In: Advanced Math
Let A ∈ Mat n×n(R) be a real square matrix.
(a) Suppose that A is symmetric, positive semi-definite, and orthogonal. Prove that A is the identity matrix.
(b) Suppose that A satisfies A = −A^T . Prove that if λ ∈ C is an eigenvalue of A, then λ¯ = −λ.
From now on, we assume that A is idempotent, i.e. A^2 = A.
(c) Prove that if λ is an eigenvalue of A, then λ is equal to 0 or 1.
(d) Set V1 = {v ∈ C n | Av = v} and V0 = {v ∈ C n | Av = 0}. Show that im A = V1 and ker A = V0.
(e) Prove that A is diagonalizable.