Diagonal line in math arrays

Question

I have some arrays in math mode, such as the one below. I would like a diagonal line in a cell, something like diagbox (diagobx package) or diaghead (makecell package). But it seems that diagbox and diaghead only work in a tabular environment. Of course a way to go consists in using tabular instead of array but this requires to put some $'s in each cell. I'm looking for a less tedious solution.

\documentclass{article}

\usepackage{diagbox}

\begin{document}

$$
\begin{array}{r|cccc}
%\diagbox{x}{y} 
& (0,0) & (0,1) & (1,0) & (1,1) \\ 
  \hline
(0,0) & \phi^{-1}  &  \frac{f_n}{f_{n-1}} - \phi^{-1} & 0  & \frac{f_{n+1}}{f_{n-1}} \\ 
(0,1) & f_{n}/f_{n-1} & 0 & f_{n+1}/f_{n-1} & 0 \\ 
(1,0) & \phi^{-1}  & \phi^{-2}  & 0 & 0 \\ 
(1,1) & 1 & 0 & 0 & 0 \\ 
\end{array} 
$$

\end{document}

Answer 1

Just encapsulate \diagbox{x}{y} inside an \hbox{}, i.e., \hbox{\diagbox{x}{y}}. Also, you can surround the text with $...$ to ensure the text is in mathmode as such: \hbox{\diagbox{$x$}{$y$}}. Off-topic, you should be using \[...\] instead of $$...$$ (have a look here).

\documentclass{article}

\usepackage{diagbox}
\begin{document}
\noindent\ Without \$...\$
\[
    \begin{array}{r|cccc}
        \hbox{\diagbox{x}{y}} & (0,0) & (0,1) & (1,0) & (1,1) \\ \hline
        (0,0) & \phi^{-1}  &  \frac{f_n}{f_{n-1}} - \phi^{-1} & 0  & \frac{f_{n+1}}{f_{n-1}} \\ 
        (0,1) & f_{n}/f_{n-1} & 0 & f_{n+1}/f_{n-1} & 0 \\ 
        (1,0) & \phi^{-1}  & \phi^{-2}  & 0 & 0 \\ 
        (1,1) & 1 & 0 & 0 & 0 \\ 
    \end{array} 
\]
With \$...\$
\[
    \begin{array}{r|cccc}
        \hbox{\diagbox{$x$}{$y$}} & (0,0) & (0,1) & (1,0) & (1,1) \\ \hline
        (0,0) & \phi^{-1}  &  \frac{f_n}{f_{n-1}} - \phi^{-1} & 0  & \frac{f_{n+1}}{f_{n-1}} \\ 
        (0,1) & f_{n}/f_{n-1} & 0 & f_{n+1}/f_{n-1} & 0 \\ 
        (1,0) & \phi^{-1}  & \phi^{-2}  & 0 & 0 \\ 
        (1,1) & 1 & 0 & 0 & 0 \\ 
    \end{array} 
\]

\end{document}

Answer 2

A better solution: since diagbox always use \tabcolsep and \arraycolsep may have a different value, we need to do this:

\newcommand\mydiagbox[2]{\hbox{\tabcolsep=\arraycolsep\diagbox{$#1$}{$#2$}}}

Then we can use \mydiagbox{X}{Y} inside array environment.