Linear Algebra/Matrix Equation
Diagonal Matrix 
A diagonal matrix, , is a square matrix in which the entries outside of the main diagonal are zero. The main diagonal of a square matrix consists of the entries which run from the top left corner to the bottom right corner.
In the example below the main diagonal are
Identity Matrix 
The identity matrix, with a size of n, is an n-by-n square matrix with ones on the main diagonal and zeros elsewhere. It is commonly denoted as , or simply by I if the size is immaterial or can be easily determined by the context.
The most important property of the identity matrix is that, when multiplied by another matrix, A, the result will be A
- and .