In Memoriam: Prof. dr. ir. Jan C. Willems

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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.

Please feel free to leave a message at the bottom of this page

68 thoughts on “In Memoriam: Prof. dr. ir. Jan C. Willems

  1. Larry Ho

    Among the many fond memories Sophia and I had with Jan was the 1970 visit by him just before Christmas in our London apartment where we were staying in our first sabbatical abroad. We were both young and in foreign land. The feeling of friendship and Comradery stay with me all these years. Larry Ho

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  2. Dimitri Jeltsema

    Although it was no secret that Jan was seriously ill, this news reaches me like a bomb. I will never forget his invitation and warm welcome to his house in Antwerp to talk a full day about our research and beyond. It maybe sounds like a cliche, but after I left, packed with the 100+ questions Jan asked me, the underlying message for myself was that I know that I know nothing. It is a comfort feeling that his influences and ideas will live fort in the many people that have been his students and that I can proudly call my collegues.

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    1. Shiva Shankar

      I feel orphaned, because Jan was my mentor (as he was for so many), and rescued me when matters were going from bad to worse at my previous position. Jan, for me, is proof that friendship and generous support are not confined by categories of nationality or race or anything else, and that true humanity transcends them all. This is, finally, what Jan taught me, by his example, and what I shall remember of him till the end of my days.

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  3. Agung Julius

    I remember one late spring 10 years ago, when Madhu Belur and I had the privilege to spend a couple of days working with Jan in Leuven. We would spend entire days doing mathematics, with Jan dividing his time between the two of us. At the end of each day, I would find my head almost exploding, while Jan was always sharp with lots of energy! I can’t agree more with Harry about Jan’s intellectual stamina. His departure is a big loss to all of us.

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  4. John Baillieul

    I cherish the memories of collaborating with Jan in 1998 on the edited Springer volume entitled Mathematical Control Theory. We also organized a workshop together that year to honor our friend and mentor, Roger Brockett. Jan was always a warm, supportive, and good humored guy who was great to work with. I shall miss him a great deal.

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  5. Maria Elena Valcher

    I have a lost a friend, a mentor and a source of inspiration.
    He was a wise, mature, and supportive man, with amazingly broad views and intuitions, and at the same time he had the fresh and untamed enthusiasm of a PhD student, always looking ahead at new results, new perspectives, new connections. He was an amazing person even outside of the research environment, and having a conversation with him on any topic was a unique and entertaining experience. I will miss him a lot. His smiling face will remain forever among my dearest memories.

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  6. W. Murray Wonham

    Jan was one of the finest researchers and scholars in the field of control and systems, to which he brought a classical blend of scientific insight, rigor of thought and expression, and appreciation of esthetics. In his personal friendship we cherished Jan’s unfailing generosity, integrity, and good humor.

    He will be remembered with respect and affection.

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  7. George. J. Pappas

    Jan was a true systems scientist, with control systems being a component of this larger agenda. He had a rare combination physical insight coupled with mathematical rigor. What I find very unique about his work is his emphasis on developing a science of modeling, as opposed to post-modeling analysis. This talent is needed today much more than ever. His vision and accomplishments will continue to guides us in the future.

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  8. Ulrich Oberst

    Jan’s death, although expected after his recent last personal email to me on August 4th, is a very sad and big loss, both for me personally and for the whole systems theory community.
    The many outstanding traits of his personality and his scientific contributions have already been described above, and I can only underline them. Jan’s seminal ideas and work on behaviors in 1985 were the starting point of my own engagement in systems theory. He was gentle to the readers and listeners of his many papers and conference talks that were always outstanding examples of clarity. His amiability and fairness towards his colleagues and in particular to me have always been appreciated and will be so for a long time in the future. I will keep Jan and his work in the best admiring memory.

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  9. Andre Tits

    From time immemorial, I have been a great admirer and a friend of Jan’s. In recent years I got to know him better when I undertook to teach an undergraduate class on the behavioral approach at Maryland. While already sick at the time, Jan seemed to always be in tune with my progress in the class, answering my questions within hours, and entering related philosophical discussions with me. (Jan knew how to make you feel like such discussion was from equal to equal, which it obviously was not.) Jan will always be an inspiration to me.

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  10. Uwe Helmke

    What a loss! I have neither been a student of Jan nor did we ever had a joint paper but I always felt close to Jan, having the greatest admiration to all he achieved. He supported me in my career on several crucial instances and was always friendly to me. The last meetings with Jan and Doke were in Würzburg 2010 at a meeting together with R. Kalman and others on Newtwork Synthesis, in Oberwolfach 2012 and in Kyoto. I have very fond memories of going out together for dinner in Kyoto with Doke and my wife Julia. These were always such pleasant evenings–I will never forget them! Is so sad that these cannot be continued!

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  11. Margreta Kuijper

    The last time I saw Jan was in the city of Budapest in 2010. He was frail, he looked pale but every bit of his personality and humour was still shining through. What a shock to hear that he passed away. He has been such an inspiration for me, his way of doing research, coming about so naturally, his enthusiasm and total dedication have set such an example. I will never forget how his face could light up when, talking about research he was “onto something”. I will miss him very much and my thoughts are with Doke and his direct family.

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  12. Bijoy K. Ghosh

    My first encounter with Jan was even before I had met him. It must have been around Oct. 1979 and I had just joined graduate school. Jan was organizing a Systems Conference in Europe, and I was interested in attending. My advisor told me – Bijoy, you cannot. You are not from a NATO country. I was shocked and somewhat offended and so I complained to Jan. His reply came in rather quickly explaining why it would not be possible to support me using conference money. The surprising part of that letter is the last paragraph that went on to say –‘ … if in spite of this, you decide to come, we can reimburse you a portion of your airfare.’ This offer from Jan came to an unknown graduate student, whom he has never met. I could not accept his offer.

