Identity Matrix

An identity matrix acts like the number 1 in matrix multiplication.

The 3x3 identity matrix is:

$$\left[\begin{array}{ccc}1& 0& 0\\ 0& 1& 0\\ 0& 0& 1\end{array}\right]$$

More generally, for a $n\times n$ matrix, the identity matrix is:

$$\left[\begin{array}{cccc}1& 0& \cdots & 0\\ 0& 1& \cdots & 0\\ ⋮& ⋮& \ddots & ⋮\\ 0& 0& \cdots & 1\end{array}\right]$$