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.*