Re: DeclareMathOperator and digit in operator name

On 17/12/2025 14:32, Vincent Lefevre wrote:
> Hi,
> 
> After a recent upgrade of TeX Live in Debian/unstable
> (2025.20251127 snapshot?), the following LaTeX file fails
> to compile:
> 
> \documentclass{article}
> \usepackage{amsopn}
> \DeclareMathOperator{\Fast2Sum}{Fast2Sum}
> \begin{document}
> $\Fast2Sum(X)$
> \end{document}
> 
> with an error on the \DeclareMathOperator.
> 
> This is due to the "2" in the operator name \Fast2Sum.
> 
> However, in
> 
> https://mirrors.ctan.org/install/macros/latex/required/latex-amsmath.tds.zip
> 
> I cannot see any corresponding change mentioned in "changes.txt".
> 
> So I'm wondering where this has changed in TeX Live and whether
> this is a change done on purpose. In the latter case, where is
> this change documented?
> 

This was never supported: you are defining \Fast to *require* being 
followed by "2Sum". There were technical changes which mean the path 
taken may change, but as this was always an error, the outcome is not 
something that is 'controlled'.

Regards,

Joseph