If anywhere at the page the sequence of characters
_ _NOMATHJAX_ _
appears without spacings between _ _ , at least once, then the sign of dollar $ is not interpreted as indication to the Latex syntax of formulas:
The <math> environment also does not work: <math>\psi</math> is not interpreted as ψ.
This go through the whole article, since the beginning till the end; this switching out cannot be switched in again with the
_ _MATHJAX_ _
command. For this reason, option with nomathjax in angular brackets is preferable.