Block matrix with partial vertical line

Question 1

I would like to create a partial block matrix, with two blocks on the bottom row, separated by a vertical line between them, and separated from the rest of the matrix by a horizontal line.

My first attempt captured the idea, but the alignment was lacking

\documentclass{article}\usepackage{amsmath}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pmatrix}      \begin{matrix}        1 & x_0 & x_0^2  & \dots & x_0^n \\          &     & \vdots &       & \\        1 & x_m & x_m^2  & \dots & x_m^n \\      \end{matrix} \\      \hline      \begin{array}{c c | c c c}        \bigzero & & & \bigid_{n-m} &      \end{array}    \end{pmatrix}  \end{equation*}\end{document}

Ideally, I would like a little more space between the horizontal line and the rows. Furthermore, the0 should be centred between the first two columns, while theI should be centred between the last three columns.

I was able to obtain a much better spacing using thenicematrix package, however I have no idea how to produce the vertical line, whichshould join with the horizontal line but not spread into the first three rows.

\documentclass{article}\usepackage{amsmath}\usepackage{nicematrix}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceArray}{CCCCC}      1 & x_0 & x_0^2  & \cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \cdots & x_m^n \\      \hline      \Block{2-2}{\bigzero} & & \Block{2-3}{\bigid_{n-m}} &  \\      & & & &    \end{pNiceArray}  \end{equation*}\end{document}

Question 2

There is a typo in your code: you wrote{\bigid_{n-m}) instead of{\bigid_{n-m}}.

Question 3

@F.Pantigny Thanks, fixed.

Question 4

Possibly related:tex.stackexchange.com/questions/99238/compact-block-matrix

Question 5

@ABlueChameleon I read that question while doing my due diligence, but not only it is not immediately clear how to adapt the answer to this particular case, it also seems a lot more convoluted than usingnicematrix.

Question 6

@A.P. I didn't mean to say that I thought this was a duplicate or anything, but rather that if someone was looking for ways to typeset block matrices, there were additional options over there.

Question 7

You can draw the rule you want with a TikZ instruction:

\documentclass{article}\usepackage{nicematrix}\usepackage{tikz}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceMatrix}      1 & x_0 & x_0^2  & \Cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \Cdots & x_m^n \\      \hline      \Block{2-2}{\bigzero} & & \Block{2-3}{\bigid_{n-m}} &  \\      & & & &      \CodeAfter      \tikz \draw (4-|3) -- (6-|3) ;    \end{pNiceMatrix}  \end{equation*}\end{document}

With recent versions ofnicematrix, you can merely use the keyborders of the command\Block:

\documentclass{article}\usepackage{nicematrix}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceArray}{ccccc}      1 & x_0 & x_0^2  & \cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \cdots & x_m^n \\      \Hline      \Block[borders=right]{2-2}{\bigzero} & & \Block{2-3}{\bigid_{n-m}} &  \\      & & & &    \end{pNiceArray}  \end{equation*}\end{document}

Question 8

I miss similar example in package documentation. With some explanation, how vertical line is drawn. Also, is there a way to draw\hline˙ that it will not touch delimiter parenthesis (use optionmargin move vertical lines between block)?

Question 9

@Zarko: You are right. It's not possible to understand how this answer works because the documentation ofnicematrix is incomplete. I will complete that documentation soon. In a few words, the recent versions ofnicematrix create new nodes, a node for each row and a node for each column corresponding to the position of the (potential) rules in the array.

Question 10

@Zarko: It's not possible to change the horizontal rule created by\hline. The command\hline has not been modified bynicematrix. However, it's possible to use\cmidrule ofbooktabs to have an horizontal rule a bit shorter.

Question 11

Thank you for explanation in the first comment. Solution proposed in second comments works (fine), but to my opinion is to rude. Use simple\hline in conjunction withmargin option, if they will work properly, seems to be more consistent with existed solution (which not use new nodes for rows and columns). Anyway, +1 for answer!

Question 12

Unfortunately, I cannot get this to run (I copied your code as-is). I get two errors:Undefined control sequence. \CodeAfter andPackage pgf Error: No shape named col-3 is known (plus similar ones forrow-4 androw-6. I tried upgradingnicematrix to the latest available revision (54758 according to mytllocalmgr), to no avail.

Question 13

You can also use two instructions\multicolumn:

\documentclass{article}\usepackage{amsmath}\usepackage{nicematrix}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceMatrix}      1 & x_0 & x_0^2  & \Cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \Cdots & x_m^n \\      \hline      \Block{2-2}{\bigzero} & \multicolumn{1}{C|}{} & \Block{2-3}{\bigid_{n-m}} &  \\      & \multicolumn{1}{C|}{} & & &    \end{pNiceMatrix}  \end{equation*}\end{document}

Question 14

While I like this solution because it is in a sense simpler in not requiringtikz, I prefer the more centred alignment of the vertical bar in your other answer.

F. Pantigny 52.5k4 gold badges60 silver badges99 bronze badges · Accepted Answer · 2024-04-01 15:47:22Z

You can draw the rule you want with a TikZ instruction:

\documentclass{article}\usepackage{nicematrix}\usepackage{tikz}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceMatrix}      1 & x_0 & x_0^2  & \Cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \Cdots & x_m^n \\      \hline      \Block{2-2}{\bigzero} & & \Block{2-3}{\bigid_{n-m}} &  \\      & & & &      \CodeAfter      \tikz \draw (4-|3) -- (6-|3) ;    \end{pNiceMatrix}  \end{equation*}\end{document}

With recent versions ofnicematrix, you can merely use the keyborders of the command\Block:

\documentclass{article}\usepackage{nicematrix}\newcommand{\bigzero}{\mbox{\normalfont\Large\bfseries 0}}\newcommand{\bigid}{\mbox{\normalfont\Large\bfseries I}}\begin{document}  \begin{equation*}    \begin{pNiceArray}{ccccc}      1 & x_0 & x_0^2  & \cdots & x_0^n \\        &     & \Vdots &       & \\      1 & x_m & x_m^2  & \cdots & x_m^n \\      \Hline      \Block[borders=right]{2-2}{\bigzero} & & \Block{2-3}{\bigid_{n-m}} &  \\      & & & &    \end{pNiceArray}  \end{equation*}\end{document}

I miss similar example in package documentation. With some explanation, how vertical line is drawn. Also, is there a way to draw\hline˙ that it will not touch delimiter parenthesis (use optionmargin move vertical lines between block)?
@Zarko: You are right. It's not possible to understand how this answer works because the documentation ofnicematrix is incomplete. I will complete that documentation soon. In a few words, the recent versions ofnicematrix create new nodes, a node for each row and a node for each column corresponding to the position of the (potential) rules in the array.
@Zarko: It's not possible to change the horizontal rule created by\hline. The command\hline has not been modified bynicematrix. However, it's possible to use\cmidrule ofbooktabs to have an horizontal rule a bit shorter.
Thank you for explanation in the first comment. Solution proposed in second comments works (fine), but to my opinion is to rude. Use simple\hline in conjunction withmargin option, if they will work properly, seems to be more consistent with existed solution (which not use new nodes for rows and columns). Anyway, +1 for answer!
Unfortunately, I cannot get this to run (I copied your code as-is). I get two errors:Undefined control sequence. \CodeAfter andPackage pgf Error: No shape named col-3 is known (plus similar ones forrow-4 androw-6. I tried upgradingnicematrix to the latest available revision (54758 according to mytllocalmgr), to no avail.

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Block matrix with partial vertical line

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