Aligning one of the elements of an equation in an array environment

Question

Consider the following equation -

\documentclass{article}
\usepackage{array}

\begin{document}
\begin{equation}
A =  \\ \nonumber \left\{
\begin{array}{lllr}
A_{1} & & \mbox{} m=1, n=1 \\[3pt] 
A_{2} & & \mbox{} 2 \leq m \leq M, n=1 \\[3pt]
A_{3} & & \mbox{} m=1, 2 \leq n \leq N \\[3pt]
A_{4} & & \mbox{} 2 \leq m \leq M, \\[3pt] 
& & \mbox{} 2 \leq n \leq N
\end{array}
\right.
\end{equation}
\end{document}

I would like to shift $A_{4}$ a bit lower along the left column, so that it lies in between its constraints. I tried using {\raise -1ex \hbox{}} but that threw some errors. Is there an easier/smarter way to handle this?

Answer 1

With the help of cases and aligned:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}\nonumber
 A = 
  \begin{cases}
    A_{1} &   m=1, n=1 \\
    A_{2} &   2 \leq m \leq M, n=1 \\
    A_{3} &   m=1, 2 \leq n \leq N \\
    A_{4} &  \begin{aligned} &2 \leq m \leq M, \\  &2 \leq n \leq N \end{aligned}
  \end{cases}
\end{equation}
\end{document}