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