Centering column in alignat environment?

Question

How do I center the contents of each cell in the middle column? I am approximating it with \phantom, but there's got to be a better way.

\begin{alignat*}{3}
  &E&\quad&\mathrm{par}&&\quad\Sigma,\\
  &\Psi&\quad&\phantom{p}\mathrm{"}\phantom{r}&&\quad\frac{\epsilon V}{1+4\pi\epsilon F},\\
  &\Psi&\quad&\phantom{p}\mathrm{"}\phantom{r}&&\quad\frac{(1+4\pi\epsilon F)^2}{\epsilon}.
\end{alignat*}

thanks

Answer 1

At its heart the alignat environment is built on TeX's \halign. In that environment you can ignore the formatting specifications for a given column by issuing \omit. Thus writing

\omit\hfill text \hfill

will give you a centered column of text, since the column specification not only included the left/right alignment but also that the meterial is set as mathematics. It is probably best to create a separate command to provide this construction, which may then be used anywhere in an alignat, align or similar enviroment, at the beginning of a cell:

\documentclass{article}

\usepackage{amsmath}

\newcommand{\ditto}{{''}}
\newcommand{\ccol}[1]{\omit\hfill #1\hfill}

\begin{document}

\begin{alignat*}{3}
  &E&\quad&\ccol{par}&\quad&\Sigma,\\
  &\Psi&&\ccol{\ditto}&&\frac{\epsilon V}{1+4\pi\epsilon F},\\
  &\Psi&&\ccol{\ditto}&&\frac{(1+4\pi\epsilon F)^2}{\epsilon}.
\end{alignat*}

\begin{align}
  E &= mc^2\\
  F &\ccol{is another quantity}\\
  G &= u^2 + v^2 - w^2 + x^2 - y^2 + z^2
\end{align}

\end{document}

Sould you wish to span multiple columns the relevant command is \multispan, e.g. \multispan{3}\hfill text \hfill will place text spread over three columns and centered.

Alternatively, as Harish Kumar suggests you can use array or tabular:

\documentclass{article}

\usepackage{amsmath}

\newcommand{\ditto}{{''}}

\begin{document}

\begin{equation*}
  \begin{array}{r@{\quad}c@{\quad}l}
  E&\mathrm{par}&\Sigma,\\
  \Psi&\mathrm{\ditto}&\displaystyle\frac{\epsilon V}{1+4\pi\epsilon F},\\
  \Psi&\mathrm{\ditto}&\displaystyle\frac{(1+4\pi\epsilon F)^2}{\epsilon}.
  \end{array}
\end{equation*}

\begin{center}
  \begin{tabular}{r@{\quad}c@{\quad}l}
    \( E \)&par&\( \Sigma, \)\\
    \( \Psi \)&\ditto&\( \displaystyle\frac{\epsilon V}{1+4\pi\epsilon F}, \)\\
    \( \Psi \)&\ditto&\( \displaystyle\frac{(1+4\pi\epsilon F)^2}{\epsilon}. \)
  \end{tabular}
\end{center}
\end{document}

However, the default line spacing from alignat is better and you don't need to worry about adjusting the math styles.