Definition:Matrix/Diagonal/Antidiagonal

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

The antidiagonals of $A$ are the diagonal of $\mathbf A$ lying perpendicular to the main diagonal of $\mathbf A$.

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