    For reasons that I do not completely remember, Jan organized a set of seminars in the Pierce Hall, conference room. The year was roughly 1980 and Jan was talking about his newly invented almost versions of the invariant subspace theory. One year into graduate school, I had never seen a seminar so charged with arguments and counter arguments with a somewhat elderly person from behind the lecture room making comments that did not look like a ‘friendly discussion’. For me, that was the introduction to geometric control theory introduced by Hermann and Willems, and I am not sure if the arguments were ever settled.

    Much later, during mid eightees, I submitted a paper with a coauthor to System and Control Letters. Associate Editor promptly rejected the paper saying one of the proofs was too long. It was something like a six page paper with one theorem that was eight pages long. I asked for reconsideration and the conclusion was not changed. The matter was placed before Jan (he was the managing editor) and it took him only about a month to write to me saying: ‘Bijoy, your paper was placed before four additional reviewers……… I am sorry, the paper cannot be accepted. The fact that the paper was finally rejected was not so surprising. The surprising part was the final comment that Jan made in that same letter (I wish I had kept that letter). ‘…. if in spite of our decision, you think that the paper should be accepted, let us know.’ Needless to say, that was the first and the only time, I rejected my own paper.

    Jan, I will always miss you.

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  13. Stefano Stramigioli

    Unfortunately we knew we were going to get this bad news, but still it is hard to believe Jan is not with us anymore. His never ending energy for discussions, openness and pleasant attitudes was special and appreciated by many people. I will never forget the many discussions had in different occasions and his inspiring role for me when I was a young Ph.d. student. Jan is not with us anymore, but the memories and his contributions will stay with us for ever. Jan, thanks a lot for everything you have done!

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  14. Hans Zwart

    Although I did my studies in Groningen, I only got to know Jan better when I did Ph.D. Although I was not a student of him, and infinite dimensional systems theory was not his topic, I could always bother him with questions. He definitely thought me not to be afraid of asking questions. At colloquia, when I felt stupid because I could not follow the presentation anymore, it was always Jan who raised his finger, and posed a question. This attitude of genuine, but critical, interest is what I will always remember of Jan.

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  15. Hans Hellendoorn

    Jan willems played a very important role in the control community, in particular in the Netherlands and also in Delft. His focus on mathematical precision has changed the minds of a whole generation of control engineers who nowadays teach their students to formulate exactly and always double check mathematical equations. Frequently, I hear people in the department praise the lessons and the attitude they learned from Jan. Not only because he was an excellent scientist but also because he was a warm personality, open and modest, and always prepared to give support or attend a meeting. We will miss him very much and cherish his memory.

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  16. Jacob Engwerda

    I wish you Doke, Mark and Mia and Doke’s (grand) children strength in this difficult period.
    I hope you will be strengthened by the fact that this loss is shared by so many people
    around the world.

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  17. David Hill

    I feel a great sense of loss. I met Jan at conferences and on visits to Australia, and we shared a few dinners over the years – most recently at my house then in Canberra, but the impact he had on my view of systems and control started earlier and is somewhat impossible to summarise. He was ‘my hero’ when working on dissipativity as a student and I eagerly adopted his circuits-systems-control view. Later when I met him (Australia is a long way from Groningen), I was captivated by his intellectual but humanised approach to almost everything, always looking for the essence but with some sense of humour. It is really hard to imagine he has left us.

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  18. Frank Allgöwer

    Although expected, Jan’s passing away is a great shock and a very big loss.
    Jan was a most admired champion of the field, who constantly questioned everything, and this way acted as the key to insight and understanding. We will miss his advise, the long discussions about god and the world, his mentorship, and especially his nice presence.
    The control field just lost one of its giants and we all lost a dear friend and mentor.
    Thanks very much Jan. We will miss you!!

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  19. Paul van den Bosch

    We all in the field of control have benefitted considerably from his drive, ambitions, personality, networks and knowledge. He was the excellent teacher and showed us the way how to behave and do research. He is one of the giants in control on which shoulders, we, as dwarfs, can continue. His vision to start the Network on Systems and Control and later DISC had improved our field in The Netherlands considerably such that we are really visible in the world. A major achievement.
    We will miss him.
    Paul van den Bosch.

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  20. Bruce Francis

    Among his many endearing qualities, Jan was an engaging letter writer. What follows are some excerpts from his email correspondence of 2012.

    … I am myself quite confused by the issue of causality. Is this concept signal-oriented or should one think of system interaction (like: positive feedback causes instability)? I have done little serious thinking on this matter. In my notorious CSM article, I make some superficial comments about causality in the ‘sidebar’ on pages 56-57. Whatever else, I do hope you enjoy the cynical comment by Bertrand Russell at the end. But what transpires in this analysis is (i) the fact that yours truly seems hopelessly confused about the issue of causality, and (ii) the fact that one needs pure delays (as in discrete-time systems and probably in electromagnetics) in order to talk meaningfully about non-anticipation.

    … I do not consider the theory around normed spaces, Banach and Hilbert, distributions, even Fourier and Laplace, “applied”. In my view a treatment of such topics is “fundamental” (or “pure”) mathematics. I would like to reserve the term “applied” to topics where there is a clear application domain from the beginning (like “how do we deal with chance?” or “how do we deal with interconnected systems?”, etc.) .[Needless to say that for the same reason I find Schwartz’ title of his famous book in which he introduces distributions, Méthodes Mathématiques pour les Sciences Physiques, a misnomer.]

