An **idempotent matrix** is a square matrix which when multiplied by itself, gives the resultant matrix as itself. In other words, a matrix **P** is called idempotent if P^{2} = P.

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## Idempotent matrix Examples

The examples of 2 x 2 idempotent matrices are

1.

2.

This second matrix is the identity matrix.

The examples of 3 x 3 idempotent matrices are

1.

2.

## How do you know if a matrix is idempotent?

It is easy to check whether a matrix is idempotent or not. Simply, check that square of a matrix is the matrix itself or not i.e. P^{2} = P, where P is a matrix. If this condition is satisfied then the matrix is idempotent. If the condition is not satisfied then the matrix is not idempotent.

Matrix one is Idempotent but it is not symmetric. I believe it projects arbitrary vectors onto the span of its column space. But what does it project along?