A square matrix A is said to be idempotent if
A2 = A. Let A be an
idempotent matrix.
- Show that I − A is also
idempotent.
- Show that if A is invertible, then A =
I.
- Show that the only possible eigenvalues of A are 0 and
1.(Hint: Suppose x is an eigenvector with
associated eigenvalue λ and then multiply
x on the left by A twice.)
- Let W = col(A). Show that
TA(x) =
projW x and
TI−A(x)
= projW ⊥ x.