Centering \vdots in a system of many equations

Question

I have a system of equations with an arbitrary number of equations (k). I'd like to use \vdots to compactly describe the system, like so:

\begin{align*}
  R(-1) &= \sum_{i=1}^m A(i)R(i-1) \\
  R(-2) &= \sum_{i=1}^m A(i)R(i-2) \\
        &\vdots                    \\
  R(-k) &= \sum_{i=1}^m A(i)R(i-k)
\end{align*}

I'd like the dots, however, to be centered with = or the entire equation. Is there an elegant way of centering a column or row with AMSMath? I'm currently using an unholy combination of whitespace operators (\; \, etc.) to get the job done.

Answer 1

\documentclass{article}
\usepackage{amsmath,calc}
\begin{document}

\begin{align*}
  R(-1) &= \sum_{i=1}^m A(i)R(i-1) \\
  R(-2) &= \sum_{i=1}^m A(i)R(i-2) \\
        &\mathrel{\makebox[\widthof{=}]{\vdots}} \\
  R(-k) &= \sum_{i=1}^m A(i)R(i-k)
\end{align*}

\end{document}

Answer 2

Consider the package mathtools, which provides several corrections for and additions to amsmath.

\usepackage{mathtools}

It also provides a comfortable solution for your problem. You can even choose between a normal (\vdotswithin) and a short (\shortvdotswithin) distance.

\begin{align*}
  a &= b \\
  & \vdotswithin{=} \\
  & = c \\
  \shortvdotswithin{=}
  & = d
\end{align*}

The result convinces.

More details can be found in the documentation of the package, section "Centered \vdots", where also the example above is taken from.

Answer 3

Here is another option (not very popular among TeXans):

\begin{eqnarray}
  R(-1) &=& \sum_{i=1}^m A(i)R(i-1) \\
  R(-2) &=& \sum_{i=1}^m A(i)R(i-2) \\
        &\vdots& \\
  R(-k) &=& \sum_{i=1}^m A(i)R(i-k)
\end{eqnarray}