Proving Idempotency and Nonsingularity of Matrix A

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SUMMARY

The discussion focuses on proving properties of an idempotent matrix A, specifically that I - A is also idempotent and that I + A is nonsingular. The participants confirm that if A is idempotent (A^2 = A), then I - A maintains idempotency. Additionally, they establish that I + A is nonsingular, with its inverse given by (I + A)^(-1) = I - (1/2)A. This conclusion is reached through determinant properties and manipulation of matrix equations.

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Homework Statement



Let A be an idempotent matrix (A^2=A)
a) show that I-A is also idempotent
b) show that I+A is nonsingular and (I+A)^(-1)=I-(1/2)*A


Homework Equations





The Attempt at a Solution



I managed to solve part a, but have a little problem solving b.

I tried
(I+A)*(I+A)^(-1)=(?)I

but didn't know how to treat (I+A)^(-1)

Thanks,
Roni.
 
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You are given that A2 = A.
Adding I to both sides gives us
A2 + I = A + I
==> |A2 + I| = |A + I|

Can you do anything with that?
 
Mark44 said:
You are given that A2 = A.
Adding I to both sides gives us
A2 + I = A + I
==> |A2 + I| = |A + I|

Can you do anything with that?

oh, nice approach.
Thanks for the help.
I'll try the second part of B
 

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