How to write matrices with dimensions in latex?

Question

I am just starting to use latex, and with all the internet help i figured out how to do many things myself, but now i am stuck. How can I write what you all can see in the picture? This is what i want to get

Thanks for the help!!!

Welcome to TeX.SX! Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. — , Jun 03 '15 at 16:04
Related/duplicate: What's the best way to write matrix together with its dimensions on top and left? — Werner, Jun 03 '15 at 16:14

score 35 · Answer 1 · edited Apr 13 '17 at 12:36

At its simplest, this might be the easiest way.

\documentclass{article}
\usepackage{amsmath} 
\begin{document}
\begin{equation}
\underset{n\times 1}{\mathrm{Y}} =  \underset{n\times p}{X} \times 
\underset{p\times 1}{\theta} + \underset{n\times 1}{\varepsilon}
\end{equation}
\end{document}

enter image description here

As egreg points out, putting the matrix dimensions in \scriptscriptstyle may give a better appearance. That can be added to each underset of the above solution (or, as he notes, by creating a helper macro), or one could try this alternate stackengine approach, which gives added control over the depth of the underset.

\documentclass{article}
\usepackage{stackengine} 
\stackMath
\begin{document}
\begin{equation}
\def\sss{\scriptscriptstyle}
\setstackgap{L}{8pt}
\def\stacktype{L}
\stackunder{\mathrm{Y}}{\sss n\times 1} =  \stackunder{X}{\sss n\times p} \times 
\stackunder{\theta}{\sss p\times 1} + \stackunder{\varepsilon}{\sss n\times 1}
\end{equation}
\end{document}

enter image description here

If one wants a more visual interpretation of this result, I would note this related question, Matrix decomposition dimensions diagram

\documentclass{article}
\usepackage{stackengine}
\stackMath
\newlength\matfield
\newlength\tmplength
\def\matscale{1.}
\newcommand\dimbox[3]{%
  \setlength\matfield{\matscale\baselineskip}%
  \setbox0=\hbox{\vphantom{X}\smash{#3}}%
  \setlength{\tmplength}{#1\matfield-\ht0-\dp0}%
  \fboxrule=1pt\fboxsep=-\fboxrule\relax%
  \fbox{\makebox[#2\matfield]{\addstackgap[.5\tmplength]{\box0}}}%
}
\newcommand\raiserows[2]{%
   \setlength\matfield{\matscale\baselineskip}%
   \raisebox{#1\matfield}{#2}%
}
\newcommand\matbox[5]{
  \stackunder{\dimbox{#1}{#2}{$#5$}}{\scriptstyle(#3\times #4)}%
}
\parskip 1em
\begin{document}
$\renewcommand\matscale{.6}
\matbox{7}{2}{n}{1}{\mathrm{Y}} = 
\matbox{7}{4}{n}{p}{X} \raiserows{1.5}{\matbox{4}{2}{p}{1}{\theta}} +
\matbox{7}{2}{n}{1}{\varepsilon}$
\end{document}

enter image description here

@ChristianHupfer Damned if I do...damned if I don't :^) I might add it as an alternative. — Steven B. Segletes, Jun 03 '15 at 16:10
It was my first idea, but you're definitely quicker than me, so go ahead ;-) — , Jun 03 '15 at 16:10
Possibly \underset{\scriptscriptstyle n\times p}{X} gives less prominence to the dimensions. — egreg, Jun 03 '15 at 16:12
@egreg Oh, I agree... but if I have to type out \scriptscriptstyle, I might as well do it with stackengine ;^b — Steven B. Segletes, Jun 03 '15 at 16:13
\newcommand{\matrixdim}[2]{\underset{\scriptscriptstyle#2}{#1}} and \matrixdim{X}{n\times 1} is easy (and preferable anyway, even if we don't add the style selection). — egreg, Jun 03 '15 at 16:15

MyUserIsThis · Answer 2 · 2015-06-03T17:16:36.190

3

I add my little answer in the form of a tabular (much more dirty than the math ones, but may be useful if a lot of complexity is added to the expression)

\documentclass{article}

\begin{document}
\begin{tabular}{ccccccc}
Y & $=$ & $X$ & $\times$ & $\theta$ & $+$ & $\epsilon$ \\
$n\times 1$ & & $n\times p $ & & $p\times 1$ & & $n\times 1$
\end{tabular}
\end{document}

edited Jun 03 '15 at 17:16

answered Jun 03 '15 at 16:56

MyUserIsThis

1,201

1

This is a workable solution, thanks for contributing :-) I'll note to you and others though that if you're a stickler for typography, the spacing is waaay off for mathematics. By the way, you might be able to use a matrix or array environment to avoid going in and out of math mode all the time :-) – Sean Allred Jun 03 '15 at 20:27

How to write matrices with dimensions in latex?

2 Answers2