equationsnumbering
I do have a matrix but I can't add a reference number for it as shown in read in the photo below .. any help?
\[
A=
\begin{pmatrix}
0.3 & 0.4 & 0.3 & 0.0 \\
0.4 & 0.3 & 0.0 & 0.3 \\
0.3 & 0.0 & 0.4 & 0.3 \\
0.0 & 0.3 & 0.3 & 0.4
\end{pmatrix}
\]
\label{e1}
Here is a small sample of numbering equations.
\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{align} % numbers equations automatically
x &= (1-x) + 2x \\ % \\ ends the line
&= (x+y) - y \nonumber \\ % \nonumber removes the equation number
&= \frac{x}{2} + \frac{x}{2} % last line does not have \\
\end{align}
\begin{align*} % use of * removes all numbers in this block
x &= (2x + 5) - (x+5) \\
&= yx - (y-1) x
\end{align*}
\end{document}
This sample produces
Use a single column array:
\documentclass[]{article}
\usepackage{amsmath}
\usepackage{multicol}
\newcommand{\tsub}[1]{_{\mathrm{#1}}}
\begin{document}
\begin{multicols*}{2}
Using Poisson's equation for gravity,
\begin{equation} \label{eq: phi}
\Phi(r) =
\left\{
\begin{array}{@{}r@{}}
\dfrac{GM\tsub{tot}}{R\tsub{vir}}
\biggl[\ln\biggl(\frac{r}{R\tsub{vir}}\biggr) - 1\biggr]
\\
r < R\tsub{vir}
\\[2ex]
\dfrac{-GM\tsub{tot}}{r} \hfill r > R\tsub{vir} .
\end{array}
\right.
\end{equation}
\end{multicols*}
\end{document}
Textual subscripts should be in upright type and so I set them.
On the other hand, with the twocolumn option the text width is wider and you can use a better way.
\documentclass[twocolumn]{article}
\usepackage{amsmath,mathtools}
\newcommand{\tsub}[1]{_{\mathrm{#1}}}
\begin{document}
Using Poisson's equation for gravity,
\begin{equation} \label{eq: phi}
\Phi(r) =
\begin{dcases}
\dfrac{GM\tsub{tot}}{R\tsub{vir}}
\biggl[\ln\biggl(\frac{r}{R\tsub{vir}}\biggr) - 1\biggr]
&
r < R\tsub{vir}
\\
\dfrac{-GM\tsub{tot}}{r} & r > R\tsub{vir} .
\end{dcases}
\end{equation}
\end{document}
Best Answer
You should use an
equation: