“…Let H be a real Hilbert space with scalar product • | • and associated norm • , let x 0 ∈ H, let U and V be closed vector subspaces of H with projection operators proj U and proj V , respectively, and let p ∈ V . The basic best approximation problem minimize x − x 0 subject to x ∈ U and proj V x = p (1.1) covers a wide range of scenarios in areas such as harmonic analysis, signal processing, interpolation theory, and optics [3,22,32,35,38,40,43,52,59]. In this setting, a function of interest x ∈ H is known to lie in the subspace U and its projection p onto the subspace V is known.…”