Second-order conditions for spatio-temporally sparse optimal control via second subderivatives

We address second-order optimality conditions for optimal control problems involving sparsity functionals which induce spatio-temporal sparsity patterns. We employ the notion of (weak) second subderivatives. With this approach, we are able to reproduce the results from Casas, Herzog, and Wachsmuth (ESAIM COCV, 23, 2017, p. 263-295). Our analysis yields a slight improvement of one of these results and also opens the door for the sensitivity analysis of this class of problems.