# Linear Algebra/Matrix Equation

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## Diagonal Matrix[edit]

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[edit]

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 .