Legacy of Mohammed Dahleh

By: Munther A. Dahleh

LIDS, MIT, Cambridge MA

My intention from the write-up below is to give the reader a glimpse of Mohammed's life and experiences that contributed to his career in academia. Clearly the point of view I am presenting is skewed by my particular relationship to him. This paper is by no means a summary of Mohammed's major research contributions nor it is a list of his accomplishment, but rather it is a chronological description of his life briefly accounting for historical incidents that contributed to Mohammed's overall career.

Mohammed led a broad research program that manifested itself in: key publications within and outside the control community, in a number of students with superb multidisciplinary training, a number of organized workshops that attracted researchers from all over the world, and friendships within the control community that became an integral part of Mohammed's life. In this article, I will attempt to describe aspects of Mohammed's broad and diverse legacy in our community.

Childhood (1961-1979)

On the 12th of February, 1961, Mohammed was born in the small town of Tulkarim in the West Bank, Palestine, together with a twin sister, who died shortly after from a vicious virus. Mohammed grew up in a well educated family; his father Abdullah was an Engineer who was educated in the US and his mother, Wisam, was an English teacher, who left her teaching career to attend to her children.

Mohammed had two siblings, an older sister who is currenlty living in Jordan and is a high school English literature teacher, and myself. Mohammed attended a Catholic nursery school in the West Bank town of Nablus, and after 1966, he attended the Islamic Scientific college (ISC) in Amman, Jordan, where he received all of his early schooling.

Mohammed developed a special reputation at the ISC. While his brilliance was observed by both the teachers and the students in the school, it was his unconventional character that was most striking about him. Mohammed did not fit the model of the "good student" who was studious, obedient, and conformist, but rather he showed signs of intellectual and character independencies very early. Before high school, Mohammed simply ignored the rigid structure of the school. He paid moderate attention to his homework, he did not engage the teachers, and he ignored exams and rankings as well as all extra curricular activities imposed by the school. He pursued his own vast interests with vigor. His curiosity spanned many areas including Sciences, mathematics, Philosophy, languages, and especially geography. With the absence of effective youth programs at that time in Jordan that may have nurtured such broad interests, Mohammed took up reading as a venue for self-education which became a passion in his life that he shared with close friends and family.

This unconventional style was accompanied with an extremely daring behavior which may surprise those who knew Mohammed only in the past 20 years. As a child, he repeatedly used the back of a bus as a method of transportation. One time he fell and broke his hand. He routinely climbed buildings using the collapsing external pipes-once to rescue a cat that was trapped on the second floor when the pipe broke. Surprisingly, this behavior was associated with a great sense of responsibility and loyalty to his friends and family. His generosity and kindness, with his intensity and brilliance made him a focal point among his associates and attracted many people to him with vastly different styles or interests.

While the education at ISC was quite rigorous, it was quite conventional, and in indirect ways, very limiting. Mohammed recognized this early on and handcrafted his education while utilizing the school in very creative ways. He built strong relationships with specific teachers who were open minded about education, particularly the western view. While he rebelled against dogmatic interpretations of religion and culture, he took it upon himself to introduce progressive writings by both eastern and western scholars and encouraged students and teachers to critique their work. Because of his immense love for poetry, which he inherited from his mother, he organized poetry competitions in the school that became a tradition of the ISC during their various celebrations. Although Mohammed benefited tremendously from the rigorous mathematical and scientific training provided by the school. Nonetheless, he was critical of the lack of parallel experimental training which was a handicap to his education that took several years to overcome.

Mohammed's curiosity transcended subjects to people. Most of his early friends were extremely talented, but not in ways that were recognized by the society.

