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Ingo Wegener passed away on November 26, 2008, after a threeyear struggle with brain cancer. A farewell celebration was held in Bielefeld on December 4, the day on which Ingo would have turned 58. This is a tremendous loss for all those who knew him personally, for Computer Science in Germany, in Europe, and in the world at large, and for many other communities in which he played a prominent role. Ingo is survived by his wife, Christa.
Ingo studied mathematics at the University of Bielefeld, where he graduated with a "Diplom" in 1976, received his PhD in 1978, with a thesis on circuit complexity, and obtained his "Habilitation" in 1981. He held a professorship in computer science in Frankfurt am Main from 1980 to 1987, when he became a full professor of Computer Science (in the area of Efficient Algorithms and Complexity Theory) at the Technische Universität Dortmund, the position he held until his death.
Ingo had an amazing talent for identifying interesting research areas and for asking exactly the right questions. More often than not, he also came up with the answers himself, by providing novel and efficient algorithms and proof techniques. His versatility and comprehensive knowledge in many different areas was astounding. Many younger researchers will know Ingo from his work in the years starting from the early 1990s, when he started to work in a newly created research area which might be called "Formal analysis of metaheuristics". It was his conviction that the huge field of optimization algorithms based on metaheuristics like Evolutionary Algorithms, Simulated Annealing, and so on, should be studied using the methods from the theory of efficient algorithms and complexity theory. Previously, the standard approach to measuring the efficiency of the heuristics had been to simply implement them and run tests. This new, theoretical, approach turned out to bevery fruitful, yielding a much deeper understanding of the limitations of such metaheuristics. It was documented in a large number of scientific papers by Ingo, by members of his research group, by his scientific cooperators in Germany and abroad, and many others.
Equally important was the work of Ingo and his group in the area of algorithms for and complexity of branching programs and Binary Decision Diagrams. Ingo thoroughly studied this fundamental method for representing Boolean functions, interesting as an object of theoretical study and eminently useful in practice. He posed many important and ultimately fruitful questions, and introduced several important variants of the basic OBDD model. He furthered the accessibility of this area to researchers all over the world by writing the monograph "Branching Programs and Binary Decision Diagrams  Theory and Applications", guiding the reader to the edge of current research. Before coming to Dortmund in 1987, Ingo had already finished his first influential monograph, "The Complexity of Boolean Functions"  a must on the bookshelf of every complexity theorist of that time. This book was based on his experience in the area of circuit and branching program complexity, to which he contributed several important results. Before that period, he did successful research in the area of "search problems", a topic that he revisited in his later work on search heuristics. How he, in addition to these achievements, found the time to do research on algorithms and data structures, like bottomup heap sort, or knight's tours on chessboards, is anyones guess.
Besides and beyond his scientific achievements, Ingo was a gifted and devoted teacher who managed to fascinate his students in an astounding way. His lectures covered all aspects of Efficient Algorithms and Complexity Theory, ranging from secondyear algorithms classes over "Formal Languages, Computability, and Complexity" through advanced graduate courses to very specialized researchrelated topics. Ingo's lectures were extremely well prepared, many of them accompanied by lecture notes  not written by scribes onthefly, but by himself before the semester started. When he delivered his lectures, complicated thoughts came across very lighthandedly. He made them very easy to grasp, never frustratingly difficult. Even his very tough mandatory classes were popular among the students. Ingo was the only professor at TU Dortmund who won the University Price for Excellence in Teaching twice. (The students nominate the prize winner.) He wrote several attractive textbooks for undergraduate students, and he was a devoted advisor for the students who wrote their diploma theses under his guidance (there were more than 130 of them). His enthusiastic manner also made him an ideal mentor for his many PhD students.
For his scientific achievements Ingo was awarded the KonradZuseMedaille in 2006, the most prestigious German computer science award. He was a member of some of the most important Academies in Germany, notably the German Academy of Sciences Leopoldina. In 2004 he was appointed to the German Council of Science and Humanities (the most important scientific advisory committee to the German government), a great honor and also a source for a lot of work that he applied himself to with great enthusiasm. His advice was valued both within his university  he was a long time member of the academic senate  and in the several steering committees of important German Computer Science endeavors, like the scientific directorate of Schloss Dagstuhl, in which he participated. On a European level, starting 2005, he was a member of the Council of the EATCS.
The basis of all these formally acknowledgeable and countable achievements was the fact that Ingo was simply as devoted a scientist as he was a teacher. He could not help but search for new questions to answer. Dealing with him was pleasurable, because of his calm and pragmatic way of approaching issues, and because of his open and friendly manner. Ingo will be missed by many  students, colleagues, and friends  and for quite a few students and colleagues he was a friend and an important source of thoughtful advice over the years. But he will also live on  in the results of his work, but also in the minds of all those who had the fortune to work with him.