identity matrix

the identity matrix $\text{[math]}$ is the $\text{[math]}$ -matrix with 1's along the main diagonal (the diagonal from top left to bottom right) and 0's elsewhere.
if $\text{[math]}$ is an $\text{[math]}$ matrix, then
$\text{[math]}$ [cite:@calc_hubbard_2015 definition 1.2.10]