The LaTeX command for mathcal R (ℛ) is \mathcal{R}.
\mathcal{R}Writing a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \mathcal{R} 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.
\mathcal{R} 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.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\mathcal{B}\mathcal{C}\mathcal{D}\mathcal{G}\mathcal{M}\mathcal{N}\mathcal{P}\mathcal{S}\mathcal{T}\mathcal{X}\mathscr{A}\mathfrak{g}\mathfrak{h}\mathfrak{n}More math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \mathcal{R} in math mode; it needs no extra package. For example, $\mathcal{R}$.
None: \mathcal{R} is built into LaTeX, so no extra package is required.