Paragraph ended before \align was complete for following code

Question

\begin{align}\label{flicker4}
\[{\rm{rank}}\left[ {\begin{array}{*{20}{c}}
E&0&{{B_d}}\\
C&{{D_d}}&0\\
0&0&{{D_d}}
\end{array}} \right] = n + {\rm{rank}}{D_d} + {\rm{rank}}\left[ {\begin{array}{*{20}{c}}
{{B_d}}\\
{{D_d}}
\end{array}} \right],\]

\end{align}

Answer 1

There are a few problems with your code. First of all:

• Empty lines are not allowed inside a display math environment
• align is already a math environment, so you shouldn't use \[ .. \] inside it.

The code below has some additional suggestions, including using bmatrix instead of array, using equation instead of align because there is only one line and no alignment needed, removing a lot of unnecessary braces, and defining a new operator instead of using \rm (which has been deprecated for twenty-odd years).

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\rank}{rank}
\begin{document}
\begin{equation}\label{flicker4}
\rank
\begin{bmatrix}
E & 0   & B_d \\
C & D_d & 0   \\
0 & 0   & D_d
\end{bmatrix} = n + \rank D_d +
\rank \begin{bmatrix}
B_d\\
D_d
\end{bmatrix},
\end{equation}
\end{document}