25

I want to figure out how I can align a system of linear equations very nicely.

At the moment I am using the following command:

\systeme{x_1=2r + s -t,x_2= r, x_3=-2s +2t, x_4=s, x_5=t}

But this gives me something ugly in staircase form, like:

enter image description here

Any way I could fix it so the x_i's are all at the left side, nicely underneath each other, and possibly with the equation signs also aligned?

Here is a MWE:

\documentclass[11pt,a4paper,openany]{report}
\usepackage{amssymb,amsmath,amsthm}
\usepackage[dutch]{babel}
\usepackage{mdframed}
\usepackage{systeme,mathtools}
\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
  \hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{#1}}
\makeatother
\usepackage{lipsum}
\usepackage{relsize}
\newcommand\md{\ }

\begin{document}

\systeme{x_1=2r + s -t,x_2= r, x_3=-2s +2t, x_4=s, x_5=t}

\end{document}
16
  • normally one uses align or one of its variants from amsmath but it is impossible to comment on your code as \systeme is not a standard plain or latex command and you give no indication of its definition. Jan 4, 2015 at 23:31
  • I'm using TexnicCenter, and I just use the package: \usepackage{systeme,mathtools}. There's no special definition, it's just standard.
    – Kamil
    Jan 4, 2015 at 23:32
  • I need to have \systeme because I want that there appears an array on the left to make it clear to the reader that it is a system of equations.
    – Kamil
    Jan 4, 2015 at 23:37
  • 1
    You have given no indication of what \systeme is, I had never heard of the command searching around suggests you are using a package of the same name and using latex, but you really should provide a complete example from \documentclass to \end{document} by editing the above question Jan 4, 2015 at 23:41
  • 1
    Use \systeme*
    – egreg
    Jan 5, 2015 at 0:00

5 Answers 5

25

The command to use for this is \systeme*:

\documentclass[11pt,a4paper,openany]{report}
\usepackage{amsmath}
\usepackage{systeme}

\begin{document}

I want to figure out how I can align a system of linear equations very nicely.
At the moment I am using the following command:
\[
\systeme*{x_1=2r + s -t,x_2= r, x_3=-2s +2t, x_4=s, x_5=t}
\]

\end{document}

enter image description here

By setting the value of \syslineskipcoeff you can modify the spacing; the default value is 1.25:

\[
\syslineskipcoeff{1}
\systeme*{x_1=2r + s -t,x_2= r, x_3=-2s +2t, x_4=s, x_5=t}
\]

enter image description here

5
  • 1
    Wonderful! Appreciate it.
    – Kamil
    Jan 5, 2015 at 0:08
  • The letters $r, s, t$ aren't aligned.
    – Bernard
    Jan 5, 2015 at 0:11
  • @Bernard That's not required by the OP
    – egreg
    Jan 5, 2015 at 0:13
  • 1
    @egreg: it's one of the main intrests in systeme. Else a simple align* environment would do the job. Or did I miss something?
    – Bernard
    Jan 5, 2015 at 0:16
  • @Bernard If one replace systeme* by systeme the letters $r, s, t$ will be aligned. Do you agree with me?
    – Student
    Jan 13, 2021 at 10:20
22

Here's a solution that uses only the array package and an array environment. In case you're curious about what's going on in the preamble of the array environment:

  • The four columns that contain variables are of type r

  • To get the correct spacing around the = symbols and the + and - signs, the amount of intercolumn whitespace (governed by the length parameter \arraycolsep) is first set to 0pt.

    • The = symbols are inserted automatically; the directive @{{}={}} tells LaTeX to treat = as an object of type mathrel.

    • The directives >{{}}c<{{}} tell LaTeX to center-set the column contents (which will be either +, -, or blank) and to treat them as objects of type mathbin.

enter image description here

\documentclass[11pt,a4paper,openany]{report}
\usepackage{array}
\begin{document}
\[
\left\{
\setlength\arraycolsep{0pt}
\begin{array}{ r @{{}={}} r  >{{}}c<{{}} r  >{{}}c<{{}}  r }
x_1 & 2r &+&  s &-&  t \\
x_2 &  r               \\
x_3 &    &-& 2s &+& 2t \\ 
x_4 &    & &  s        \\
x_5 &    & &    & &  t \\
\end{array}
\right.
\]
\end{document}
14

The systeme command is designed for the "matrix-like" (if you can't tell, I'm an engineer, not a mathematician) portion of the system to be on the left-hand side, not the right-hand side.

If you can accept a simple LHS->RHS swap of your input, systeme works out of the box:

\documentclass{article}
\usepackage{systeme}

\begin{document}
\systeme{2r + s -t=x_1, r=x_2, -2s +2t=x_3, s=x_4, t=x_5}
\end{document}

enter image description here

It is likely possible to create a new command in the spirit of \systeme{} with the aligned and unaligned sides reversed, but the code is above my head. ;-)

9

A simple hack with the aligned environment:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{systeme}
\usepackage{amsmath}

\begin{document}

\[ \left\{\begin{aligned}x_1 & = \\x_2 & =\\x_3 & =\\x_4 & =\\x_5 & =\\ \end{aligned}\sysdelim. . \systeme[rst]{2r + s -t , r , -2s +2t, s, t}\right.
 \]%

\end{document} 

enter image description here

3

Using a TABstack. In this case the "Center" of \tabbedCenterstack refers to vertical centering, the [r] refers to horizontal right alignment of each column, \stackMath processes the data in math mode, and \TABbinary inserts a {} prior to and following each cell, so as to give the + and - the proper horizontal spacing.

\documentclass[11pt,a4paper,openany]{report}
\usepackage{tabstackengine}
\stackMath
\begin{document}
\[
\left\{
\TABbinary\tabbedCenterstack[r]{
x_1 =& 2r &+&  s &-&  t \\
x_2 =&  r               \\
x_3 =&    &-& 2s &+& 2t \\ 
x_4 =&    & &  s        \\
x_5 =&    & &    & &  t
}\right.
\]
\end{document}

enter image description here

2
  • +1. What would you say are the main differences between using \TABbinary\tabbedCenterstack[r] and using an array environment?
    – Mico
    Jan 5, 2015 at 15:12
  • @Mico 0pt intercolumn gap is default in tabbed... stacks; Center-stacks are long stacks, thus all lines are forced to be equal baselineskips apart (though short stacks are more akin to arrays in this regard); interrow vertical baselineskip is easily set with \setstackgap{L}{length}; more options for how to automatically treat math operators with \TABbinary, \TABbinaryRight, and \TABbinaryLeft; \fixTABwidth{T} is available to force all columns to equal width; while centerstacks are vertically centered, stacks and understacks are not, which is often very useful. Jan 5, 2015 at 15:28

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