    … Mathematicians are weird fellows (I worked with them for 30 years). But you and I are not. To call the theory of vector, Banach, and Hilbert spaces, ‘applied’ is strange to me. Useful for applied mathematicians, absolutely, definitely, undoubtedly, but not applied itself. Otherwise, what mathematics is not applied? Number theory can be viewed as a preparation for cryptography, the theory of PDEs as a preparation for Electromagnetics, spectral theory of unbounded linear operators as a preparation for QM, etc.

    … Please bear with me a few moments, while I dish out some pedagogical/philosophical musings and wisdom. In my view one of the serious shortcomings of mathematical science is that the mathematical formalizations do not go back all the way to the ‘physics’ which they claim to address. Let me start with our own field. It pretends to deal with interconnected systems but it formalizes systems as signal processors. Signal processors and sensor/actuator feedback controllers are exceedingly important special cases, but merely special cases. With arrow-ridden signal flow diagrams one does not get off the ground for modeling even simple electrical circuits, the paradigmatic example of an interconnected system. When we interconnect two terminals of two physical systems, we invariably identify more than one variable (two currents and two potentials, two forces and two positions, two pressures and two mass flows, two temperatures and two heat flows, etc.) and yet we so often find signal flow diagrams where only one variable is carried from one subsystem to another. I find it so difficult to accept that the IO framework, which is obviously deficient for simple systems as interconnected circuits or lumped mechanical systems, will suddenly become the framework of choice for biological systems.

    It is a mystery to me how the IO point of view could have been so robust throughout the 20th century, and also now. My only explanation is sociological. The lure of mathematics / that is what we learn when we are young / that is how we converse / that is how our puzzles are formulated / we are in a hurry and under pressure to write our next paper and a paradigm shift will only slow us down, etc. In addition, there are so many people in the field (at least in my circles) with a pure mathematical training and without the experience to reason physical examples really through. Mathematics is in a sense easy: one can learn it and become very good at it, without having to trot into badly formulated domains.

    An area where this shortcomings of mathematical thinking is also obvious is probability and stochastics. It is typically taught, also to electrical engineers and economists, without entering into the interpretation at all, or at best with a couple of vague paragraphs referring to relative frequencies. The mathematics is there, but the methodology is not explained. Is a Kalman filter really trying to cope with additive stochastic noise? Is the problem with system identification really that there is an unmeasured input that has stochastic regularity?  Unknown, at least unmeasured, but known that it is stochastically regular?  Why have these stochastic formulations been preferred to simple least squares?

    I could go on and on.

    That is why I am suspicious in letting the boundary even slide further. Yes, Heaviside and Dirac used delta functions in a non-rigorous way. But that does not mean that making distributions rigorous is mathematical physics. Of course in an exposition of mathematical physics one may need to explain some of this mathematics, but without the physics and with only the mathematics, one does not have mathematical physics. With only measure theory, one does not explain how to deal with random phenomena. With only Hilbert spaces, linear operators etc. one does not have an account of applied mathematics. With a dictionary, a thesaurus, a book of grammar and one a book of syntax, we do not have literature appreciation or Shakespeare.

    Well, I guess that is enough for one Monday morning. Thank you for listening.

    … In my passion, I cannot resist in adding last word. Frankly, I also do not care very much for what is applied and what is pure.

    But I believe that one should not close one’s eyes for counterexamples. With IO thinking one cannot get off the with eg electrical circuits. There exists a perfectly well thought out alternative (generalization if you like) and yet the community chooses to ignore these problems and, worse yet, many textbooks will use precisely electrical circuits as illustrations for IO thinking…

    Kind regards, Jan

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  21. Sandro Zampieri

    I was young when I met Jan the first time. He was one of the great masters of systems theory and I had a lot of respect for him. But Jan broke immediately this distance allowing only friendship between us. He liked being together with young students and he had a special gift in communicating with them.
    It’s so sad to know that I’ll not see Jan anymore stopping me along the corridors that connect CDC conference rooms and giving me a break chatting pleasantly about everything.

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    1. Marco Signoretto

      I first met Jan in the early days of my phd at ESAT. Back then, I did not known how important his contribution to system and control was. Nevertheless, I immediately realized that his insightful, passionate and constructive comments were the clear sign of a true scientist. Although my topic was somewhat far from his domain we met many times in his office animately discussing fundamental mathematical aspects. I cherish those memories as one of the best moments in my experience as a researcher.

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  22. Karel Keesman

    Jan, I will always remember your open mind, inspiring lectures and interest in the real applications I have been working on.

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  23. Keith Glover

    I first met Jan on September 24th 1969, the week after his 30th birthday, when he gave the first lecture in the course ‘6.60: Analysis of Dynamical Systems’ at MIT where I had just arrived as a graduate student and he had just started his second year as an Assistant Professor. His clarity of exposition and enthusiasm for systems and control had me hooked and essentially determined my subsequent career. It was a privilege for me to become his (joint-)first PhD student and I have treasured his opinions, company and impeccable sense of taste ever since.

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  24. Henk Broer

    I know Jan since he came to Groningen in the early 1970’s. He always was a charming person to have discussion with. And fun. At a very early stage he recognized the `crisis in mathematics’ as he called it, something that is still going on today and that we are experiencing the effects of

    It is sad that he is gone now and I wish all those close to him strenght and courage.

