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

The elements $a_{j j}: j \in \closedint 1 {\min \set {m, n} }$ constitute the main diagonal of $\mathbf A$.

That is, the main diagonal is the diagonal of $\mathbf A$ from the top left corner, that is, the element $a_{1 1}$, running towards the lower right corner.

Diagonal Elements

The element of the main diagonal of a matrix are called the diagonal elements.

Also defined as

Some sources define the main diagonal only for a square matrix.

Also known as

The main diagonal of a matrix is also known as:

the principal diagonal
the leading diagonal.

Also see

  • Results about the main diagonal can be found here.