
\mathscr{A} is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ๐ in a formula gives the same symbol.
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\mathcal{M}\mathcal{N}\mathcal{P}\mathcal{R}\mathcal{S}\mathcal{T}\mathcal{X}\mathfrak{g}\mathfrak{h}\mathfrak{n}\dagger\ddagger^\circ\sharpMore math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \mathscr{A} in math mode after adding \usepackage{mathrsfs} to your preamble. For example, $\mathscr{A}$.
It comes from the mathrsfs package: add \usepackage{mathrsfs} to your preamble.