Question: I am trying to get a double tilde symbol over a letter but I can't find a well suiting solution. I tried

```
$\tilde{\tilde{z}}$

```

and

```
$\overset{\approx}{z}$ 

```

but both solution look a bit off.
I am trying to find something with less vertical space between the two tildes but a similar size like `\tilde{}`.

*edit:* vertical spacing

Answer: 
A variation on my answer here: [double tilde symbol under letter](https://tex.stackexchange.com/questions/229543/double-tilde-symbol-under-letter/229598#229598). Here, `\tsup` takes an optional argument specifying the number of symbols to overset (default 2).

```
\documentclass{article}
\usepackage{stackengine}
\stackMath
\newcommand\tsup[2][2]{%
 \def\useanchorwidth{T}%
  \ifnum#1>1%
    \stackon[-.5pt]{\tsup[\numexpr#1-1\relax]{#2}}{\scriptscriptstyle\sim}%
  \else%
    \stackon[.5pt]{#2}{\scriptscriptstyle\sim}%
  \fi%
}
\begin{document}
\[
\tsup[1]{z}\neq\tsup{\Lambda}\neq\tsup[3]{\Delta}\neq\tsup[4]{\psi}
\]

\end{document}

```

[![enter image description here](https://i.stack.imgur.com/cmi8i.jpg)](https://i.stack.imgur.com/cmi8i.jpg)

Here's a version that uses the proper tilde glyph and works across 10-12pt fontsize range:

```
\documentclass[10pt]{article}
\usepackage{stackengine}
\stackMath
\newcommand\tsup[2][2]{%
 \def\useanchorwidth{T}%
  \ifnum#1>1%
    \stackon[-1.3ex]{\tsup[\numexpr#1-1\relax]{#2}}{\mathchar"307E}%
  \else%
    \stackon[-1ex]{#2}{\mathchar"307E}%
  \fi%
}
\begin{document}
\[
\tilde z=\tsup[1]{z}\neq\tsup{\Lambda}\neq\tsup[3]{\Delta}\neq\tsup[4]{\psi}
\]
\fontsize{12pt}{14pt}\selectfont
\[
\tilde z=\tsup[1]{z}\neq\tsup{\Lambda}\neq\tsup[3]{\Delta}\neq\tsup[4]{\psi}
\]
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/gsCFw.jpg)](https://i.stack.imgur.com/gsCFw.jpg)

In both MWE's the only issue missing is the italic kerning of the overset. That could be somewhat addressed globally (not for each particular character) with a little `\kern`

```
\newcommand\tsup[2][2]{%
 \def\useanchorwidth{T}%
  \ifnum#1>1%
    \stackon[-1.3ex]{\tsup[\numexpr#1-1\relax]{#2}}{\mathchar"307E\kern-.5pt}%
  \else%
    \stackon[-1ex]{#2}{\mathchar"307E\kern-.5pt}%
  \fi%
}

```

[![enter image description here](https://i.stack.imgur.com/JwW5F.jpg)](https://i.stack.imgur.com/JwW5F.jpg)
The `tipa` package has `\doubletilde`. However, it is not made for math mode, so you have to cheat a bit:

```
\documentclass{article}
\usepackage{amsmath}
\usepackage[extra]{tipa}

\begin{document}

$\doubletilde{\kern-1pt$z$}$

\end{document}

```

[![enter image description here](https://i.stack.imgur.com/yt222.png)](https://i.stack.imgur.com/yt222.png)