TITLE: Is there a symbol for the antidiagonal matrix that has 1 as every entry?
QUESTION [9 upvotes]: Is there a symbol for the antidiagonal matrix that has 1 as every entry? For example, in two dimensions 
\begin{equation}
\left(
\begin{array}{cc}
0 & 1 \\
1 & 0
\end{array}
\right)
\end{equation}

REPLY [7 votes]: A  row reversed identity matrix ($I_n$) can be denoted as $$J_n=\left( \begin{array}{} 0 & 0 & \ldots & 0 & 0 & 1 \\ 0 & 0 & \ldots & 0 & 1 & 0 \\ 0 & 0 & \ldots & 1 & 0 & 0 \\ \vdots & \vdots & & \vdots & \vdots &\vdots \\ 0 & 1 & \ldots & 0 & 0 & 0 \\ 1 & 0 & \ldots & 0 & 0 & 0 \\ \end{array} \right)$$
See here for further infomation.
$J_n$ is also used to notate an all-ones matrix (see comment). On the other hand the all-ones matrix is often notated as $\textbf 1_n$. To avoid any confusion you have to define notations.