Definition:Matrix/Diagonal/Subdiagonal

Definition
Let $\mathbf A = \sqbrk a_{m n}$ be a matrix.

The subdiagonals of $A$ are the diagonal of $\mathbf A$ lying parallel to and above the main diagonal of $\mathbf A$.

That is, the elements $\map a {r + k, s + k}$ where $s > r$.