Cfrgtkky

I am creating a block-diagonal matrix and want to use boxes along the diagonal but the boxes don't touch each other, so it looks funny.

What's a workaround for this?

Here's my code:

documentclass{article}

usepackage{amsmath}

begin{document}

[

  begin{bmatrix}       

    fbox{$B_1$} & 0 & 0  \

     0 & fbox{$B_2$} & 0 \

     0 & 0 & fbox{$B_3$} \

   end{bmatrix}

]

end{document}

something like the matrix in the middle ...of this answer (but without the red and color filling)

Squares in Matrix

Thanks,

The space between the columns is controlled by the dimen register arraycolsep:

documentclass{article}

usepackage{amsmath}

begin{document}

[

  setlength{arraycolsep}{0pt}

  begin{bmatrix}

    fbox{$B_1$} & 0 & 0  \

    0 & fbox{$B_2$} & 0 \

    0 & 0 & fbox{$B_3$} \

  end{bmatrix}

]

end{document}

Without touching the delimiters:

documentclass{article}

usepackage{amsmath}

begin{document}

[

  setlength{arraycolsep}{0pt}

  begin{bmatrix}

    ,fbox{$B_1$} & 0 & 0,  \

    ,0 & fbox{$B_2$} & 0, \

    ,0 & 0 & fbox{$B_3$}, \

  end{bmatrix}

]

end{document}

The size of the delimiters are at least delimiterfactor per mille of the formula height or the height of the formula minus delimitershortfall. Full size is achieved by either

delimiterfactor=1000relax

or

setlength{delimitershortfall}{0pt}

Example:

documentclass{article}

usepackage{amsmath}

begin{document}

[

  setlength{arraycolsep}{0pt}

  % delimiterfactor=1000 %

  setlength{delimitershortfall}{0pt}

  begin{bmatrix}

    ,fbox{$B_1$} & 0 & 0,  \

    ,0 & fbox{$B_2$} & 0, \

    ,0 & 0 & fbox{$B_3$}, \

  end{bmatrix}

]

end{document}

Colored boxes would also be possible as in Zarko's answer:

documentclass{article}

usepackage{amsmath}

usepackage{xcolor}

begin{document}

[

  setlength{arraycolsep}{0pt}

  setlength{delimitershortfall}{0pt}

  newcommand*{myfbox}[1]{%

    fcolorbox{red}{red!20!white}{$#1$}%

  }

  begin{bmatrix}

    ,myfbox{B_1} & 0 & 0,  \

    ,0 & myfbox{B_2} & 0, \

    ,0 & 0 & myfbox{B_3}, \

  end{bmatrix}

]

end{document}

Like this?

Well, this matrix is drawn ...

documentclass[border=3mm,tikz,preview]{standalone}

usetikzlibrary{matrix}



begin{document}

[

    begin{tikzpicture}

matrix (m1)    [matrix of nodes,

                 left delimiter={[}, right delimiter={]},

                 row sep=-0.5pt,column sep=-0.5pt,

                 every node/.style={inner sep=5pt}

                 ]

{

|[draw]| $B_1$ &   0           &   0             \

 0            & |[draw]| $B_2$ &   0             \

 0            &   0           & |[draw]| $B_3$   \

 };

    end{tikzpicture}

]

end{document}

by tikz with TikZ library matrix. Distance between cells of matrix as well to its brackets are set by inner sep=5pt. Cells with "frame" have option |[draw]|.

Addendum:
Since the matrix is now actually image, it is simple to present on more fancy ways. For example as:

MWE for above version of matrix is:

documentclass[border=3mm,tikz,preview]{standalone}

usetikzlibrary{matrix}



begin{document}

[

    begin{tikzpicture}

matrix (m1)    [matrix of nodes,

                 left delimiter={[}, right delimiter={]},

                 row sep=-0.5pt,column sep=-0.5pt,

                 every node/.style={inner sep=5pt},

                 F/.style = {draw=red, line width=0.5pt, fill=red!20}

                 ]

{

|[F]| $B_1$ &   0           &   0           \

 0          & |[F]| $B_2$   &   0           \

 0          &   0           & |[F]| $B_3$   \

 };

    end{tikzpicture}

]

end{document}