Regularity of multipliers and second-order optimality conditions for semilinear parabolic optimal control problems with mixed pointwise constraints

A class of optimal control problems governed by semilinear parabolic equations with mixed pointwise constraints is considered. We give some criteria under which the first and second-order optimality conditions are of KKT-type. We then prove that the Lagrange multipliers belong to \(L^p\)-spaces. Moreover, we show that if the initial value is good enough and boundary \(\partial\Omega\) has a property of positive geometric density, then multipliers and optimal solutions are H\"{o}lder continuous.