This book constitutes the proceedings of the 23rd International Symposium on Fundamentals of Computation Theory, FCT 2021, held in Athens, Greece, in September 2021. The 30 full papers included in this volume were carefully reviewed and selected from 94 submissions. In addition, the book contains 2 invited talks. The papers cover topics of all aspects of theoretical computer science, in particular algorithms, complexity, formal and logical methods.

The 6th Workshop on Approximation and Online Algorithms (WAOA 2008) focused on the design and analysis of algorithms for online and computati- ally hard problems. Both kinds of problems have a large number of appli- tions from a variety of ?elds. WAOA 2008 took place in Karlsruhe, Germany, during September 18-19, 2008. The workshop was part of the ALGO 2008 event that also hosted ESA 2008, WABI 2008, and ATMOS 2008. The pre- ous WAOA workshops were held in Budapest (2003), Rome (2004), Palma de Mallorca (2005), Zurich (2006), and Eilat (2007). The proceedings of these p- viousWAOA workshopsappearedasLNCS volumes2909,3351,3879,4368, and 4927, respectively. Topics of interest for WAOA 2008 were: algorithmic game theory, appro- mation classes, coloring and partitioning, competitive analysis, computational ?nance, cuts and connectivity, geometric problems, inapproximability results, mechanism design, network design, packing and covering, paradigms for design and analysis of approximationand online algorithms, randomizationtechniques, real-world applications, and scheduling problems. In response to the call for - pers, wereceived56submissions.Eachsubmissionwasreviewedbyatleastthree referees, and the vast majority by at least four referees. The submissions were mainly judged on originality, technical quality, and relevance to the topics of the conference. Based on the reviews, the Program Committee selected 22 papers. We are grateful to Andrei Voronkov for providing the EasyChair conference system, whichwasusedtomanagetheelectronicsubmissions, thereviewprocess, and the electronic PC meeting. It made our task much easier. We would also like to thank all the authors who submitted papers to WAOA 2008 as well as the local organizers of ALGO 2

This book provides a good opportunity for computer science practitioners and researchers to get in sync with the current state-of-the-art and future trends in the field of combinatorial optimization and online algorithms. Recent advances in this area are presented focusing on the design of efficient approximation and on-line algorithms. One central idea in the book is to use a linear program relaxation of the problem, randomization and rounding techniques.

This state-of-the-art survey contains 11 carefully selected papers that cover some classical problems of scheduling, of packing, and of graph theory, but also new optimization problems arising in various applications like networks, data mining or classification.

This book constitutes the thoroughly refereed workshop post-proceedings of the 17th International Workshop on Approximation and Online Algorithms, WAOA 2019, held in Munich, Germany, in September 2019 as part of ALGO 2019. The 16 revised full papers presented together with one invited paper in this book were carefully reviewed and selected from 38 submissions. Topics of interest for WAOA 2018 were: graph algorithms; inapproximability results; network design; packing and covering; paradigms for the design and analysis of approximation and online algorithms; parameterized complexity; scheduling problems; algorithmic game theory; algorithmic trading; coloring and partitioning; competitive analysis; computational advertising; computational finance; cuts and connectivity; geometric problems; mechanism design; resource augmentation; and real-world applications.

This book constitutes the refereed proceedings of the 14th International Symposium on Experimental Algorithms, SEA 2015, held in Paris, France, in June/July 2015. The 30 revised full papers presented were carefully reviewed and selected from 76 submissions. The main theme of the symposium is the role of experimentation and of algorithm engineering techniques in the design and evaluation of algorithms and data structures. The papers are grouped in topical sections on data structures, graph problems, combinatorial optimization, scheduling and allocation, and transportation networks.

This book constitutes the thoroughly refereed post-workshop proceedings of the 12th International Workshop on Approximation and Online Algorithms, WAOA 2014, held in Wroclaw, Poland, in September 2014 as part of ALGO 2014. The 22 revised full papers presented were carefully reviewed and selected from 49 submissions. They cover a wide range of topics such as coloring and partitioning, competitive analysis, network design, packing and covering, paradigms for design and analysis of approximation and online algorithms, randomization techniques, real-world applications, and scheduling problems.