Since Mignot proved in 1976 the necessity of a system which is equivalent to strong
stationarity, it has been an open problem whether such a system is still necessary in the
presence of control constraints. Using moderate regularity of the optimal control and an
assumption on the control bounds (which is implied by u_a<0≤u_b quasi-everywhere in Ω
in the case of an upper obstacle y≤ψ), we can answer this question in the affirmative. We …