This is a little LaTeX hack that I would like to document. For something that I am writing right now I have a big table, where one column contains a certain abelian group. All these groups are direct sums of two other abelian groups. Wouldn't it be nice if the column was aligned on the $\oplus$? At the end of this post there is a comparison to see what I mean in practice.

These abelian groups are the Picard group of certain singular curves, and they are direct sums of $\mathrm{Pic}_0$ and a number of copies of $\mathbb{Z}$ depending on the number of irreducible components. But this is not very important for the remainder of the post.

Using the dcolumn package it is possible to align columns on a decimal point. Luckily, this decimal point can be chosen in a rather broad sense. The column specifier 

D{+}{\oplus}{-1}

tells you that we wish to align on the + symbol in the LaTeX code, and that we wish to display \oplus as our decimal point. The -1 is not important for us, because we don't have actual decimals.

A few things to keep in mind:

  • the dcolumn package automatically turns everything into math mode, hence you don't need to use $'s in this particular column;
  • so if you need to align actual text using this method, you can always wrap things in \text{};
  • if you have a column header, you do not with to apply the dcolumn column specifier to it, which is possible by using \multicol{1}{c}{...}.
Compare: 
\begin{tabular}{c}
  \toprule
  \multicolumn{1}{c}{$\quad\Pic_0C$} \\
  \midrule
  $C\oplus\mathbb{Z}$ \\
  $\Ga\oplus\mathbb{Z}$ \\
  $\Gm\oplus\mathbb{Z}$ \\
  $\Gm\oplus\mathbb{Z}^{\oplus2}$ \\
  $\Ga\oplus\mathbb{Z}^{\oplus2}$ \\
  \bottomrule
\end{tabular}
 
\begin{tabular}{D{+}{\,\oplus\,}{-1}}
  \toprule
  \multicolumn{1}{c}{$\quad\Pic_0C$} \\
  \midrule
  C+\mathbb{Z} \\
  \Ga+\mathbb{Z} \\
  \Gm+\mathbb{Z} \\
  \Gm+\mathbb{Z}^{\oplus2} \\
  \Ga+\mathbb{Z}^{\oplus2} \\
  \bottomrule
\end{tabular}

Notice that I had to move the column header to the right a little. I should figure out a better solution, especially because this creates a little extra margin on the left. But this nitpickery has gone on long enough now.