How to prove that an orthogonal projection matrix is idempotent

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SUMMARY

The discussion focuses on proving that an orthogonal projection matrix \( P \) is idempotent, specifically demonstrating that \( P^2 = P \). The equation \( P = A(A^T A)^{-1} A^T \) is provided, where \( A^T \) denotes the transpose of matrix \( A \). Participants emphasize the need to understand the definition of an orthogonal projection matrix to establish the proof effectively.

PREREQUISITES
  • Understanding of linear algebra concepts, particularly matrix operations.
  • Familiarity with orthogonal projection matrices and their properties.
  • Knowledge of matrix transposition and inversion.
  • Experience with proof techniques in mathematics.
NEXT STEPS
  • Study the properties of orthogonal projection matrices in detail.
  • Learn about matrix idempotency and its implications in linear transformations.
  • Explore examples of orthogonal projections in various vector spaces.
  • Review proof techniques used in linear algebra, focusing on matrix identities.
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Students and educators in mathematics, particularly those studying linear algebra, as well as anyone interested in the theoretical foundations of matrix operations and projections.

TheBuddha11
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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.
 
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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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