    Henk

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  25. Anna Bosgra

    It was a great shock for me, to hear that Jan Willems has passed away, only a few month after my husband Okko. For Okko Jan was an important model, with a profound professional knowledge. But above all he was such a charismatic and loving personality, that when you ever met him, you will never forget him. I remember the day, in september 2009, on the symposium on the occasion of the retiring of Okko: Jan was ill, but he was there, with his fine contribution! I know that Okko felt a lot of thankfulness for all the inspiration he got from Jan.

    I wish Doke and the family all the strength and courage they need in de coming time.
    Anna Bosgra.

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  26. Hans Schumacher

    In 1978, Jan delivered a series of lectures at the Mathematical Centre in Amsterdam on the emerging discipline of mathematical system theory. As a PhD student in applied mathematical analysis at the Vrije Universiteit in Amsterdam, I cycled to the MC each week to follow these lectures. My expectations were more than fulfilled. Jan talked about geometric control theory and opened up a world of research vistas. Needless to say that he had a tremendous influence on my thesis, and on my later research work as well. He was a natural leader, full of inspiring ideas, capable like few others to question what most people would take for granted. Always looking for the fundamentals, he would not give up until the essence was made clear. On social occasions, it was a pleasure to talk to him and to share in his nonconformist views, whether it concerned science, politics, or any other topic of interest. It is saddening that his voice will not be heard anymore. Jan’s passion for true understanding will remain a source of inspiration.

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  27. Aart de Zeeuw

    I was about to finish my studies (in statistics) in Groningen when Jan arrived. I needed a few last credits and signed in on his first course. This had several effects. I switched to systems and control, Jan opened my eyes for the beauty and the challenge of science, I continued for a PhD (in differential games and economics), I embarked on an academic career and I got the support of Jan through life. I will never forget him, the discussions with him, his inspiration, his open mind and his friendship.

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  28. Yutaka and Mamiko Yamamoto

    Jan,

    I and Mamiko miss the days we spent together, in Japan and everywhere we met. You were a great friend, in a sense also a mentour, and I have learnt a great deal from you. We shared lots of fond memories together, and we adore these occasions that we can no longer have.

    You were a true scholar, and it was always a genuine pleasure to discuss science with you. Your unbiased thinking, penetrating deeply into the very basics of the issues is always a great inspiration
    to me. This is deeply rooted in your fundamental attitude toward life, and this virtue is prevalent in every phase of your life.

    You were one of (if not the) the most charming person I’ve ever known, with a great sense of humor and wit. This is also supported by your fundamental philosophy toward life. You are an extremely
    fair person, and do not wish to treat yourself in any way different from others. You were a true liberal and a gentleman.

    Jan, I miss the days we discussed science, mathematics, system theory, religion, randomness, probability, uncertainty, culture, the sense and nonsense of what we do and we don’t. Your love for clarity always impressed me, and I’d like to think I shared some of it during our discussion. Those were wonderful days.

    Thanks again for all these days, Jan. You were a great person, and we are proud to have had this privilege of knowing you and sharing these great occasions together.

    We wish that your soul rests in peace forever.

    Best wishes,

    Yutaka and Mamiko

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  29. Petar Kokotovic

    Already as an MIT graduate student Jan showed extraordinary maturity in selecting research problems which he pursued with great originality. When I first met him at the 1966 Allerton Conference, organized annually by the University of Illinois, Jan impressed us all with his treatment of a fundamental problem of feedback, hotly debated by leading authors in the field. Soon after, with his comprehensive study of the Riccati equation in 1971 and his beautiful general theory of dissipative systems in 1972, Jan emerged as a leading scholar of system theory, whose work has continued to have major impact to this day. Jan’s dissipative systems paper was included, thirty years after its publication, in the IEEE collection “Twenty Five Seminal Papers, 1932-1981″. Jan’s role as a builder of the scientific foundations of our field is immense.
    Over the years I continued to be fascinated by the brilliance of Jan’s intellect, the warmth and openness of his personality. Anna and I enjoyed having Doke and Jan as dear friends and cherish the memory of the times we spent together, especially our delightful tour of vineyards in Portugal.

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  30. Martin Weiss

    Being among those who had the chance of working three years in an office next to Jan, one year even as a postdoc under his supervision, I feel extremely privileged to have known in Jan Willems one of the greatest names in the field of Systems and Control. He was certainly the smartest person that I ever encountered in my entire life. Even as the time I was in direct contact to him was relatively short, his influence was essential for my career and I am most thankful for that.

    With the regret that during the last years I could not find an opportunity to visit him again, I can only join the large group of people that will miss him dearly in the time to come.

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  31. Roberto Tempo

    I have been shocked to hear the tragic news.

    Many nice memories of meetings with Jan around the world come out these days. I also have vivid memories of his visit to Torino in 1997, where he taught a short course. During that course, and afterwards, I have learned from Jan the beauty of difficult mathematical problems.

    The systems and control community is going to miss him immensely.

    Roberto Tempo

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  32. Bo Wahlberg

    Jan has been a regular participant at the workshops of the European Research Network System Identification. ERNSI was launched by the European Commission as a training network for PhD students, and has had an annual workshop since 1992. The idea is to have a small workshop at a nice historical place with very lively and open discussions. Jan has very much contributed to the friendly atmosphere of ERNSI and will be greatly missed. A large number of system identification researchers, including myself, have been inspired by Jan’s critical and constructive way to discuss research challenges in modeling of dynamical systems.

