Let
$A$
be a square matrix of order
$n$
. Then
$A$
is called an **idempotent** matrix if
$AA = A$
.

If a matrix $A$ is idempotent, it follows that $A^n = A, \forall n \in \mathbb{N}$ . All Idempotent matrices except identity matrices are singular matrices.

One way to make idempotent matrices is
$A = I - u u^T$
, where
$u$
is a vector satisfying
$u^T u = 1$
. In this case
$A$
is symmetric too.

```
#Idempotent matrices in Julia
using LinearAlgebra
u = rand(5)
u = u/norm(u) #Force u^T * u = 1
A = I - u*u'
isapprox(A^100,A) # true
isequal(A,A') # true (test for symmetricity)
```

Cross posted to https://pythonjulia.blogspot.com/2022/03/100-julia-exercises-with-solutions.html

## Top comments (4)

Hey! This is cool to see, it would be nice to also add some context for folks about why you would need this, etc etc. You can also set the canonical URL if you edit this post to point to the original blog post above.

You can also set the canonical URL if you edit this post to point to the original blog post above.

How to do it.

I went ahead and fixed it for you, but next to the publish button there's a blue circle-ish thing, that's where you can set the canonical URL.

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