arraysmatrices
I'm having trouble aligning submatrices and elements within the same matrix. Essentially, what I want to do is this (excuse the terrible paint-job):
Where the submatrix Q is located at the middle of the upper-left four matrix cells. I'm primarily familiar with the array environment: is this a possibility there?
A flexible way of creating bordermatrix is the blkarray package. It allows to position delimiters in arbitrary positions only on some lines with a {block} environment. Here’s the same example as in Herbert’s answer:
\documentclass{article}
\usepackage{blkarray}
\begin{document}
\[
\begin{blockarray}{cccccc}
& & & i & & \\
\begin{block}{c\Left{}{(\mkern1mu}ccccc<{\mkern1mu})}
& 0 & a & b & c & d \\
& 0 & A & B & C & D \\
\cline{2-4}
\begin{block*}{c(ccc|cc)}
j & s & t & u & v & w \\
& t & & v & & \\
& u & & w & & \\
& V & W & z & Y & Z \\
\end{block*}
\end{block}
\end{blockarray}
\]
\end{document}
To avoid the \cline to go past the left parenthesis, I had to use \Left{}{(\mkern1mu} instead of ( as the left delimiter and, for symmetry, I added on the right a <{\mkern1mu}.
I think, the problem is rather that the minus sign has a large width, larger than the digits. Because of this the minus sign becomes more weight in centered alignments.
The following example only uses the digit width of the minus sign for the aligment and reduces the total width of the minus sign to 110% of the digit width. Usually there is enough white space at the left of the number thus that the protruding does not hurt to much.
\documentclass[11pt,a4paper]{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{graphicx}
\newcommand*{\matminus}{%
\leavevmode
\hphantom{0}%
\llap{%
\settowidth{\dimen0 }{$0$}%
\resizebox{1.1\dimen0 }{\height}{$-$}%
}%
}
\begin{document}
\[
A = \begin{pmatrix*}
\matminus1 & \matminus10 & 1 \\
100 & 5 & 16 \\
13 & 7 & 7
\end{pmatrix*}.
\]
\end{document}
The suggestion above is a compromise between different requirements. Of course, also the numbers can also be centered without taking into account the minus sign without changing the column margins. But then the cell entries are not independent any more, the typesetting depends on the values in the other rows:
\llap{$-$} sets the minus sign and lets TeX think, the width is zero. That matches the centering requirement. But depending on the other values in the other rows, additional
white space needs to be inserted at the left of the column.\hphantom{-} does not set a minus sign, but only the horizontal space. It can help for the proper alignment of positive and negative numbers, but there might be the need to reduce the white space at the left side of the column depending on the other values in the column.
Best Answer
How about a matrix inside a matrix?