"Frank " <firstname.lastname@example.org> wrote in message <email@example.com>... > > > Is the solution unique? ============
Without the constraints |x_i|=1, the solution will always be non-unique, because an (M-2)-by-M can never be full-column rank.
Even with the constraints, though (and assuming you're looking for a least squares solution), the solution can be non-unique. As a simple example, take A=eye(M-2,M) and b=zeros(M-2,1). All x satisfying the constraints are solutions in this case.