Aligned and \left \right

Question

The following aligned equation gives an error 'Extra }, or forgotten \right.':

\begin{equation}
\begin{aligned}
U&=\mathbb{R}^{M N}\\
V&=\mathbb{R}^{2 M N}\\
W&=\mathbb{R}^{4 M N}\\
\min_{v\in V} \left\{E_p(v)&=\|\nabla v\|_{W,1}+\lambda \|\rho(v)\|_{U,1} \right\}
\end{aligned}
\end{equation}

Removing the \left and \right commands avoids the error:

\begin{equation}
\begin{aligned}
U&=\mathbb{R}^{M N}\\
V&=\mathbb{R}^{2 M N}\\
W&=\mathbb{R}^{4 M N}\\
\min_{v\in V} \{E_p(v)&=\|\nabla v\|_{W,1}+\lambda \|\rho(v)\|_{U,1} \}
\end{aligned}
\end{equation}

I don't understand why having the alignment marker inside a \left \right command is an error. How do I align an equation with the alignment inside a \left \right command?

Answer 1

You need to close the \left before the alignment with \right. and open another \left:

But this may be problematic if there are different vertical sizes on either side. In this case the easier solution is to use the fixed size delimiters such as \big, \Big, etc.

Code:

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
\begin{equation}
\begin{aligned}
U&=\mathbb{R}^{M N}\
V&=\mathbb{R}^{2 M N}\
W&=\mathbb{R}^{4 M N}\
\min_{v\in V} \left{E_p(v)\right.&=\left.|\nabla v|{W,1}+\lambda |\rho(v)|{U,1} \right}
\end{aligned}
\end{equation}
\end{document}