His best friend, a neighbor, who was several years older than Mohammed was a school drop out, whose sole interest was to become a mechanic. Another school friend was an orphan raised by his grandmother who had a talent in writing comedy books. Mohammed watched both of these friends get destroyed by a very rigid system that had low tolerance to unconventional career paths. Often he spoke to his teachers about reforming such a system but to no avail. Mohammed mentioned often that most of his close friends from elementary school never made it beyond high school. I strongly believe that these experiences shaped his thinking as he thought about ways to realize his interests.

The political situation in Jordan influenced Mohammed's perspective on life. As he jokingly used to put it when confronted with a difficult situation: "I have been through three wars, I can deal with this problem". Black September in 1970 possibly had the most critical effect on Mohammed. Many people will have heroic stories to tell about their survival, while Mohammed's repeated reaction was: "I am thankful to be alive". His recognition of the failure of war in resolving fundamental conflicts came at that early stage and shaped his thinking on addressing conflicts at all levels. He vehemently opposed war anywhere in the world and passionately supported peaceful resolutions. This was Mohammed's approach to any conflict situation he encountered. Mohammed experienced a sudden turn around in high school following a short religious phase.

He began to focus on excelling in sciences and mathematics as he planned to pursue an academic career in research and teaching that would begin with higher education in the US. His independent study put him at least two years ahead of his class. He wrote his first review article when he was 16 years old summarizing the various methods of integration, and presenting a set of challenging problems for students to prepare for the general Baccalaureate examination after high school. This examination, known as Tawjeehe, determined the student's field of study and consequently their future career path if they chose to study in Jordan or neighbouring Arab countries. Ironically, students who did not perform well were forced to look for educational opportunities abroad, and in some cases, ended up with a better education than their fellow students who did well on the exam. When Mohammed took the Tawjeehe, he ranked as the number one student in the nation which included over sixty thousand students.  His reputation spread nationwide as he emerged as one of the most talented students in the country. Despite the fact that Mohammed could choose to study anywhere in the Arab world and any topic he pleased, he still carried on with his plan and headed to the United States to get his education and pursue his dream. A dream he realized but unfortunately was not given the chance to fully enjoy.

College Years (1979-1987)

 

Mohammed arrived at Texas A & M (TAMU) with an intense desire to learn and explore. He chose electrical engineering as a major, although mathematics was his passion. After his freshman year, he was already known among many math professors as a star student, and by the end of his sophomore year, he was known among the electrical engineering faculty. He constructed his own education by planning all his courses and petitioning to replace required classes with ones that he deemed more appropriate or interesting. During his junior year, he was already taking graduate level courses in electromagnetic theory. He took on a project with Professor Tsang on understanding reflections of electromagnetic waves over periodic structures which became the topic of his BS thesis. This work resulted in Mohammed's first journal publication in the Journal of Applied Physics. This was Mohammed's first experience in research and he was thrilled by it.

At TAMU, Mohammed gained a great education. Apart from choosing excellent courses to take, he exploited the option of taking independent study classes from professors on topics of their research. Again, he built very close relationships with his teachers and engaged them in a process to help him build a very strong background necessary to pursue his plans. Mohammed also took the opportunity to develop a strong experimental and computational training by taking advanced courses with laboratory components.

Four years later, Mohammed left TAMU and headed to Princeton to pursue his Ph.D. in applied mathematics. The decision to continue his studies in mathematics  was a difficult one given that, on the one hand, Mohammed had a choice to pursue engineering at the top schools in the country, and on the other hand, because his early education was in a system that valued only two disciplines; engineering and medicine. Nevertheless, Mohammed did not allow this bias to prevent him from pursuing his dream of having an academic career in the area he passionately enjoyed.

It was at Princeton that Mohammed set forth his future career. He started his Ph.D. in the field of continuum mechanics, but after his general examination, he recognized that this was not what he really enjoyed. Mohammed struggled to find a thesis topic in the applied mathematics program that he felt strongly about.

He began doubting his choice, and started considering other alternatives.

