I have a parametric equation, which is numbered automatically by the begin{equation} environment. My issue is, the number appears on the next line down, below the equation. Since there is enough space on the second line of the split equation, I want the equation number to appear there instead. Does anyone know how I could do this?

This is similar to some previous questions, but I have tried their solutions of \notag for the top line, and using negative \hspace, neither of which work, I think because I use an array. My code is below:

```
\documentclass[a4paper,twoside,12pt]{article}
\usepackage{multicol}
\begin{document}
\begin{multicols*}{2}
Using Poisson's equation for gravity,
\begin{equation} \label{eq: phi}
\Phi(r) = \left\{
\begin{array}{lr}
\frac{GM_{tot}}{Rvir}[\ln\left(\frac{r}{R_{vir}}\right) - 1] : r < R_{vir} \\
\\
\frac{-GM_{tot}}{r} : r > R_{vir} .
\end{array}
\right.
\end{equation}
\end{multicols*}
\end{document}
```

Thanks in advance!

EDIT: I can't use a different document format (e.g. [twocolumn]) since parts of my report are done in a single column. I do not wish to have my r < Rvir on a different line. I was thinking ideally that I could shift along the r > Rvir clause, so that the Equation number could be in line with the second equation, since there is room on that line. Can anybody do this?

## Best Answer

Use a single column array:

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.