Somehow SE's MathJax doesn't like `\DeclareMathOperator` in subscripts or superscripts, this doesn't render on SE:
```tex
$\DeclareMathOperator{\two}{two}$
$\two+\two=\two^\two$
```
(I can't directly demonstrate the failure here, since MathJax seems not available in this community.)

But in the [MathJax demo](https://www.mathjax.org/#demo) it renders just fine:

![](https://i.sstatic.net/YtJ1L.png)

What's the reason? Different version of MathJax?