I was reading a paper where the authors used the following notation:
$||b - \mathbf{A}x||^2_D = (b - \mathbf{A}x)^t \mathbf{D} (b - \mathbf{A}x)$
where $\mathbf{D}$ is a diagonal matrix
I was curious about the subscript $D$ when taking the norm-$2$. Does this notation represent something special or is it just the author's way of expressing the quantity in the right hand side? Have you guys encountered this notation before?
Thanks!