amsmath package error using SIAM LaTeX template files

Question

I am preparing paper using the latest SIAM template siamart_0516.zip downloaded from http://www.siam.org/journals/auth-info.php.

I am having errors in math equations containing tag{} command with special characters (e.g., mathbf{}, bar{} and etc.) in math mode. To be clear, the following code:

documentclass[review]{siamart0516}
usepackage{amsmath}

begin{document}
begin{equation}
bar{x} = x + y
tag{$bar{x}$}
label{eq:x}
end{equation}

end{document}

produce an error:

Package amsmath Error: bar allowed only in math mode. end{equation}

while using only tag{$x$} without bar{} works fine:

documentclass[review]{siamart0516}
usepackage{amsmath}

begin{document}
begin{equation}
bar{x} = x + y
tag{$x$}
label{eq:x}
end{equation}

end{document}

Finally, initial non-working code using the standard article documentclass, i.e., replacing documentclass[review]{siamart0516} with documentclass{article} works like a charm.

Any guesses what is causing this issue and how to overcome it?

egreg · Accepted Answer

For very strange reasons, ntheorem, when used with the thmmarks and amsmath options, redefines maketag@@@ (an internal command of amsmath) to do, among other things

settowidth{tagwidth}{$##1$}%

This is utterly wrong, because ##1 is substituted with the tag text, which is supposed to be in text mode.

Now, what happens when you do tag{$x$}? The settowidth macro puts the second argument in an hbox, so what results is hbox{$$x$$} which is legal, because $$ in restricted horizontal mode just typesets an empty math formula, and what's measured is a text mode “x”. But $bar{x}$ (or any other inherently math), the code hbox{$$bar{x}$$} tries to typeset bar{x} in text mode, and the error is shown. Moreover, something like tag{a*} would end up to be wrongly measured, because the width of a* in text mode is quite different from the width of $a*$.

Solution: patch the relevant command to remove the nasty $ characters.

documentclass{article}
usepackage[leqno]{amsmath}
usepackage{etoolbox}

usepackage[amsmath,thmmarks,hyperref]{ntheorem}

patchcmd{SetTagPlusEndMark}{$}{}{}{}
patchcmd{SetTagPlusEndMark}{$}{}{}{}

begin{document}

begin{equation}
bar{x} = x + y
tag{$bar{x}$}
label{eq:x}
end{equation}

end{document}

The same works with siamart0516.cls:

documentclass[review]{siamart0516}
usepackage{amsmath}

usepackage{etoolbox}

patchcmd{SetTagPlusEndMark}{$}{}{}{}
patchcmd{SetTagPlusEndMark}{$}{}{}{}

begin{document}

begin{equation}
bar{x} = x + y
tag{$bar{x}$}
label{eq:x}
end{equation}

end{document}

Przemysław Scherwentke · Answer

If it may be an aswer only for the second part of yor question, putting into mbox seems to solve the problem:

documentclass[review]{siamart0516}
usepackage{amsmath}

begin{document}
begin{equation}
bar{x} = x + y
tag{mbox{$bar{x}$}}
label{eq:x}
end{equation}

end{document}

David Carlisle · Answer

Looks like a "feature" of ntheorem package

You can reproduce the error with

documentclass{article}
usepackage[leqno]{amsmath}

RequirePackage[amsmath,thmmarks,hyperref]{ntheorem}%[1.33] (bad use of this argument in siam class)

begin{document}
begin{equation}
bar{x} = x + y
tag{$bar{x}$}
label{eq:x}
end{equation}

end{document}

could trace exactly where it goes wrong but I'd be tempted to simply accept the other answer and use mbox workaround.

amsmath package error using SIAM LaTeX template files

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