colortbl and amsmath conflict

Question

I have a problem with colortbl and amsmath. Is someone here aware of a conflict between these two packages?

\documentclass[10pt]{article}
\usepackage{amsmath}
%\usepackage{colortbl}

\begin{document}

  \begin{tabular}[b]{cc}
    Correct & Incorrect\\
    \parbox[b][]{0.4\linewidth}{
    \begin{align*}
      3(2x-5)^2&=3(4x^2-20x+25)\\
               &=12x^2-60x+75
    \end{align*}
    } &
        \parbox[b][]{0.5\linewidth}{
        \begin{align*}
          3(2x-5)^2&=(6x-15)^2\\
                   &=36x^2-180x+225
        \end{align*}
        }\\
\end{tabular}

\end{document}

This compiles fine but when I uncomment \usepackage{colortbl} it throws errors.

Here is the error log.

! Misplaced \omit.
\math@cr@@@ ...@ \@ne \add@amps \maxfields@ \omit 
                                                  \kern -\alignsep@ \iftag@ ...
l.14 }
       &
I expect to see \omit only after tab marks or the \cr of
an alignment. Proceed, and I'll ignore this case.

! Misplaced \omit.
\math@cr@@@ ...@ \@ne \add@amps \maxfields@ \omit 
                                                  \kern -\alignsep@ \iftag@ ...
l.14 }
       &
I expect to see \omit only after tab marks or the \cr of
an alignment. Proceed, and I'll ignore this case.

I have to mention that the problem seems to crop up only when I use align inside the parbox in the table.

Answer 1

This is groping in the dark a bit, because I couldn't find anything significant by tracing, but then these alignment constructs are so fragile it really goes over my head.

By staring hard at the colortbl code, I cooked up the following patch. Please try the modified MWE:

\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage{colortbl}

%%%%%%%%%% HERE COMES THE PATCH %%%%%%%%%%
\usepackage{etoolbox}

\makeatletter
\pretocmd\start@align
{%
  \let\everycr\CT@everycr
  \CT@start
}{}{}
\apptocmd{\endalign}{\CT@end}{}{}
\makeatother
%%%%%%%%%% END PATCH %%%%%%%%%%

\begin{document}

  \begin{tabular}[b]{cc}
    Correct & Incorrect\\
    \parbox[b][]{0.4\linewidth}{
    \begin{align*}
      3(2x-5)^2&=3(4x^2-20x+25)\\
               &=12x^2-60x+75
    \end{align*}
    } &
        \parbox[b][]{0.5\linewidth}{
        \begin{align*}
          3(2x-5)^2&=(6x-15)^2\\
                   &=36x^2-180x+225
        \end{align*}
        }\\
\end{tabular}

\end{document}

I don't know whether this has any averse effects on the colortblness of the outer tabular though. Please test.

Answer 2

I am not sure what the actual source of the problem is, but one way to work around this problem is to just use aligned instead:

Notes:

• As egreg pointed out, using a \parbox is no longer necessary when using the aligned environment.
• I would not recommend using an = when things are not equal as is the case for the incorrect column. I have not made that change here as then you need to decide what to do with the following equal.

Code:

\documentclass[10pt]{article}
\usepackage{colortbl}
\usepackage{amsmath}
\begin{document}
\begin{center}
\begin{tabular}[b]{cc}
Correct & Incorrect\
$\begin{aligned}
3(2x-5)^2&=3(4x^2-20x+25)\
&=12x^2-60x+75
\end{aligned}$
 &
$\begin{aligned}
3(2x-5)^2&=(6x-15)^2\
&=36x^2-180x+225
\end{aligned}$
\
\end{tabular}
\end{center}
\end{document}