Theoretical Computer Science in Italy Preface

Association for Theoretical Computer Science (EATCS). Its objective is to favor the dissemination and promotion of scientific research in all fields of theoretical computer science in Italy, by involving both national and international experts in the area. The 20th edition of the conference took place in Como, on September 9?11, 2019. The scientific program included 29 contributed papers. This volume contains a selection of extended versions of seven papers presented at the conference. All included papers were accepted only after being reviewed according to the journal standards. The paper by Cantone, Maugeri and Omodeo provides a complete taxonomy of the polynomial and the NP-complete fragments of set theory consisting of all conjunctions that involve, besides variables intended to range over the von Neumann set-universe, a collection of constructs drawn from the Boolean set operators ?, ?, \ and the membership relations ? and /?. Mantaci, Restivo, Romana, Rosone, and Sciortino focus on the combinatorial properties of the notion of string attractor, showing that the size of the smallest string attractor of a word is not a monotonic measure and that a circular variant of it characterizes the conjugacy classes of standard Sturmian words. The paper by de? Liguoro and Treglia studies a Curry style type assignment system for untyped ?-calculi with effects, based on a monadic approach. The work by Buro and Mastroeni shows that the frameworks of contextfree grammars, many-sorted signatures and order-sorted signatures are essentially the same from a semantic perspective. Auletta, Ferraioli and Greco study a problem of opinion diffusion on social graphs under the lens of algorithm design and computational complexity. Aceto, Anastasiadi, Castiglioni, Ing?lfsd?ttir, Luttik and Pedersen show that order-insensitive bisimilarity is not finitely based over BPA with priority. Finally, Giuliani, Lipt?k, Masillo and Rizzi deal with the following problem, which has interesting applications in bioinformatics: given a string w, the challenge is to find in which positions, if any, the $-character can be inserted to turn w into the Burrows-Wheeler-Transform image of a word ending with $. We would like to thank the authors, the reviewers, and all the people who contributed to the conference and this special