Preface of STACS 2021 Special Issue

on Theoretical Aspects of Computer Science (STACS).The conference was held online, due to Covid pandemic, organised in Saarbrücken by Saarland University from March 16 to March 19, 2021.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 56 papers out of 228 submissions were accepted, putting STACS 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 2021.The topics of the chosen papers cover various areas of Theoretical Computer Science, that is, algorithmic graph theory, linear dynamical systems, parameterized complexity analysis, automata theory, complexity theory, algorithmic group theory, and distributed algorithms.In what follows, we briefly describe the contributions of the papers, ordered alphabetically by author names.In the article "The Complexity of the Distributed Constraint Satisfaction Problem", Silvia Butti and Víctor Dalmau study the distributed variant of the constraint satisfaction problem on a synchronous, anonymous network from a complexity point of view.They show that the problem is decidable in polynomial time if and only if the template is a set of relations invariant under symmetric polymorphisms of all arities.The Minimum Circuit Size Problem MCSP w.r.t. to some size bound s is the problem of deciding whether the minimum circuit size of a given Boolean function on n inputs is at most s(n).Recent works in meta-complexity exhibited "hardness magnifi-