He swiftly made a change and began working with Professor Hopkins in electrical engineering on adaptive control. He embraced the field with tremendous passion, and was able to answer some hard open questions about rapid switching of adaptive systems. His interest in robust control was rising, while he still maintained a broad perspective of the field and how it applied to various disciplines.

In his Ph.D. thesis, Mohammed considered the open problem of adaptive stabilization for delay systems. During his research, Mohammed recognized the interplay between adaptation and robustness. Consequently he introduced the idea of adaptive/robust control strategies whereby adaptation focuses on identifying and controlling systems with finite-parameter uncertainty, and the feedback control is constructed to be inherently robust to dynamic perturbations. Aspects of this formulation was published in his papers.

At Princeton, Mohammed met Anthony Peirce, a fellow student in the applied mathematics program. Mohammed and Anthony became life-time friends and collaborators. Anthony was working with Professor Rabitz, a physical chemist, on reaction diffusion problems and became aware of the complexity of the laser field design problem due to the complex molecular behavior described by Schrodinger's equation. Anthony began discussing this problem with Mohammed, who immediately recognized it as a control problem. This interaction lead to the publication of papers that initiated a whole research program in quantum control for Mohammed in subsequent years.

Mohammed emerged from Princeton as a true scholar. Not only did he build an incredible broad background in engineering and mathematics, and utilized it in an excellent Ph.D. experience, he also mastered the ethics of research whether conducted in an independent or collaborative fashion. He also broadened his education further by building up his knowledge in social sciences and languages. Mohammed exploited the very rich environment at Princeton and became affiliated with the near-eastern program, where he participated in many of their activities and seminars. Throughout this time, he built close friendships with many people from various parts of the world, many of which he maintained for many years afterwords.

At Princeton, Mohammed met his wife, Marie, who was a graduate student in the applied mathematics program. Mohammed and Marie were married in 1986, and a year later, Mohammed concluded his Ph.D. thesis and returned to his alma matter.

Dividing her time between Princeton and TAMU, Marie completed her PhD thesis in 1990.

Faculty at TAMU 1987-1990

Mohammed returned to TAMU in 1987 as an Assistant Professor. At this new position, he engaged the faculty and students with full excitement and ambition.

When he arrived, Professor Bhattacharyya and some of his students were working on problems concerning robustness with real parameter uncertainty.

The work followed the seminal work of Kharitonov characterizing stability robustness with respect to parametric uncertainty by checking the stability of four polynomials. Mohammed was interested in seeing the limit of such a theory in addressing realistic problems. In collaboration with Bhattacharyya and their student Chapellat, Mohammed derived a series of results that played a major role in the development of this area. Initially, they pursued extending the results of Kharitonov for multi-linear interval plants. But their major contribution was in deriving computable conditions for analyzing classes of mixed parametric and non-parametric uncertainty. Establishing this connection between robust control and parametric uncertainty was a fundamental contribution to robust analysis of uncertain systems.

In 1988, Mohammed attended a workshop in Torino on Robust Identification and Control. This workshop aimed at bridging the gap between the divided camps in robust control. Mohammed met two people at this workshop whose lives became intertwined with his: Alberto Tesi and Antonio Vicino. Both Alberto and Antonio became close friends and collaborators. Mohammed's work with them focused on showing that systems with parametric and sector bounded uncertainties enjoy remarkable extremal properties in robust stability and performance analysis and design of feedback. During this trip Mohammed also fell in love with Italy.

The Decade at the University of California at Santa Barbara, 1991-2000

Mohammed joined the University of California at Santa Barbara (UCSB) in 1991, during the formation of the Center for Control Engineering and Computation (CCEC) under the leadership of Petar Kokotovic. As the youngest CCEC faculty, Mohammed was also its most creative and innovative contributors. In 1995, along with the promotion to Full Professor, Mohammed was also named the Center's research director. In the Center he initiated and led new research directions in atomic force microscopy and microcantilevering, in flow modeling and control, and in quantum computation and control. He also dedicated much of time and energy recruiting and mentoring younger faculty.

