We consider optimal control problems that inherit a sparsity structure, especially we look at problems governed by measure controls. Our goal is to achieve maximal sparsity on the discrete level. We use variational discretization of the control problems utilizing a Petrov-Galerkin approximation of the state, which induces controls that are composed of Dirac measures. In the parabolic case this allows us to achieve sparsity on the discrete level in space and time. Numerical experiments show the differences of this approach to a full discretization approach.Inspired by [2] we consider the continuous minimization problemwhere the state y ∈ L q (Q) solves a heat equation with right hand side u, initial data u 0 and zero boundary values in a very weak sense, i.e.For more details we refer to [2, Definition 2.1.]. Here, Ω is an open bounded domain in IR d , d ∈ {1, 2, 3} with Lipschitz boundary Γ := ∂Ω, and Ω c ⊂⊂ Ω. For given T > 0 we consider the time interval (0, T ) and the space-time domain Q c ⊂⊂ Q := Ω × (0, T ). Let α > 0, β > 0 be suitable parameters and q ∈ (1, min{2, (d + 2)/d}). As control spaces we consider the real and regular Borel measures M(Ω c ) := C(Ω c ) * and M(Q c ) := C(Q c ) * , respectively. In this setting, (P ) has a unique solution (ū 0 ,ū) with associated stateȳ, which can be proven similar to [2, Theorem 2.7.]. For the controls we observe the following sparsity (see [2, Corollary 3.2.]):wherew ∈ L 2 (0, T ; H 1 0 (Ω)) ∩ C(Q) is the adjoint state (see [2, Theorem 3.1.]) andū 0 =ū + 0 −ū − 0 andū =ū + −ū − are the respective Jordan decompositions. Considering it the generic case thatw is not constant on sets of measure greater than zero, this implies that the support sets of the controls are of measure zero. This motivates us to suggest a discretization strategy, which preserves this sparsity structure on the discrete level in space and time.
Variational discretizationWe want to achieve the desired maximal discrete sparsity, i.e. Dirac-measures in space-time, by choosing the Petrov-Galerkinansatz and -test spaces that will induce this structure in combination with the variational discretization concept introduced in [5]. This concept, via the discretization of the test space and the optimality conditions, induces an implicit discretization of the controls (u 0 , u) ∈ M(Ω c ) × M(Q c ). This is how we control the discrete structure of the controls.In the following we will indicate discretization in space-time by index σ := (τ, h). We define the discrete state space consisting of piecewise linear and continuous finite elements in space and piecewise constant functions with respect to time