    Bo Wahlberg
    Coordinator of ERNSI

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  33. Bert van Keulen

    I was one of the many students who attended the Systems and Control graduate program at the Mathematics Institute in Groningen. After initially taking the welcoming, informal, and always scientifically curious atmosphere for granted, I realized later what a fantastic environment Jan Willems had created. This was because Jan was smart, wise and generous. A great example for all of us.

    Bert van Keulen

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  34. Rikus Eising

    Jan was a TEACHER with a good shoulder and stimulus for his students. He was one of the giants in the S&C field. Jan created a School (S&C, Philosophy, Science, Wisdom, Environment and Friendship). Without Jan no DISC. I have never met a person so motivating.
    I will cherish Jan’s memory.

    Rikus Eising

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  35. Rodolphe Sepulchre

    I have come to know Jan first through his work. I found in his papers an unmatched level of depth.
    I later came to know the vibrant man behind this work. Jan was always vibrant. There was an intense fire in each of his words and thoughts. It was the fire that accompanies the quest for truth. The fire that keeps alive, against all odds.
    Then came the illness. And with the illness, I came to know an extraordinary human being and an extraordinary couple.
    Jan will be deeply missed but Jan’s spirit will keep illuminating the fire of many, winner of his longlasting battle.

    Reply
  36. Roland Toth

    Dear Jan,
    You were one of the few good men, who left a mark on the world, which would be treasured by future generations. As a researcher, I think no one can wish more than that. I personally thank you for your guidance, the eye-opening discussions and the inspiration. Without them, I could have never got this far. I will always remember you! God bless you!

    Reply
  37. A Stephen Morse

    Jan Willems was a great engineering scientist and a great guy. He thoroughly exuded passion for his research like no one else I know did – and with good reason. His contributions in areas as diverse as dissipative systems, the Riccati Equation, modeling, and even adaptive control have left indelible marks on the field. He thoroughly understood what style in research meant and he had excellent judgement in choosing his problems. He gave lucid, exhilarating and sometimes humorous talks. Those fortunate enough to have heard him lecture using hand-written slides know that he had beautiful penmanship, learned no doubt in his early days in Brugge. He was a warm, generous, easily approachable individual with a delightful and wry sense of humor. He absolutely loved to debate – about almost anything.

    I first met Jan around the time he was finishing his PhD at MIT. What began as a mere acquaintance eventually grew into a lifelong friendship. Over the years Karin and I had the good fortune to be able to spend much quality time with Jan and with Doke not only at professional meetings, but also on holidays all over the world. I learned that Jan had sea legs, remarkable stamina as a biker, considerable prowess as a helmsman, and the diplomatic skills of a professional. We have wonderful memories of those times. Our hearts go out to Doke and we already miss Jan very, very much.

    Reply
  38. Brian Anderson

    Jan was a tremendous combination–a lovely human generous guy and a wonderful researcher. I remember him on the one side for his courtesy, his warmth, his insights about life, his concern for people and on the other side I remember him for his immense scientific contributions, especially on passive systems and behavioural descriptions of systems. He really stood like a tower in Europe for decades, casting a very big shadow that reached across oceans. It was a thrill to hear him lecture; I always learnt something, even about things as old as Maxwell’s equations.

    Reply
  39. Gregg & Nathalie Hupert

    Dear Doke,

    May the fact that you have had the privilege of having such an amazing
    companion , father of wonderful kids, and husband as Jan, bring you
    courage, strength and solace in you current situation and going forward .

    This is a loss we all share, but I believe that Jan would have wanted us
    to focus on the amazing life he has had, and the legacy he has left behind.

    On behalf of my family, may i send you our most heartfelt condolences.

    We are always there for you.

    Yours sincerely,

    Nathalie, Gregg, Yael, Matthew and Jayne.
    Carl Avet and Aldo and Sylvia Salerno-Jeuken who are here at this moment
    join in these wishes

    Reply
  40. Alberto Isidori

    Jan’s untimely death is a terrible loss for his wife Doke, for the hundred of friends that Jan had, and for a large scientific community that Jan contributed to shape and lead for more than 40 years.

    Jan’s contributions to the foundation and definition of a discipline, System Theory as we know it today, are immense. His visions, his ideas have permeated our field since the early 1970’s and continue to remain cornerstones of our daily work. In many cases, innovations and new ideas have ephemeral life and quickly fall into oblivion. This has not been the case for any single new concept introduced and developed by Jan. Today, we all speak in terms of concepts, languages, ideas that Jan has introduced over the years. Jan’s heritage is one of the few everlasting heritages in our discipline.

    I had only one paper co-authored with Jan, but this one happens to be my most-cited paper. This single example tells better than anything else how much his influence of the work of colleagues has been profound.

    Reply
  41. Frank Callier

    Thank you Jan for your contributions to Systems and Control : Nonlinear Feedback Systems, Passivity, Riccati Equation, Almost (A, B) – invariant subspaces, Behavioral Systems, …..
    Your work will live on. Moreover you were a great human being , which I happened to know since my time at Berkeley in the early seventies. You were also an excellent mentor and organizer in your field.

    Yours sincerely

    Frank Callier, Namur-Jambes, Belgium.

