Centering \vdots in a sparse matrix with an even number of rows

Question

I am trying to define only the non-zeros elements in a sparse matrix, thus using some \vdots where the entries are zeros and specifying the row when needed (using the blockarray command, with \usepackage{blkarray} in the preamble). My matrix only has 2 columns (or 4), so I can't center the \vdots like I would like (the problem would be solved if the matrix had an odd number of columns). Here's an exemple:

$\begin{blockarray}{ccc}
  \begin{block}{[cc]l}
   \vdots \\
   a & b & /K-1  \\
   c & d & /K \\
  \vdots \\
 \end{block}
\end{blockarray}$

As you can see in this example, while I specified to center the entries inbetween [ and ], the \vdots are centered on the first column. If I replace cc by any combination of r, c, and l it is still not satisfying. Also if I put the \vdots between any number of & on the right and/or on the left it still doesn't work.

The other solution I have (but not satisfactory either) is to add another column:

$\begin{blockarray}{cccc}
  \begin{block}{[ccc]l}
   &\vdots& \\
   a & & b & /K-1  \\
   c & & d & /K \\
  &\vdots& \\
 \end{block}
\end{blockarray}$

But as you can see now the non-zeros entries are too far apart and the result is not really pretty.

Thank you for your time and your help.

Answer 1

blkarray is able to do it without extra packages, just use \BAmulticolumn, which is the specific command for a blockarray:

\documentclass{article}
\usepackage{blkarray}

\begin{document}

\[
\begin{blockarray}{ccc}
  \begin{block}{[cc]l}
   \BAmulticolumn{2}{c}{\vdots} & \\
   a & b & /K-1  \\
   c & d & /K \\
   \BAmulticolumn{2}{c}{\vdots} & \\
 \end{block}
\end{blockarray}
\]

\end{document}

Answer 2

Two possibilities: the first one uses an additional column and the @{...} construct to decrease the inter-column spacing; the second one uses \tikzmark to place some marks and then places the \vdots using the marks.

\documentclass{article}
\usepackage{blkarray}
\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand\tikzmark[1]{%
  \tikz[remember picture,overlay] \node (#1) {};}

\begin{document}

\[
\begin{blockarray}{cccc}
  \begin{block}{[c@{\hspace{3pt}}c@{\hspace{3pt}}c]l}
   &\vdots& \\
   a & & b & /K-1  \\
   c & & d & /K \\
  &\vdots& \\
 \end{block}
\end{blockarray}
\]

\[
\begin{blockarray}{ccc}
  \begin{block}{[cc]l}
   \tikzmark{a} & \tikzmark{b} \\
   a & b & /K-1  \\
   c & d & /K \\
   \tikzmark{c} & \tikzmark{d} \\
 \end{block}
\end{blockarray}
\]
\begin{tikzpicture}[remember picture,overlay]
\node[yshift=0.5ex] at ( $ (a)!0.5!(b) $ ) {$\vdots$};
\node[yshift=1.7ex] at ( $ (c)!0.5!(d) $ ) {$\vdots$};
\end{tikzpicture}

\end{document}