The LaTeX command for underbrace (x) is \underbrace{x}_{n} (underbrace with label).
\underbrace{x}_{n}Writing a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \underbrace{x}_{n} 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.
\underbrace{x}_{n} is a math accent: it takes an argument and only works in math mode, while text accents such as \'e are separate commands. With unicode-math under XeLaTeX or LuaLaTeX, typing x in a formula gives the same symbol.
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\widetilde{xy}\underline{x}\hat{x}\tilde{x}\bar{x}\overline{x}\overbrace{x}^{n}\vec{x}\dot{x}\ddot{x}\acute{x}\grave{x}\check{x}\breve{x}Math accents in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \underbrace{x}_{n} in math mode; it needs no extra package. For example, $\underbrace{x}_{n}$.
None: \underbrace{x}_{n} is built into LaTeX, so no extra package is required.