Under the joint leadership of Petar and Mohammed, the Center for Control Engineering and Computation promoted an unprecedented culture of collaboration and inclusiveness. Many workshops were organized through the center that brought together people from all over the world, not only to interact on the technical work, but also to interact socially in order to benefit from their vastly different experiences.

Some of the major workshops are:

Workshop on ``Differential Games and Robust Control'', May 1991;

Workshop on ``The Modeling of Uncertainty in Control Systems'', June 1992;

Workshop on ``Robust Controller Designs and Differential Games'',May 1993;

Workshop on ``Methods in Robust Control Design'', June 1996;

Workshop on ``Dynamics, Control and Computation,'' April 2-3, 1998;

Workshop ``Vistas in Control'', April 1999, to name only a few. Numerous short courses and seminars were given by leading experts in the field.

Sharing their enthusiasm and responsibilities for the center, Petar and Mohammed developed a warm and enriching friendship seldom seem between men with a 27 year age difference. Their daily discussions often went beyond research topics into literature, religion and philosophy, music, history and politics. In this way, they also stimulated broader cross-cultural interests of their colleagues and students. Mohammed widely traveled and participated in many international meetings to build relationships with researchers from all over the world in order to enhance the research program at UCSB. For example, in 1993, Mohammed attended the IMA in Minnesota Sasha Megretski. Sasha's career path was completely altered after Mohammed facilitated his interview at Iowa state University. During that workshop and others, Mohammed had the chance to interact closely with John Doyle. That interaction grew to a close friendship and to a very fruitful collaboration between UCSB and Caltech.

Mohammed's career at UCSB reached new levels. He was recognized as an outstanding teacher by both the Mechanical Engineering Department and the university academic senate. He promoted a truly multi-disciplinary research program built around robust control theory as its core, which spanned areas including quantum mechanics, fluids, and nanotechnology. During this time,

Mohammed and Marie, who completed her PhD in 1990, had two children, Taher (1994) and Jumana (1996). Mohammed and Marie dived into parenthood with tremendous excitement and joy. It was apparent that Mohammed was experiencing a great fulfillment and flourishment in various aspects of his life.

Mohammed always believed that a strong research program needs a strong core.

Because of that, he devoted part of his time to the development of foundational issues in robust control. This included his work on controller design for mixed objectives which culminated in the publication jointly with M. Salapaka of the research monograph: "Multiple Objective Control Synthesis". Also included is his work on non-standard robustness analysis in the presence of mixed uncertainty (e.g., plant perturbations described with different norms).

Mohammed's interest in nanotechnology began with his discussions with Arun Majumdar who during 1996-1997 was at the UCSB. Arun was the head of a laboratory that housed an atomic force microscope (AFM). AFM's have revolutionized microscopy making practical imaging and manipulation of material at the atomic scale. Mohammed recognized the immense underlying potential of this micro-cantilever based technology and the contributions dynamical and control systems theory can make to take this technology to the next frontier. This led to a research program that focused on the study of complex microcantilever interactions as documented by the articles.  Mohammed supervised several students on this topic, including S. Ashhab, A. Daniele, and S. Salapaka.

The theoretical study of complex AFM dynamics, nanopositioning, and friction was well complemented by a laboratory that Mohammed initiated which eventually housed an Atomic Force Microscope and control and signal processing hardware to test the theories being developed. This effort was also well supported by equipment grants by Digital Instruments, a leading manufacturer of AFM's.

