How to have vector length with tilde X? [duplicate]

Question

Following the question in Can we have x with tilde on top?

Basically, i want to have vector lenght like this

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\begin{document}

$\| \tilde{x} \|  $   

\end{document}

The output is:

But how to have another tilde at the bottom of X?

Answer 1

Two variants with the accents package:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{accents}

\begin{document}

$ \| \underaccent{\tilde}{\tilde{x}} \| \qquad\| \undertilde{\widetilde{x}} \| $

\end{document}

Answer 2

\documentclass{article}

\usepackage{undertilde}

\begin{document}
$\| \utilde{\tilde{x}} \|$
\end{document}