Math & Science ⇒ Mathematical Objects of same Width

[Cham]

I defined three commutators, and I would like to display them with exactly the same size (width).

Here's a minimal compilable example :

Code: Select all

\documentclass[12pt,letterpaper]{article}
\usepackage{amsmath}
\usepackage{tensor}
\newcommand*{\1}{\hspace{1pt}}

\begin{document}
  \begin{align}
    [\; \tensor{\partial}{_i}, \, \tensor{\xi}{_j} \,] &= -\, \tensor{\varepsilon}{_{ijk}} \; \tensor{\partial}{_k}, \\ \nonumber \\
    [\; \tensor{\partial}{_i}, \, \tensor{\xi}{_{\textsf{dS}}} \1] &= -\, \omega \; \tensor{\partial}{_i}, \\ \nonumber \\
    [\; \tensor{\xi}{_i}, \, \tensor{\xi}{_{\textsf{dS}}} \1] &= 0.
  \end{align}
\end{document}

Here's a preview :

commutators.jpg (12.19 KiB) Viewed 6932 times

Currently, the brackets (commutators) on the left don't have the same width. I tried adding some spaces here and there using \, and \; but the results aren't satisfying. Using \phantom doesn't help much either. For aesthetical reasons, the first and third commutators should have the same size as the second one.

Any suggestions ?

[localghost]

An {array} — packed into a new command — could be an option. A very basic approach would look like this.

Code: Select all

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}    % loads »amsmath«
\usepackage{tensor}
\usepackage{array}

\newcommand{\commutator}[2]{%
  [%
  \begin{array}{
    @{}
    >{\centering $\displaystyle}p{1em}<{$}
    @{,}
    >{\centering $\displaystyle}p{1.5em}<{$}
    @{}
  }
    #1 & #2
  \end{array}%
  ]%
}

\begin{document}
  \begin{align}
    \commutator{\tensor{\partial}{_i}}{\tensor{\xi}{_j}} &= -\tensor{\varepsilon}{_{ijk}}\tensor{\partial}{_k}, \\[2\jot]
    \commutator{\tensor{\partial}{_i}}{\tensor{\xi}{_{\textsf{dS}}}} &= -\omega\tensor{\partial}{_i}, \\[2\jot]
    \commutator{\tensor{\xi}{_i}}{\tensor{\xi}{_{\textsf{dS}}}} &= 0.
  \end{align}
\end{document}

Needs perhaps some adjustment for the column width depending on the content. But this could be incorporated into the \commutator command.


Thorsten

[Stefan Kottwitz]

Another suggestion:

Code: Select all

\newcommand{\commutator}[3][1.2em]{[ \makebox[#1]{$#2$} , \makebox[#1]{$#3$} ]}

It can be used like Thorsten's suggestion: \commutator{first arg}{second arg}

It additionally supports a width argument, in case your expressions would have a different width in different formulas but should be made to the same width in each case: \commutator[width]{first arg}{second arg}. I just set 1.2em to the default.

Stefan

[Cham]

Ok. I get the idea.

Thanks to both of you.

[Cham]

Stefan,

your solution is very nice. However, I think there is too much space just before the 'comma', so I changed your commutator definition to this (notice the \!,) :

Code: Select all

\newcommand{\commutator}[3][1.4em]{[ \makebox[#1]{$#2$} \!, \makebox[#1]{$#3$} ]}

It's working, but is this the right way to reduce the extra space before the comma ? I want to be sure not to interfere with the width definition of the whole thing.

EDIT : By the way, the command to use the commutator is like this :
\commutator[width]{first arg}{second arg}
not
\commutator[width]{first arg,second arg}

Here's a sample of the commutator command in action. Very nice, isn't ?

commutators.jpg (36.99 KiB) Viewed 6895 times