“…Problems of this kind arise in the description of a multitude of scientific and engineering applications including optimal design, control, and parameter identification [8]. Examples of PDE-constrained optimization problems arise in aerodynamics [30,36], mathematical finance [10,15,16], medicine [4,26], and geophysics and environmental engineering [2,1,27]. PDE-constrained optimization problems are infinite-dimensional and often ill-posed in nature, and their discretization invariably leads to systems of equations that are typically very large and difficult to solve.…”