# In Memory of George Sell

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May 29, 2015 was a sad day in the history of modern dynamical systems. On that day, the field lost an extraordinary leader, George R. Sell, who was an originator and pioneer of many important areas in the field. Even more tragic is the fact that many people working in the field will forever miss a great mentor, an irreplaceable advisor, and a dear friend.

George R. Sell (1937–2015) [Picture by Genevieve Raugel]

An author of over 120 scientific articles and 5 popular books, George is among the most influential and respected people in the field. His pioneering work in the 1960s on the topological-dynamics formalism of a nonautonomous differential equation established a deep connection between the coefficient space and the solution space. This remarkable discovery of George completely changed the classical view of nonautonomous dynamics and has become a standard approach for the study of many intriguing time-dependent phenomena in nature. His series of works with Robert J. Sacker in the 1970s and 1980s on dynamical spectra introduced a very important characteristic and a fundamental theory into the study of linear skew-product flows. This theory, now widely regarded as the Sacker–Sell spectral theory, has led to a number of active major research areas in dynamical systems in the past three decades. His work from the 1990s on inertial manifolds, global attractors, fluids, and climate modeling brought many innovative ideas into the study of infinite-dimensional dynamical systems.

What makes George a legendary and admirable figure in modern dynamics goes far beyond his mathematical work. He was famous for his tireless devotion to professional service. Among his many important contributions to the international mathematical community, he was a founder and main organizer of several very successful research centers, including the Institute for Mathematics and Its Applications (IMA) and the Army High Performance Computing Research Center at the University of Minnesota. He was the founding Editor-in-Chief of the Journal of Dynamics and Differential Equations (JDDE), which has become a major journal in the area of dynamics of differential equations. As a scientific editor of JDDE and several other professional journals, he made an enormous contribution to dynamical systems through his hard work, good judgment, high standards and collegial interaction with his authors.

Those who knew George always remember his kindness and gentleness. He was a great teacher and mentor who was largely responsible for the career development and success of many people working in the field, including me. George was always intellectually curious and eager to engage with colleagues, especially young mathematicians. He was particularly supportive of new researchers, listening to their presentations at conferences, commenting and asking questions, and frequently meeting them at coffee breaks and social functions to further develop the discussion.

With his noble character, high professionalism, and lifelong dedication to the development of dynamical systems, George’s inspiration and influence to the field are immeasurable and will last for generations to come.

George’s professional accomplishments and contributions to the international mathematical society have been acknowledged with high academic distinctions. He was awarded an honorary doctorate by the University of St. Petersburg (former Leningrad State University) in 1990. He was invited to deliver keynote addresses at many major mathematical conferences, including ICM, AMS/MAA annual meetings, Equadiff, and Americas DENA; and he was a tutorial/mini-course lecturers for numerous graduate summer schools worldwide. Two international conferences, the International Conference on Dynamical Methods of Differential Equations” held in Medina del Campo, Spain in 2002 and the International Conference on Infinite Dimensional Dynamical Systems” held in Toronto, Canada in 2008, were dedicated to the celebrations of his 65th and 70th birthdays, respectively. George's many landmark contributions to dynamical systems, as well as areas that he led or promoted are, documented in two special journal/book volumes in his honor: the Journal of Differential Equation (JDE) special Sell volume (J. Differential Equations, Vol. 208, No. 1–2, 2005) and the Fields Institute Communication special volume Infinite Dimensional Dynamical Systems” (Fields Inst. Comm., Vol 64, 2013). The JDE special volume also contains a biography of George written by Victor A. Pliss.

Several academic events have been held or are in planning in memory of George. In November 2015, the School of Mathematics at the University of Minnesota organized a memorial meeting for George in which over ten of his colleagues, friends, and family members delivered speeches. Information on this memorial event can be found in the School’s In Memoriam” page, which also contains a brief biography of George. In July 2016, the IMA held a Special Workshop on Dynamics and Differential Equations in memory of George. Participants included about eighty researchers in the field, of whom a substantial number were young mathematicians. This workshop was cosponsored by the IMA, the School of Mathematics at the University of Minnesota, the US National Science Foundation, and Springer; and it was co-organized by M. Jolly, P. Polacik, G. Raugel, R. Temam, E. Titi, and Y. Yi. The main lectures included talks by P. Constantin (Princeton), D. Iftimie (Lyon I), R. A. Johnson (Firenze), J. Mallet-Paret (Brown), H. Matano (Tokyo), M. Marion (École Centrale de Lyon), A. Mazzucato (PennState), R. J. Sacker (USC), W. Shen (Auburn), Y. You (South Florida), L.-S. Young (NYU), and S. Zelik (Surrey). The workshop covered a wide range of important areas that George long worked on or was interested in—including nonautonomous dynamics, ergodic theory, global stability, inertial-manifold theory, fluid dynamics, computational dynamics, monotone dynamics, and dynamics of partial, functional, and stochastic differential equations. The XI Americas Conference on Differential Equations and Nonlinear Analysis—to be held in August 2017 in Edmonton, Canada—will be also be dedicated to George's memory. The Americas conference series, a major biennial event in the Americas continent in the topics of dynamics and differential equations, started in 1994 and has held meetings in Mexico, Brazil, USA, Venezuela, Canada, Chile, Colombia, Peru, and Argentina. George was a strong supporter and promoter of the conference series, and he participated in almost all of its past meetings. The Journal of Dynamics and Differential Equations will publish a special Sell Volume in 2018 to honor George’s role  in leading the success of the journal. In addition, Springer will establish a Sell lecture series” to be held bienially starting in 2017. The first Sell lecture will be delivered in conjunction with the Americas XI conference in August 2017 in Edmonton, Canada.

What George has given to the field of dynamical systems and to the mathematical community at large are invaluable assets from which we will forever benefit. It was a privilege to know him; and while we are following in his footsteps, he (like his mathematics) will be always with us.

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