I finally found a plugin of MathJax for wordpress, which has all the AMS symbols.

\[ \square x^4 \]

Everything looks okay on IPads or other tablets in horizontal mode.

You will effectively see the desktop version. On a vertical IPhone the math font is too large, and the formulas will not always fit into the display.

In horizontal mode, everything looks okay. You will see the mobile page provided by my WordPress theme.

The remaining problem is that formulas will no longer work in feed readers and other pages, which embed the content.