Equations with eqnarray and align render as an error under MathJax 4

[redeboer]

Since Sphinx 8.2, sphinx.ext.mathjax loads MathJax 4 by default (https://cdn.jsdelivr.net/npm/mathjax@4/tex-mml-chtml.js). MathJax 4 enforces a check that MathJax 3 did not: an AMS equation structure (eqnarray, align, align*, gather, multline) may not be nested inside another one. Both ways in which our notebooks produce display math do exactly that nesting:

• Sphinx wraps every math block — $$ ... $$ as well as a {math} directive, labelled or not — in \begin{equation}\begin{split} ... \end{split}\end{equation} (see sphinx.util.math.wrap_displaymath).
• IPython.display.Math wraps its input in $\displaystyle ... $.

The result is that the equation is replaced by a red Erroneous nesting of equation structures on the rendered page. Only inner environments are allowed in these positions: aligned, alignedat, gathered, split, array, pmatrix, cases. Note that the pages still rendered correctly under MathJax 3, so this regression came in silently with a Sphinx upgrade.

Affected

File	$$ blocks	Math() with a hand-written environment	sp.multiline_latex()
docs/005/index.ipynb			1
docs/009/index.ipynb			4
docs/010/index.ipynb	1
docs/011/index.ipynb			7
docs/013/index.ipynb			1
docs/014/index.ipynb			6
docs/015/index.ipynb	3	2	1
docs/017/index.ipynb			1
docs/021/index.ipynb	22		2
docs/024/index.ipynb			1
docs/027/index.ipynb	1
docs/033/index.ipynb	4

Fix

For the Markdown cells and the hand-written Math(R"\begin{eqnarray}...") strings: replace the environment with aligned and collapse eqnarray's three-column alignment &=& to &=.

For sp.multiline_latex() there is no keyword fix — its only environments are align*, eqnarray and IEEEeqnarray, and all three are illegal inside Math(). Replace those calls with ampform.io.aslatex, which renders into aligned: Math(aslatex({lhs: rhs}, terms_per_line=n)).

Since these reports are pinned and rebuilt from their own environments, the fix can be applied file by file.