1. The problem statement, all variables and given/known data S1 is in subspace of C^n. P unique orthogonal projector P : C^n -> S1, and x is in range of C^n. Show that the minimization problem: y in range of S1 so that: 2norm(x-y) = min 2norm(x-z) where z in range of S1 and variational problem: y in range of S1 so that: (x-y)*z = 0 (* is the hermitian so this is an inner product) for each z in range of S1 have the same unique solution y = Px 2. Relevant equations P^2-P = 0 Range(P) = S1 ? 3. The attempt at a solution I have no idea what's going on here. I am using Numerical Linear Algebra by Trefethen and Bau but there's nothing about minimization or variational problems in the book =/ any hint to get me started would be much appreciated. For the variational problem this might be one way but it doesn't show that it's unique and I'm not even sure it's correct: (x-y)*z = 0 Set y = Px and multiply by P (Px-P^2x)*z = 0 but P^2 = P so (Px-P^2x) = 0 and therefore LHS = RHS Clueless on the first one.