On Saturday August 31, 2013 Professor Jan Willems passed away. Although inevitable, the news has left us with a sad, empty feeling, realizing that never again we will be able to enjoy his warm, cheerful, enthusiastic and inspiring company. Jan leaves behind his wife Doke, his children Mark and Mia, Doke’s children, their children-in-law, and their grandchildren. In the first place our thoughts are with them.
Jan also leaves behind a large group of former PhD students who all experienced him as nothing less than a miracle when doing research together. Jan was the unique mix of ultimate creativity, associative power, ability of deep thinking, and broad knowledge, combined with an enormous amount of energy, enthusiasm, and perfectionism.
He also leaves behind many collaborators and colleagues from his beloved field of systems and control, a field that he helped to shape in such a prominent way, over such a long period. He influenced not only his Ph.D. students, but also his many masters students, undergraduate students and colleagues in Groningen, as well as those in the rest of The Netherlands, Europe and overseas. He had charisma, that special characteristic that young people nowadays call the X-factor. Although he became an icon for the systems and control community, he was as happy talking to young scientists as he was to top scientists. His door was always open to all. He remained a wonderful, cheerful, considerate, animated and kind person, the ideal colleague and traveling companion.
Jan Willems was born on September 18, 1939 in Bruges in Belgium. After finishing his studies in engineering at the University of Ghent, he moved to the United States. There he obtained his M. Sc. degree in electrical engineering from the University of Rhode Island in 1965, and his Ph. D. degree in electrical engineering from the Massachusetts Institute of Technology in 1968. His doctoral dissertation, on input/output stability, appeared as the monograph “The Analysis of Feedback Systems”, MIT Press, 1971. From 1968 to 1973 he worked as an assistant professor in the Department of Electrical Engineering at MIT. During this period he made fundamental contributions to the subject of optimal control, in particular, linear quadratic problems with indefinite cost, and the associated algebraic Riccati equation. His groundbreaking paper “Least squares stationary optimal control and the algebraic Riccati equation” in IEEE TAC, 1971, and his work on dissipative dynamical systems that appeared in Archive for Rational Mechanics and Analysis in 1972 led to the notions of dissipative system and linear matrix inequality (LMI), that are generally considered as the main concepts and analysis tools in the area of robust control, both in the linear and nonlinear case.
In 1973, Jan was appointed Professor at the Department of Mathematics of the University of Groningen in the Netherlands to set up the new specialism of Systems and Control. During this period his research covered subjects from differential games, realization theory and physical systems. By the end of the seventies his interests turned to the geometric approach to control, and to problems of disturbance decoupling. This research area attracted a large amount of attention during that period. In the late seventies he introduced the notions of almost controlled invariant and almost conditioned subspaces, that allowed to resolve problems of approximate disturbance decoupling by high gain feedback. During this period, Jan was also one of the founders (together with Roger Brockett) of the new journal Systems and Control Letters, which had its first issue appearing in 1981. He acted as one of the managing editors from 1981 to 1994. In addition, he also acted as editor-in-chief of the SIAM Journal on Control and Optimization from 1989 to 1993.
In the early 80’s, Jan became conscious of the limitations of input/output thinking as the framework for the analysis and synthesis of open and interconnected systems. This uneasiness eventually led him to develop what is called the behavioral approach, in which a dynamical system is simply viewed as a family of trajectories. This work also emphasizes the importance, for example in object oriented modeling, of latent variables in addition to the manifest variables which the model aims at. In the behavioral setting, interconnection is viewed as variable sharing, and control is viewed as interconnection, with feedback as an important special case. The original ideas were introduced in an early paper in the Italian journal Ricerche di Automatica. A more extensive development appeared in a three part paper in Automatica in 1986 and 1987. In 1988, he was awarded the Automatica Price Paper Award for this series of three articles in which the behavioral framework was disseminated. An important resource for his behavioral ideas is also the text book (co-authored with Jan Willem Polderman) “Introduction to Mathematical Systems Theory: A Behavioral Approach” from 1998.
In 1998 Jan received the prestigeous IEEE Control Systems Award. In the same year he was awarded the IEEE Control Systems Magazine Outstanding Paper Award for his paper “300 years of optimal control, from the brachystochrone to the maximum principle” (co-authored with Hector Sussmann).
During his period in Groningen, Jan has been of major importance to the systems and control community within the Dutch universities. Using his natural charm and skills in diplomacy and persuasion, he was one of the founders and chairperson (from 1986 to 1996) of the Dutch Network of Systems and Control. The main aim was to organize a national graduate school that offered courses in systems and control theory, one of Jan’s ambitions that came true. The network was the precursor of the Dutch Institute of Systems and Control (DISC) that was founded in 1995. From 1995 to 1999 Jan was the chairperson of the Board of the Dutch Institute of Systems and Control (DISC).
In 1993 Jan held the general chair of the European Control Conference that was held in Groningen, and acted as president of the European Union Control Association from 1994 to 1996. He was also the president of the Dutch Mathematical Society from 1994 to 1996. These are just the highlights; he was also very active in shaping policies in teaching and research at the Mathematics Institute in Groningen.
In 2003, Jan Willems became emeritus professor from the University of Groningen. Two years before, in 2001, he and Doke had already moved to Antwerpen in Belgium. There he was warmly welcomed as guest professor at the Department of Electrical Engineering, within the research group on Signals, Identification, System Theory and Automation (SISTA) at the K.U. Leuven.
Formal retirement from Groningen by no means meant that his activity level went down. Jan remained active as ever, fruitfully collaborating with many colleagues, and actively participating in conferences and workshops all over the world. Together with Doke he spend many visiting appointments in Kyoto, enjoying the splendor of the city in fall and in spring, together with fruitful scientific collaboration at the University of Kyoto. Also, until very recently he was an active participant of the annual IEEE Conference on Decision and Control, enjoying conversations with olds friends and colleagues, but also inspiring young researchers with suggestions and usually positive criticism. His unquenchable scientific energy is manifest in the nearly one hundred publications he co-authored after his official retirement. Many of these later publications show his very deep thinking, scientific maturity and clear vision on the field of systems and control. An example of this is the impressive paper “The behavioral approach to open and interconnected systems”, that appeared in the Control Systems Magazine in 2007.
It is hard to imagine a world without Jan Willems. The products of his scientific activity, the way he shaped the field of systems and control, and his influence on the scientific taste and thinking of his students will however remain.
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