How to change space between columns in matrix

Question

Im trying to write 2 matrices in Jupyter Notebook Markdown but I can't figure out how to make the 2 matrices to have the same size. I don't know if I can change the whole size or reduce the space between columns to make it look better.

This is my code:

$$ G_x = \begin{bmatrix} -1 & -2 & -1 \\  0 & 0 & 0 \\ 1 & 2 & 1 \end{bmatrix}$$
$$ G_y\begin{bmatrix} -1 & 0 & 1 \  -2 & 0 & 2 \ -1 & 0 & 1 \end{bmatrix}$$

Answer 1

The package nicematrix has a feature dedicated to that problem.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\begin{NiceMatrixBlock}[auto-columns-width]
First matrix
[G_x = \begin{bNiceMatrix} -1 & -2 & -1 \  0 & 0 & 0 \ 1 & 2 & 1 \end{bNiceMatrix}]
and second matrix
[G_y = \begin{bNiceMatrix} -1 & 0 & 1 \  -2 & 0 & 2 \ -1 & 0 & 1 \end{bNiceMatrix}]
\end{NiceMatrixBlock}
\end{document}

You need several compilations.

Answer 2

A possibility with blkarray:

\documentclass{article}
\usepackage{amsmath}
\usepackage{blkarray, bigstrut}
\begin{document}
[ 
  \begin{blockarray}{@{}r{3}{r}}
    \begin{block}{@{}c@{\enspace}[{3}{r}]} 
    & -1 & -2 & -1 \bigstrut[t] \
    G_x = {}& 0 & 0 & 0 \
     & 1 & 2 & 1 \
    \end{block}\[-1ex]
    \begin{block}{@{}c@{\enspace}[*{3}{r}]}
    & -1 & 0 & 1 \bigstrut[t] \
    G_y = {}& -2 & 0 & 2\
     & -1 & 0 & 1 \
    \end{block}
  \end{blockarray}
]
\end{document} 

Answer 3

Here are two additional solutions:

• use \phantom{-} directives to insert a more space, and use a bmatrix* environment rather than a bmatrix environment

  • pro: easy to use
  • con: need to insert 1 \phantom directive for each columns whose width needs fine-tuning

• load the siunitx package and use its S column type in a a bespoke array environment

  • pro: no more fine-tuning required
  • con: for easy-and-straightforward application, column types must be fairly simple

\documentclass{article}
\usepackage{mathtools} % for 'bmatrix*' environment
\usepackage{siunitx} % for 'S' column type
\usepackage{array}   % for '\newcolumntype' macro
\newcolumntype{T}{S[table-format=-1.0]}  % <-- bespoke version of 'S' column type
\begin{document}
\begin{align}
G_0 &= 
\begin{bmatrix}[r]
  -1 & -2 & -1 \  0 & 0 & 0 \ 1 & 2 & 1 
\end{bmatrix} \
G_1 &= 
\begin{bmatrix}[r] % <-- note use of 'bmatrix' env.
  -1 & \phantom{-}0 & \phantom{-}1 \  -2 & 0 & 2 \ -1 & 0 & 1 
\end{bmatrix} \
G_2 &=
\left[ \begin{array}{@{} TTT @{}} %      <-- use the 'T' column type for all columns
  -1 & 0 & 1 \  -2 & 0 & 2 \ -1 & 0 & 1 % <-- note: no fine-tuning needed
\end{array}\right]
\end{align*}
\end{document}

Answer 4

With tabularray:

\documentclass{article}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\NewDocumentEnvironment{mymatrix}{+b}{
    \begin{+bmatrix}[columns={1.3em, r, colsep=2pt}]
    #1
    \end{+bmatrix}
    }{}
\begin{document}
    [
    G_x = 
    \begin{mymatrix}
        -1 & -2 & -1 \  0 & 0 & 0 \ 1 & 2 & 1 
    \end{mymatrix}
    ]
    [
    G_y = 
    \begin{mymatrix} 
       -1 & 0 & 1 \  -2 & 0 & 2 \ -1 & 0 & 1 
    \end{mymatrix}
    ]
\end{document}