Before revision:  \to  We study several properties of \mathbf{Ind}_A(\mathcal{S}) in relation to A and \mathcal{S}.
After revision:  The problem is also such that when A is positive and S is a Krylov subspace, it reduces to CG for \omega = 0 and to MINRES for \omega\to We study several properties of \mathbf{Ind}_A(\mathcal{S}) in relation to A and \mathcal{S}.