It was Bassam Bamieh who got Mohammed interested in the Fluids area. At Mohammed's initiative supported by Petar, Bassam came to UCSB as a visitor for one year. At that time, Bassam has developed the theory behind the design of distributed controllers for spatially invariant systems, and became interested in fluids dynamics as an application domain. Studying existing approaches to modelling transition to turbulence  he found the literature on what is called "non-normal growth" and the "pseudo-spectrum" (e.g., the work of Farrell, Butler, Henningson, Schmidt, and Trefethen et al.). Bassam presented these papers to Mohammed, and they both recognized the resemblance of this machinery to robust control theory. In fact, these researchers were gradually reinventing this machinery from scratch. For example, the pseudo-spectrum is related to the robust stability problem, and the non-normal growth results aim at quantifying transient growth of energy (which can be related to $H_2$ norms). Mohammed and Bassam looked at a particular paper of Farrell and Ioannou (on energy amplification), and they interpreted it as computing the norm of a spatio-temporal system. They realized that they can analytically prove the growth of energy amplification as well as compute analytically part of the spatial frequency response of the system (the part that shows the dominance of the so-called streamwise vortices and streaks). These calculations were done numerically in the Farrell and Ioannou paper, but the analytical computations clarified the underlying amplification mechanisms, and in particular why this occurs in 3D in contrast to 2D flows. This paper was published in the journal Physics of Fluids, and was the starting point of this collaboration on the fluids problem. The conference version of this paper was published at the ACC and received the Hugo Shuck prize. This work drew the attention of many control theorists such as John Doyle and Petar Kokotovic who quickly started advertising it and making the connections to the appropriate experimentalists in the Fluids area.

Mohammed also pursued the work on quantum control, playing the role of a reformist for the physics community. His paper in Science with Rabitz and Warren created a splash among physical chemists and generated a lot of interest in robust control. Mohammed continued to analyze the robustness problems that arise in designing laser fields while being constrained to only open loop strategies due to physical constraints preventing the use of feedback in such setting.

This research sprung into many other directions as Mohammed mentored Ph.D. students working in this area.

Mohammed is to be credited for an unprecedented period of progress in the Mechanical Engineering Department at UCSB. Apart from his research and teaching efforts, he served as a vice-chair of the Department. During that time, he recruited three key faculty members in the dynamics and control area: Igor Mezic, Anna Stefanopoulou, and Bassam Bamieh. He utilized the Center for Control and Computation to integrate the activities of his department with electrical engineering. He also worked extremely hard at finding ways to support the best international students to come to UCSB. He promoted free thinking and broad education. I will take the liberty here to share with you something that Mohammed wrote in response to a criticism to academic institutions: {\it well, I disagree with the claim that we don't do anything useful.  First of all the main mission of the university is to teach analysis, critical thinking, and creativity.  To achieve this goal successfully, I believe, that there must be research that is not bound completely by the desires and requirements of the society. It should draw inspiration from life, in all its forms, and in all its complexity.  That's why in a university, people study poetry, literary criticism, cosmology, and yes, even pornography.  The product of the university is the educated, analytical student, and not a transistor radio.  Maybe a transistor radio can come out, and indeed it did, from a university research environment, but that is not the intended product it was just a nice byproduct. I try to model myself, if I can, after the young English teacher in David Lodge book.  I argue constantly against my engineering colleagues in support of free thinking research in all aspects of university life including engineering. I hate to think of the university as a factory.  If it turns into one I will take a very early retirement.}

Mohammed continued his extensive interaction with the Italian control community through exchanging visits with Alberto Tesi, Antonio Vicino, and Laura Giarr\'e and their research team, and through organizing joint workshops and seminars. Because of this strong technical interaction, and his interest in the Italian language and culture, Mohammed took a sabbatical leave and spent it in Siena in 1995. This collaboration continued after he went back to the U.S. as he started and promoted (informally) an exchange program with Italian Ph.D. students visiting UCSB or other faculty in the U.S. A. De Angelis, F. Villoresi, A. Garulli, D. D'Alessandro, M. Basso, M. Napoli, M. Giannelli, G. Bianchini, L. Zaccarian etc.).  Some of these students were encouraged to pursue a Ph.D. degree in the U.S and later on to continue their academic career in the U.S (e.g., D. D'alessandro). Mohammed used to always say jokingly that he will make sure he has at least one Italian student working with him at any given time. At conferences, Mohammed became the focal point of an international group of researchers known as the ''Panettone connection'' (the name came from the special Italian Cake that became a tradition to eat at conferences) who interacted both technically and socially and assisted many young researchers in building their careers.

