Models: Concepts, Theory, Logic, Reasoning, and Semantics - Essays Dedicated to Klaus-Dieter Schewe on the Occasion of His 60th Birthday

• Atif Mashkoor
• Qing Wang
• Bernhard Thalheim

We are pleased to dedicate this Festschrift to Prof. Klaus-Dieter Schewe on his 60th Birthday. This Festschrift contains a collection of papers contributed by his former students, collaborators and colleagues - most are researchers whose academic careers have been strongly influenced by him. We offer this Festschrift as a token of acknowledgment of his scholarly achievements, services and contributions to teaching.

Prof. Klaus-Dieter Schewe received a Ph.D. in Mathematics from the University of Bonn in 1985. His Ph.D. thesis was in the area of group representation theory. After that, he worked in various ICT establishments in different roles such as IT consult, industrial developer, and industrial researcher. In 1990, he decided to return to an academic career, and subsequently obtained a Habilitation degree (Dr.rer.nat.habil.) in Computer Science, from the Brandenburg University of Technology in 1995. His habilitation thesis was in the area of formal semantics, in particular topos theory and intuitionistic logic. Since then, Prof. Klaus-Dieter Schewe has worked in a number of universities and scientific institutions over the world, including Technical University of Clausthal in Germany, Massey University in New Zealand, and Software Competence Centerr Hagenberg GmbH in Austria. This helped him build up a wide research spectrum covering on one side theoretical foundations and on the other side their practical applications.

This Festschrift contains three themes: formal methods, databases and conceptual modeling, because Prof. Klaus-Dieter Schewe has made important contributions in all these research areas.

• In the area of formal methods, the early work of Prof. Klaus-Dieter Schewe was on the integration of algebraic and model-based specifications and types expressing semantics via topoi, which was the major theme in his habilitation thesis. While his research was originally inspired by guarded commands, predicate transformers and thus the B language, Prof. Klaus-Dieter Schewe later became more interested in Abstract State Machines (ASMs). This change was motivated by the theoretical foundations within ASMs as reported by Gurevich's seminal work on the sequential ASM thesis, and the potential of ASMs for supporting real-world applications as exemplified in the area of software engineering. Together with his collaborators, Prof. Klaus-Dieter Schewe has studied behavioral theory and logic of different classes of algorithms such as (synchronous) parallel algorithms, concurrent algorithms, and reflective sequential algorithms. One of his articles on “Concurrent Abstract State Machines” was listed in the 21st Annual Best of Computing in 2016 under the category “F. Theory of Computation” by ACM.
• In the area of databases, Prof. Klaus-Dieter Schewe concentrated on object-oriented databases or more abstractly speaking databases that use rational trees as building blocks. One of his major interests concerns integrity constraints and in particular data dependencies, and the largely neglected area of updates. With his students and collaborators, axiomatizations for several classes of dependencies in presence of various constructors have been achieved. In addition to these, he also stimulated research on a physical architecture for object-oriented databases emphasizing distribution design, linguistic reflection, a stack-based approach to the integration of query and programming languages, persistent storage, and multi-level transactions.
• In the area of conceptual modeling, Prof. Klaus-Dieter Schewe created the co-design approach to Web Information Systems development, which combines semi-formal usage-oriented modeling with formal reasoning techniques derived from dynamic and deontic logics, and algebraic term rewriting.

Although Prof. Klaus-Dieter Schewe has conducted his research on diversified topics, almost all of his scientific contributions are directly or indirectly linked to formal, mathematical foundations.

In addition to his many contributions to the aforementioned research areas, Prof. Klaus-Dieter Schewe has been an awesome colleague and mentor, reading and commenting on the work of us and giving encouragement to young students. He has always been the key driver for inspiring his colleagues and students to pick up problems and to collaborate on them. Most importantly, he has empowered his colleagues and students to grow and realize their potential.