# Unknown error with align

What is wrong with this align? I get error message when I compile it. Below is the code:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amsthm,amssymb,mathrsfs,latexsym,paralist}
\usepackage[makeroom]{cancel}

\begin{document}

\begin{align*}
\phi(f_0) &=
\frac{\cancel{\phi(f_1)}\phi(g)}{\phi(e)}
e^{i\pi(\cancel{\theta_{21}}-\cancel{\theta_{11}})}
= \frac{\phi(f_1)^\cancel{2}}{\phi(f_2)}
e^{i\pi(\cancel{\theta_{21}}+\theta_{12}-\theta_{22}-\cancel{\theta_{11}})}\\
& \Rightarrow \frac{\phi(g)}{\phi(e)} = \frac{\phi(f_1)}{\phi(f_2)}e^{i\pi(\theta_{12}-\theta_{22})} . . . (\star)
\end{align*}

\end{document}


This is the output error in ShareLatex:

*Missing { inserted.

\mathchoice l.194 \end{align*} A left brace was mandatory here, so I've put one in. You might want to delete and/or insert some corrections so that I will find a matching right brace soon.*

## 1 Answer

You can't put a command like \cancel into a superscript directly. You need to put curly brackets around it.

\documentclass{article}
\usepackage{mathtools,cancel}
\begin{document}
\begin{align*}
\phi(f_0) &= \frac{\cancel{\phi(f_1)}\phi(g)}{\phi(e)}
e^{i\pi(\cancel{\theta_{21}}-\cancel{\theta_{11}})}
= \frac{\phi(f_1)^{\cancel{2}}}{\phi(f_2)}
e^{i\pi(\cancel{\theta_{21}}+\theta_{12}-\theta_{22}-\cancel{\theta_{11}})}\\
& \Rightarrow \frac{\phi(g)}{\phi(e)}
= \frac{\phi(f_1)}{\phi(f_2)}e^{i\pi(\theta_{12}-\theta_{22})} . . . (\star)
\end{align*}
\end{document}

• Curly brackets around what? Apr 23 '15 at 4:22
• Look at the seventh line in the code above: = \frac{\phi(f_1)^{\cancel ... Apr 23 '15 at 4:37
• Yes, that fixes it. Awesome, freaking spent like an hour on this. Cool. Thank you! Apr 23 '15 at 5:00
• You might want to accept this answer by putting your mouse cursor near the upper left of the answer and clicking on the check mark. Apr 23 '15 at 5:20