This matrix has only '1's on its main diagonal:
$A = \begin{pmatrix} 1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 1 & 0\\ 0 & 0 & 0 & 1 \end{pmatrix}$
But whats the opposite of main diagonal?
I want to say something like: "There are only '0's on the NOT-main-diagonal"
Can someone help me with a word?