@bep I tried with .Inner but it didn’t helped either. Calling the shortcode
<script type="math/tex" >{{ .Inner }}</script>
in my post with
{{% katex %}}S_n = a \times \frac{1-r^n}{1-r}{{% /katex %}}
doesn’t fix it. The output is still:
<p><script type="math/tex" >"S_n = a \\times \\frac{1-r^n}{1-r}"</script></p>.
@dimo414 I did a bit of research and found similar problems on Stackoverflow that required mostly a custom function/filter. This article shows pretty good, that the quotation is added to prevent XSS-attacks and other abuse.
There I would suggest to add a filter named saveJS to Hugo.