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mpegwmvavi
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Homework Statement
Suppose A and I are n*n matrices and I is a unit matrix ,and A is an idempotent matrix,ie, A=A^2 .
Show that if rankA=r and rank(A-I)=s,then r+s=n
Homework Equations
no
The Attempt at a Solution
I know that if A is an idempotent matrix ,it will have eigenvalues either 0 or 1.
(Proof: Ax=(A^2)x ,and Ax=λx so(A^2)x = A(Ax)=Aλx=λ(Ax)=(λ^2) x
thus, λx=(λ^2) x →(λ-1)λx=0. Suppose x is a nonzero eigenvector, λ = 1 or 0. )
that is, if x1 and x2 are eigenvectors associated with eigenvalue 0 and 1 respectively,
then, A(x1)=0(x1)=0, (a)
and A(x2)=1(x2)=x2
so(A-I)(x2)=0 (b)
Now, I have (a) and (b) , how to show that rankA+rank(A-I)=n?
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