Malaguti, L., S. Perrotta, and V. Taddei. “Exact Controllability of Infinite Dimensional Systems With Controls of Minimal Norm”. Topological Methods in Nonlinear Analysis, vol. 54, no. 2B, Nov. 2019, pp. 1001-2, https://apcz.umk.pl/TMNA/article/view/TMNA.2019.087.