[Tex/LaTex] \widetilde{} alternative for formula over the tilde

Question

I'd like to put some formula over the tilde in math mode and adjust the tilde length to the length of that formula. It seems that \widetilde{} is almost what I need, however it puts the formula below the tilde. Is there a way to actually put the formula above?

An example of the formula to put above is:

\widetilde{(p_1',p_2') \in C}

Edit: I wasn't exactly clear with the expected effect. I'd like to have tilde in the middle of line (like \sim does), however with the formula above.

Answer 1

I took my answer at Big tilde in math mode and made two changes: 1) I changed it from an overstack to an understack; and 2) I increased the vertical stacking gap to \LMpt, which is 1pt scaled to the local mathstyle.

\documentclass{article}
\usepackage{scalerel}[2014/03/10]
\usepackage{stackengine}

\newcommand\reallywidetilde[1]{\ThisStyle{%
  \setbox0=\hbox{$\SavedStyle#1$}%
  \stackengine{\LMpt}{$\SavedStyle#1$}{%
    \stretchto{\scaleto{\SavedStyle\mkern.2mu\sim}{.5467\wd0}}{.7\ht0}%
%    .2mu is the kern imbalance when clipping white space
%    .5467++++ is \ht/[kerned \wd] aspect ratio for \sim glyph
  }{U}{c}{F}{T}{S}%
}}

\def\test#1{$%
  \reallywidetilde{#1}\,
  \scriptstyle\reallywidetilde{#1}\,
  \scriptscriptstyle\reallywidetilde{#1}
$\par}

\parskip 1ex
\begin{document}
$x = \reallywidetilde{(p_1',p_2') \in C}$

$\scriptstyle x = \reallywidetilde{(p_1',p_2') \in C}$

$\scriptscriptstyle x = \reallywidetilde{(p_1',p_2') \in C}$
\end{document}

If, by some chance, you actually wanted the text over the tilde (rather than the tilde under the text), that is also easy:

\documentclass{article}
\usepackage{scalerel}[2014/03/10]
\usepackage{stackengine}

\newcommand\reallywidetilde[1]{\ThisStyle{%
  \setbox0=\hbox{$\SavedStyle#1$}%
  \stackengine{\LMpt}{%
    \stretchto{\scaleto{\SavedStyle\mkern.2mu\sim}{.5467\wd0}}{.7\ht0}%
    }{$\SavedStyle#1$%
%    .2mu is the kern imbalance when clipping white space
%    .5467++++ is \ht/[kerned \wd] aspect ratio for \sim glyph
  }{O}{c}{F}{T}{S}%
}}

\def\test#1{$%
  \reallywidetilde{#1}\,
  \scriptstyle\reallywidetilde{#1}\,
  \scriptscriptstyle\reallywidetilde{#1}
$\par}

\parskip 1ex
\begin{document}
$xyz \reallywidetilde{(p_1',p_2') \in C} abc$

$xyz {\scriptstyle\reallywidetilde{(p_1',p_2') \in C}} abc$

$xyz {\scriptscriptstyle\reallywidetilde{(p_1',p_2') \in C}} abc$
\end{document}