    Reply
  42. Erik Verriest

    Writing down these lines proved to be a difficult task, as words don’t come easily to describe the sense of loss we all feel. Jan was a teacher, a mentor, and above all a good friend. We first met in Stockholm at the occasion of the 1985 MTNS, at the beginning of good behavior, so to speak. As a young researcher, I got fascinated with his enthusiasm in going back to first principles. It had a contagious spell that made you look forward to another conference, to meet again and discuss new and old viewpoints. It was saddening when he wrote that his pain and tiredness kept him from attending any more.
    His opinions were always valued, and colleagues have already described in great length how much he inspired us all. I consider myself very fortunate then when in 2008, I could visit the KUL for a semester and interact directly with Jan. At that time his interests focused on the behavior of circuits. While a “classical theory”, he felt that there still were some shaky issues that needed attention. Sharing his office, our activities merged into a continuous brainstorm, spilling over the too small whiteboard on the wall. Discussions continued during the walks to the Alma for lunch. Nothing was ever left unturned, and I learned to question everything.
    In fact, what Jan taught me was to be critical, skeptical, and not hesitate to leave accepted present day approaches and start anew, from the roots up, in a Russellian way:
    “We want to stand upon our own feet and look fair and square at the world — its good facts, its bad facts, its beauties, and its ugliness; see the world as it is and be not afraid of it.”
    Thank you, Jan.
    I met Doke only a few times, but her warmth could be felt. Jan and Doke were good companions. My thoughts are with her and the family.

    Reply
  43. Ruth Curtain

    I first met Jan in 1972 at a symposium organised by The Control Theory Centre at the University of Warwick. Little did I think that we would later be colleagues at the University of Groningen from 1997 till 2001. Jan was very much influenced by his period in America and the intertwining of “work and play”.
    On my arrival in Groningen, my apartment was ready and Jan and Margherita helped me settle in. Jan even took me along to his bridge club. In the American tradition, Jan did not keep home and work separate, but opened the family home to colleagues from Groningen and abroad. I spent many pleasant evenings with lively conversations initiated by Jan and delicious meals prepared by Margherita. Our masters’ students will also have fond memories of evenings at the family home. Although we never wrote a single paper together, I was deeply influenced by Jan’s drive, research and hopes, as were my students. Jan was great with students, both in lectures and during lunch and coffee breaks. He loved talking to them and they loved listening to him. Although the Anglo-Saxon (mostly American) approach to university education in The Netherlands is now more or less accepted as “good practice”, that was certainly not the case in the 70’s and 80’s. Ph.D. students were fewer and were the responsibility of the promotor who was not always the actual supervisor. Jan’s dream was to introduce a national graduate programme for System’s Theory in The Netherlands. This idea was supported by the systems community and in 1986 became one of the first Dutch Graduate Schools. Moreover, we succeeded in becoming a National Network with the accompanying status and financial support. Those were exciting times and formed a firm basis for the future development of Systems and Control Theory in The Netherlands and for the present research school DISC. Jan had a deep passion for systems theory and he invested an enormous amount of energy, not only in writing papers, supervising students, but also in networking. He accepted every opportunity to lecture passionately on his current research topic. In latter years this was his “behavioural approach” which he believed was the true approach and his lectures became even more passionate. As attested by the many entries in this memoriam, Jan was and remains an icon in Systems Theory.

    Of course, this is but one aspect of a man of many facets. Jan and Doke were popular members of our local bridge club. Jan was certain to brighten up the evening with animated discussions and competitive bridge. When Jan and Doke moved to Antwerpen, the whole bridge club was welcomed into their very elegant home for some memorable bridge weekends. He will be missed by many in many different ways. My deep sympathy goes out to his family.

    Reply
  44. Peter Caines

    Jan Willems’s contributions to systems and control theory were both fundamental and elegant, moreover the profound perspectives he promulgated on the subject were grounded in his knowledge of its history, its relationship to the physical sciences and engineering and, indeed, to the philosophy of science. As so many know, the wonderfully enjoyable quality of discussions with Jan on almost any topic was his combination of complete intellectual honesty, incisiveness, humour and sheer common sense humanity. I am deeply saddened by his death; I have lost both a friend and a companion in the world of ideas.

    Reply
  45. Vahid Tarokh

    Although Jan Willems was a control theorist, his behavioral approach was also used in representations of algebraic structures such as algebraic codes and lattices in coding theory.

    It may not be bad if the coding theory community is also informed of this big loss

    He was a genius and gentleman and will be greatly missed.

    Reply
  46. kurian C kurian

    He was a great inspiration for me. His papers and results inspired us to aspire to the uncommonly bright way of control systems. His clarity in dispensing with the unremarkable and common place drove me to re invent myself to perfection.

    Reply
  47. Ilhan Polat

    First with Okko Bosgra and now with Jan Willems, it’s awful to acknowledge that two of my idols that I look up to for their upright characters passed away in a very short amount of time.

    I can’t say I have been really in contact with Jan Willems other than casual chats in some conferences and a few email exchanges when I was trying out different avenues for my research. However, this does not change the fact that I literally tried to copy his attitude towards approaching a problem as laid out wonderfully by Bruce Francis’ cited letter above. He has affected even the most distant circles of the field by providing explanations to very cumbersome notions with extreme lucidity. Not to mention his provocative and humorous style.

    I feel obliged to pay my tribute for this great man and wish his family power and strength for enduring his absence.

    Reply
  48. Sasanka Gottimukkala

    I have first met Jan during his 70th birthday, where a colloquim was organised at Brugge. We had a brief conversation. I had just started my PhD and was reading a paper written by him, which indeed played a central role in my thesis. Later I had opportunity to discuss with him about my research at his place in Antwerpen. I was really surprised the way Jan and Doke treated me. They were very warm and hospitable. The moments I had then discussing with Jan not only about research but on various other aspects left a great and unforgettable memories for me. That trip instilled in me a fresh confidence and inspiration which helped quite a lot during my PhD. It is a great loss to scientific community and I pray for his soul to rest in peace. I also pray that for stregth to his family in these tough times.

