Preface of STACS 2019 Special Issue

International audience; This special issue contains five articles which are based on extended abstracts presented at the 35th Symposium on Theoretical Aspects of Computer Science (STACS). The conference was held at the Technical University of Berlin from March 13 to March 16, 2019. The extended abstracts were chosen among the top papers of those which were selected for presentation in a highly competitive peer-review process (after which only 54 papers out of 260 submissions were accepted, putting it among the most competitive conferences in Theoretical Computer Science). Compared with the original conference papers, the articles have been extended with a description of the context, full proofs, and additional results. They underwent a rigorous reviewing process, following the TOCS journal standards, completely independent from the selection process of STACS 2019. The topics of the chosen papers cover various areas of Theoretical Computer Science, that is, algorithmic graph theory, automata theory, linear dynamical systems, parameterized complexity analysis, and distributed algorithms. In what follows, we briefly describe the contributions of the papers, ordered alphabetically by author names. Significantly extending the results of the conference version, the article "First-Order Orbit Queries" by Shaull Almagor, Joel Quaknine, and James Worrell studies fundamental reachability questions, so-called orbit problems: Here, for example, we are given a square matrix A of dimension d over the rationals and two semialgebraic