How to prove that an orthogonal projection matrix is idempotent

  • #1

Homework Statement


Prove that [P]^2=[P] (that the matrix is idempotent)


Homework Equations





The Attempt at a Solution



A(A^T*A)^-1 A^T= (A(A^T*A)^-1 A^T)^2

Where A^T is the transpose of A. I have no idea.
 

Answers and Replies

  • #2
D H
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What is the relationship between your matrix A and your matrix P?

Backing up a bit further, what is the definition of an orthogonal projection matrix?
 

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