Troubleshooting Code for 'Cases'

Question

I was trying to do the following:

$$

\begin{cases}

a, & n=1\\

$\Phi^{-1}x(n-1)$, & x(n)=x(n-1)

$\Phi^{-1}\x(n-1)+c$, & x(n)=x(n-1)+1

\end{cases}

$$

But I keep getting errors. Is there an obvious syntax error that I'm not aware of?

Answer 1

When using the cases environment, you don't need to have inline math delimiters wrapped inside.

One can simply write,

\documentclass{article}
\usepackage{mathtools}
%-----The font code I use----------------
\usepackage[utf8]{inputenc}
\renewcommand{\rmdefault}{ppl}                 
\linespread{1.05}    
\usepackage[scaled]{helvet}
\usepackage{courier}                                                           
\usepackage{eulervm}   

\normalfont     
\usepackage[T1]{fontenc}                                                           
\usepackage{textcomp}
%------------------------------------
\begin{document}
\begin{displaymath}
    \begin{cases}
        a, & n=1\\
        \Phi^{-1}x(n-1), & x(n)=x(n-1)\\
        \Phi^{-1}x(n-1)+c, & x(n)=x(n-1)+1
    \end{cases}
\end{displaymath}
\end{document}

and if you would like to have the math inside the cases to be display style, you can change the environment to \begin{dcases}...\end{dcases}.

Also, you had \Phi^{-1}\x(n-1)+c which could be causing other errors unless you have defined \x somewhere.

Per Gonzalo Medina's comment, we need \usepackage{mathtools} to use the dcases environment and cases should be used in the displaymath environment or \[...\].