What is Idempotent Matrix? Examples & Properties

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 P2 = P.

Idempotent matrix Examples

The examples of 2 x 2 idempotent matrices are

1.

    \[\begin{bmatrix} 0 &0 \\ 0& 0 \end{bmatrix}\]

2.

    \[\begin{bmatrix} 1 &0 \\ 0& 1 \end{bmatrix}\]

This second matrix is the identity matrix.

The examples of 3 x 3 idempotent matrices are

1.

    \[\begin{bmatrix} 2 &-2 &-4 \\ -1& 3 &4 \\ 1&-2 & -3 \end{bmatrix}\]

2.

    \[\begin{bmatrix} 1 &0 &0 \\ 0& 1 &0 \\ 0&0 & 1 \end{bmatrix}\]

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. P2 = 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.

1 thought on “What is Idempotent Matrix? Examples & Properties”

  1. 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?

    Reply

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