View source for Identity matrix

A square matrix $I$ with entries $1$ on the main diagonal and $0$ otherwise:
$$
I_{ij} = \delta_{ij} = \begin{cases} 1  & \text{if}\ i =j \\ 0 & \text{otherwise} \end{cases} 
$$
where $\delta$ is the [[Kronecker symbol]].

If $R$ is a [[ring with identity]] and 0 and 1 are interpreted as elements of $R$, then $I$ is the [[identity element]] in the [[matrix ring]] $M_n(R)$.