    Reply
  49. Jochen Trumpf

    It is difficult to relay the feeling of privilege I had when I first met Jan at a workshop organized by Paul Fuhrmann in 2004. I was a fresh post-doc at the time and I had previously only known Jan from his celebrated written work. He turned out to be not only approachable as a person but even humble and interested in my thoughts as much as in his own. I guess it is an expression
    of his true greatness that he never dwelled on what he had achieved in the past but kept pushing boundaries. Jan was like nobody else I know. Always focused on the big picture, on deep understanding and on clear thinking. “Des Pudels Kern” as Dr Faust would have said. I will very much miss his humor and his wise words. Our community is poorer for it. Over the years Jan has become a friend as well as an admired colleague and teacher. I will greatly miss him.

    Reply
  50. Mike and Mary Counihan

    Jan Willems

    I first met Jan 50 years ago when we began graduate school at the University of RI. Over the coming months we became closer. He spent the first of many Thanksgiving dinners with my parents and continued even after the children came. His brother, Jacques, was a student at MIT at the same time and we traveled many times to Cambridge. We went hiking and skiing in NH. Jan bought a used car that was very used and spent more time in the shop than on the road.

    Jan and I shared an apartment in our second year. We left URI the next year, Jan went to MIT and I went to a job in Buffalo, NY. Over the following years we married and had children but continued to spend time together. He was our best man in 1967. In 1969 Mary and I moved to Mass and spent more time together. Two of our most memorable trips were to the Italian Alps in 1974 and ten years later on a week long bike ride in the Dutch countryside with our children.
    .
    After Jan moved to Groningen, he frequently showed up in the Boston area. Mary and I visited them in Groningen several times.

    In 2002 we visited Jan and Doke in Antwerp. He seemed quite happy and content, though he did rib Doke much the way he had Margarita. We will always remember his gentle voice, his questioning ways, his mischievous look. Jan was a very distinguished and distinctive person. He lived his life to the fullest and left two beautiful children and a host of grandchildren as a result. And he’s left some people on this side of the pond who will always have him in their hearts and thoughts.

    Reply
    1. Chieu T Nguyen

      Talking about the University of Rhode Island in 1963-64 time frame when I was working on my MSEE degree with Jan and Mike. It was a pleasure taking Quantum Mechanics courses with Jan who easily got A’s thanks to his strong mathematics formation from Belgium. It was also great to share the Graduate Student apartment with him and Mike in the summer of 1964. He taught me how to eat raw hamburger and I taught him how to cook chicken! It was too bad I had to go back to Viet Nam in the middle of the war there after finishing my thesis at the end of that summer. I had a chance though to see him and Margherita again at Cambridge when I had a fellowship at Oxford in 1971. It was nice of him to have taken me and my newly wed wife to visit Stonehenge and to row a boat on the Cambridge River. Now living in Hawaii and writing a book, I wish he could be around to share some thoughts, not about control systems, but about life. Life is so short.

      Reply
  51. Peter Heuberger

    As many have already mentioned, there are simply no words to describe the loss for both family and the scientific world, with Jan’s passing. He fascinated me since his first lectures during my BSc in Groningen, during his supervison of my graduation and in all the years after that. Not only was he able to teach me valuable lessons till recently, but he was also such a nice and warm person. I have never in more than 35 years been dissappointed by Jan. He was always there when I needed him. He will be missed dearly, but will always be with me. Without Jan I would not be whom I am today. I thank him dearly and wish Doke and family all the strenght they need to cope with this loss.

    Reply
  52. Karen Rudie

    This terrible news just reached me. Jan was a mentor and friend to me during my postdoctoral year at the IMA. Some of my happiest memories—both academic and social—were in Jan’s company. My heart aches at this news.

    Reply
  53. Peter Smulders

    Soon after he settled down in Groningen Jan joined a loosely connected group of people who came together once a month to play bridge in the home of one of their members. Later on this group adopted the name “Huis Tuin en Keuken Bridge Club” and it is still very much alive today.

    During his time in Groningen Jan also took an active part in social activities outside of bridge. After he left Groningen our group spent a few memorable long weekends in Antwerp where Jan and Doke gave us a warm reception. He really enjoyed playing the host and seemed to light up with people around him.

    Yesterday at our bridge meeting we commemomerated Jan with a few words and a moment of silence.

    Around 1997 Jan and I took a more serious interest in bridge and we joined the local bridge club Filarski where we took part in the weekly competition, and we soon became club champions. Jan was the best bridge partner I could have wished for, both for his insight into the game as for his way of handling disappointments.

    Above all, Jan was a very amiable person with a good sense of humour and it was always a treat to spend the evening with him.

    Reply
  54. Felipe Pait

    Jan Willems was generous with that which is most precious: his time. We all have learned so much from listening to him and from reading his papers! I hope that the memory of his generous company will be stronger, for each of us, than the sadness of the loss.

    Reply
  55. Bozenna Pasik-Duncan

    Jan was a remarkable scholar, a wonderful advisor and mentor, and a true friend.
    Every discussion with Jan was the most memorable and inspirational for each of
    us in the Duncan family. This is a tremendous loss.

    Reply
    1. Ian Petersen

      I first met Jan not long after I arrived at ANU as a Postdoc in 1983. I remember the excitement when John Kaye mentioned that “Uncle Jan” would be visiting. Jan’s deep thinking has been a major influence on me throughout my career and he will be sadly missed.