Mohammed's Sickness

The joy of success was not long lived as Mohammed was diagnosed with colon-liver cancer in January, 1999. Mohammed's family and friends all came together to help him through this tragic sickness. He got admitted to UCLA and began receiving treatment by one of the best oncologists in the nation. Mohammed faced his sickness with his usual poise and courage. With an intense determination to survive, Mohammed was able to begin a reversal process, and within 3 months, he recovered much of his health. To all of us who lived through his struggles, it was clear that Mohammed's love for life, his determination, his patience, and his faith, much more than the medication, got him through the initial sickness.

I don't think Mohammed ever felt he fully recovered, but he was clearly thankful for this opportunity and he shared this time fully with his kids, family, and friends.  To all of us, this time was a great gift we dearly cherished.  Mohammed then lived for the next year with an exemplary posture as a father, husband, son, brother, advisor, and friend.  The joy did not last too long, and Mohammed's attempted liver resection operation failed in Febuary 2000.

With a broken spirit, and a beaten up body, Mohammed continued to fight the disease, however, also taking the opportunity to say his goodbyes to his family and friends. Mohammed passed away on July 29, 2000.

Final Remarks

Mohammed's legacy is only partially in his technical papers and the books that he authored. His full legacy is in his two gorgeous children, Taher and Jumana, and in the memory all of his family, friends, students and colleagues have of him. It is in what he left to all of us; the love of life, the value of friendship,the excitement about small beautiful things. I would like to share a quotation from Mohammed:

But, what is it that brings happiness and joy? To me real joy comes from the simplest things and almost unnoticed episodes...they never come from intense and excessive emotional experiences...it could be a smile, Taher's reluctant smile as I touch his hands on the way to school in the morning, the memory of a beautiful song or poem or a face....to me the day's monotonous experiences mean very little, and my heart and soul is constantly searching for these rare and gentle moments... when I think back on my life, these are the only feelings I maintain and my life seems utterly affected and punctuated by them...

Mohammed is missed, not only for his generous support of his associates, but mostly because of his dynamic creative personality and calming presence. I will take the liberty here to quote some of his friends as they talked about him in his memorial service:

Mustafa  Khammash:

Mohammed was a unique person who had many good qualities that one rarely finds in a single individual. He was a warm and kind person, a true gentleman. On top of all, he was a generous person, and he gave a lot of himself to others. He was at once very broad and very deep. No matter what topic was being discussed, he always brought to any discussion a new and interesting point of view that would carry the discussion to a higher level.

Antony Pierce:

Mohammed was not only a brilliant student, but a compassionate human being, intensely curious not only about partial differential equations, but also about history, politics, movies and literature. He was as comfortable debating the meaning of life with the more philosophically inclined, as explaining a subtle point of Functional Analysis to a fellow student, or arguing an obscure point about Arabic Grammar with an Arabic Scholar.

Laura Giarre:

Mohammed was usually saying that in a friendship (or in general in life) it is not the quantity of time you spend together that matters but the quality.

In any of his meeting with friends or colleagues or students or with the last person known on a bus coming from the airport he was giving the same enthusiasm, the same intensity and the same attention, as if the person he was talking to was the most important in the world.

Petar Kokotovic:

Never have I worked with someone so naturally and genuinely generous, with such an ability to share with others his time, his immense energy, his deep insights, his intensity of thought, his breadth of talent, his intelligence and his love of life. As long as I live, I will remember him as a great humanist of our time.

With him disappeared a vital source of inspiration and optimism. I experienced his untimely death as a personal tragedy darkening my old age.