The LaTeX command for widehat is \widehat{xy}.
\widehat{xy}Writing a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \widehat{xy} 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.
\widehat{xy} is a math accent: it takes an argument and only works in math mode, while text accents such as \'e are separate commands. Not to be confused with \hat{x}.
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\hat{x}: hat / estimate\bar{x}: overbar / mean\overline{x}: overline (longer than \bar)\dot{x}\ddot{x}\acute{x}\grave{x}\check{x}\breve{x}\mathring{x}\widetilde{xy}\underline{x}\hat{x}\tilde{x}\bar{x}\overline{x}\overbrace{x}^{n}Math accents in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \widehat{xy} in math mode; it needs no extra package. For example, $\widehat{xy}$.
None: \widehat{xy} is built into LaTeX, so no extra package is required.