Definition:Array/Diagonal

Definition
Let $\mathbf A = \left[{a}\right]_{m n}$ be an array.

The diagonals are the lines of elements of $\mathbf A$ running from:
 * $(1): \quad$ the element in the first row and first column running downwards and to the right
 * $(2): \quad$ the element in the first row and last column running downwards and to the left
 * $(3): \quad$ the element in the last row and first column running upwards and to the right
 * $(4): \quad$ the element in the last row and last column running upwards and to the left

If $m = n$, that is, if $\mathbf A$ is a square array, then $(1)$ and $(4)$ coincide, and $(2)$ and $(3)$ also coincide.

Also defined as
Some sources define the diagonal elements only for a square array.