Off Diagonal matrix

Question

How to write off diagonal matrix in latex? I tried this

 \begin{align*}
   \left[
     \begin{array}{ccccc}
        &  &  &  & 1 \\
        &  &  & \ldots  \\
       1 &  &  &  &
     \end{array}
   \right]
  \end{align*}

Answer 1

You can use the bmatrix environment from amsmath to typeset the matrix, and the command \iddots from the package mathdots for the dots.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathdots}
\begin{document}
\[
R_n = \begin{bmatrix}
    & & 1 \\
    & \iddots & \\
    1 & &
\end{bmatrix}
\]
\end{document}

Answer 2

This could be a solution with less code and beauty:

\documentclass{article}

\usepackage{mathdots} % http://ctan.org/pkg/mathdots

\begin{document}

\[
R_n = \left[
\begin{array}{ccccc}
        &  &  &  & 1 \\
        &  & \iddots &  & \\
       1 &  &  &  &
\end{array}
\right]
\]

\end{document}

Output:

Update

Similar types of dots can be found: \iddots from mathdots, \adots from yhmath or \reflectbox{$\ddots$} from the graphicx package.

Check this out:

\documentclass{article}

\usepackage{mathdots}
\usepackage{yhmath}
\usepackage{graphicx}
\usepackage{MnSymbol}

\begin{document}
Different types of diagonal dots:

\[
  \begin{array}{ll}
    \verb!\ddots! & \ddots \\
    \verb!\udots! & \udots \\
    \verb!\iddots! & \iddots \\
    \verb!\adots! & \adots \\
    \verb!\reflectbox{$\ddots$}! & \reflectbox{$\ddots$}
  \end{array}
\]

\end{document}

Snippet:

This could be more nicer using nicematrix.

\documentclass{article}
\usepackage{nicematrix}

\begin{document}
\begin{equation*}
R_n =\begin{bNiceMatrix}
           &    &          & 1 \\
           &    & \Iddots  &   \\
           &    &          &   \\
        1  &    &          &  
    \end{bNiceMatrix}
\end{equation*}
\end{document}

Output:

Answer 3

Here is what you can do with {bNiceArray} of nicematrix.

\documentclass{article}
\usepackage{nicematrix}

\begin{document}

\begin{align*}
\begin{bNiceArray}{cccc}
    &  &         & 1 \\
    &  & \Iddots     \\
    \\
  1 \\
\end{bNiceArray}
\end{align*}

\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).