[Tex/LaTex] Aligning two columns in one equation!

Question

Consider the following

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}


\begin{document}
\begin{align}
X_{n}=\begin{cases}
               X_{n}^{I}= t_{1}\nu_{1}^{n}+r_{1}\nu_{2}^{n}, \quad -N\leq n<-d, \\
               X_{n}^{II}= t_{2}\nu_{1}^{n}+r_{2}\nu_{2}^{n}, \quad \ -d<n<-1, \\
               X_{n}^{III}=t_{3}\nu_{1}^{n}+r_{3}\nu_{2}^{n}, \quad \quad 1<n<d, \\
               X_{n}^{IV}=t_{4}\nu_{1}^{n}+r_{4}\nu_{2}^{n}, \quad \quad d<n\leq N, \\
\end{cases}
\end{align}

\end{document}

how can I make sure that the = sign in the left column is aligned as well as the inequalities on the right?

Answer 1

One option is to try the array environment instead of the cases one.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}    
\begin{document}

\begin{equation}
a_{n}=\left\{%
       \begin{array}{@{}r@{{}={}}l@{\quad}r@{{\,n\,}}l@{}}
       X_{n}^{I}  & t_{1}\nu_{1}^{n}+r_{1}\nu_{2}^{n}, & -N \leq& <-d, \\
       X_{n}^{II} & t_{2}\nu_{1}^{n}+r_{2}\nu_{2}^{n}, & -d    <& <-1, \\
       X_{n}^{III}& t_{3}\nu_{1}^{n}+r_{3}\nu_{2}^{n}, &  1    <& <d, \\
       X_{n}^{IV} & t_{4}\nu_{1}^{n}+r_{4}\nu_{2}^{n}, &  d    <& \leq N, \\
\end{array}\right .
\end{equation}

\end{document}