      Reply
  56. David Delchamps

    I first met Jan in the late ’70s when he visited Harvard while I was a Ph.D. student there. I felt as if I’d known him for a long time, having studied his algebraic Riccati equation paper to death. On paper he was all clarity and style and literary flair, a real inspiration to me. In person, though, he was even more inspiring. His charm, charisma, and humility — and the mischievous glint in his eye — make me smile now as I recall those days.

    Reply
  57. K. Maciej Przyluski

    Jan was always extremely friendly and helpful to me.
    I should say: ”nothing compares, nothing compares to you”.
    KMP

    Reply
  58. Malcolm Smith

    It is a measure of Jan’s greatness that he influenced so many people far beyond his immediate circle of colleagues, students and co-workers. We greatly miss his energy, vitality and humanity. Simply being with Jan would spur us on to greater curiosity and liveliness of mind. His example would encourage us to try to emulate the rigour and elegance of his ideas. Colleagues know well that his work and influence on the field will have a lasting place. But so also our vivid and happy memories of him will live on and be cherished!

    Reply
  59. Ricardo Z. Yoe

    I knew about this terrible news while I was diving into Internet. The news makes me reflect about how I was interested in Jan’s research. That was in 1994-1995 when I was finishing my M.Sc.in Control Eng. in Mexico City. I loved his book “The analysis of feedback systems”. I decided to start a PhD with him. I had to wait for some years until once he said he could receive me. But rather, let me comment the following about some happy anecdotes I had with Jan. Unfortunately for me, although he was my original supervisor, he retired early (in 2002) but we kept contact by email and during sometimes he visited Groningen from Belgium. In spite of his wisdom and importance he was very humble. I recall when I met him (for first time in my life) at the Cafeteria (Acloo) at RuG (Zernike Complex) some days after my arrival to the city. I told him: “Prof. Jan Willems, it’s my pleasure to meet you”.He said smiling: “I´m just Jan. Welcome”. Some weeks later, I asked him whether he had a copy of the “yellow book” (“Introduction to mathematical systems theory: A Behavioral approach”). I told him that I wished to pay him for the copy but what I really wanted was some dedication words in that book. He sent me his OWN book (for free). He apologized because there were some pages with coffee drops.
    After that present, I kept contact by email and just a few times personally during some visitations he did to Groningen. I recall specially one when Bart de Moor gave a talk at RuG, the Johannes Bernoulli talk . Some days before that I knew that Jan had to give a talk at the University of Leuven (the Chair Francqui, 2003). Before the Bernoulli talk at Zernike Complex, I asked Jan about a good hotel to spend the night in Leuven because I wished to attend his talk. He answered : “No problem. You can stay at my place.Bart will drive back to Belgium and you can come with us also”. Amazing…!
    We did it so, and the following morning I was at 8:00 next to him in one of his computers. He was finishing his talk for that day (the aforementioned Chair Francqui). He gave me a huge book about Numerical Linear Algebra and we went to take a seat to have breakfast. There, I met Doke, his wife. I recall that I gave them a big portrait with a Mexican landscape and bottle of wine (no, unfortunately I did not have TEQUILA). We started to eat some “tosti”. Doke was very formal eating with fork and knife. I did the same . Suddenly she stand up and went to the kitchen. During her absence Jan asked me :” Do you eat tosti with for and knife”, And I answered him. “No, I did it because of you”. He smiled saying: “Me neither”.
    Some minutes later we aboarded their car (the three of us) and Jan started to drive to Leuven. He said: “We have chosen this way in order to show you some countryside” .Of course , I was very happy just watching and talking with them, but specially , watching that beautiful landscape. About one hour later, Jan asked Joke: “Are you guiding me correctly with that map? I think you are guiding me wrongly! She replied: “I think you are driving wrongly in spite of my right indications”. Funny, but the time to be in Leuven was very close. Finally, they found the right road and we arrived to the university hall Jan had to give his talk. A person was already in the middle of his talk when we could be sat in the hall, then Jan opened his laptop and continued preparing his presentation. I asked him: “Jan, you have not finished your presentation and you have just about five minutes to start!”. “He (very relaxed) said (talking about the person who was finishing his talk before his): “What this guy is saying is partially true because , bla, bla, …”. It was amazing that he was preparing his Chair Francqui talk, talking to me and listening the speaker at the same time. Finally , he gave his talk, the day finished and I went back to home in Groningen.
    After that time I met Jan once or twice. But I kept touch by email . The time went by and I finished my PhD under Kees Praagman supervision. I came back to Mexico and I still had some contact after my return to Mexico. Here, I devoted my research to modeling and control of electromechanical systems and bioengineering systems. It was a pleasure to type write Jan’s website in order to read about some new publication, I really loved to read not about Behavioral App. (I already knew) but rather about his reflections and thinking about Control history, thermo dynamical systems, comments about some book and so on.
    I think he was a science philosopher and naturally, a Control Systems philosopher.
    I think he still live among us in a very subtle, fine way. Although I just shared a few moments in my life with him, since the 90’s I followed his research and philosophy . His influence will remain in me definitely. I don´t say good bye, but so long Jan.

    Reply
  60. Dennis Bernstein

    Jan was *the* paradigm of learning, scholarship, brilliance, and, especially, humility. He interacted with everyone as an equal, despite the giant that he was. He never hesitated to probe and question, and admit that he was puzzled. He had unbounded curiosity and integrity. In everything he did, he set an example for others.

    Reply

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