TITLE: What are the diagonals in the matrix called?
QUESTION [9 upvotes]: How correctly to designate these diagonals? They are highlighted in different colors.
\begin{pmatrix}\color{green}1&\color{orange}2&\color{red}3&\color{blue}4\\
\color{orange}5&\color{red}6&\color{blue}7&\color{green}8\\
\color{red}9&\color{blue}{10}&\color{green}{11}&\color{orange}{12}\\
\color{blue}{13}&\color{green}{14}&\color{orange}{15}&\color{red}{16}\end{pmatrix}
$\color{green}{1, 14, 11, 8}$
$\color{orange}{5, 2, 15, 12}$
$\color{red}{9, 6, 3, 16}$
$\color{blue}{13,10, 7, 4}$ - secondary diagonal

REPLY [2 votes]: A cyclic generator matrix ( a generator for the cyclic group of order 4 ):
$$ {\bf C} =\left[\begin{array}{cccc}0&1&0&0\\0&0&1&0\\0&0&0&1\\1&0&0&0\end{array}\right]$$
if we multiply to the left of 
$${\bf M} = \left[\begin{array}{cccc}0&0&0&\color{blue} 4\\0&0&\color{blue}3&0\\0&\color{blue}2&0&0\\\color{blue}1&0&0&0\end{array}\right]$$
It generates ${\bf CM} = \left[\begin{array}{cccc}0&0&\color{red}3&0\\0&\color{red}2&0&0\\\color{red}1&0&0&0\\0&0&0&\color{red}4\end{array}\right], {\bf C}^2{\bf M} = \left[\begin{array}{cccc}0&\color{orange}2&0&0\\\color{orange}1&0&0&0\\0&0&0&\color{orange}4\\0&0&\color{orange}3&0\end{array}\right]$ , ${\bf C}^3{\bf M} = \left[\begin{array}{cccc}\color{green}1&0&0&0\\0&0&0&\color{green}4\\0&0&\color{green}3&0\\0&\color{green}2&0&0\end{array}\right]$
These are the ones you color, therefore we could call it something with cyclic: cyclic diagonals or cyclically generated antidiagonals.