Dear John (cc. the org-mode list)

Thanks to your help I have succeeded in finding the right configuration for html export :

#+HTML_HEAD: <script type="text/javascript"
#+HTML_HEAD:    src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_CHTML-full.js">
#+HTML_HEAD: </script>

it works perfectly for the export of formulas. I have used the documentation on mathjax.org

I hope that this cc. will be maybe useful for others.

Best wishes

Jo.