Ted Janssen

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August 13, 1936 – September 29, 2017 On Friday, September 29, 2017, emeritus Professor T.W.J.M. (Ted) Janssen passed away at the age of 81 years. Ted Janssen was appointed associate professor at Radboud University in 1972, extra- ordinary professor in Utrecht in 1993 and full professor of theoretical physics at Radboud University in 1994. With the latter appointment he has strongly contributed to shaping the new group of Theoretical Condensed Matter, part of the Research Institute for Molecules and Materials (IMM) of the Faculty of Science. Ted obtained his PhD degree in 1968 from Nijmegen on ’Crystallographic Groups in Space and Time’ as first PhD student of prof. Aloysio Janner, with whom he subsequently collaborated very fruitfully during his whole career. During that career he also had many guest professorships, such as in Leuven, Dijon, Lausanne, Paris, Nagoya and Sendai. Together with Aloysio Janner he was one of the founders of the higher dimensional superspace approach in crystal structure analysis for the description of quasiperiodic crystals and modulated structures. For this work he received in 1988 the Aminoff prize of the Royal Swedish Academy of Science (with P.M. de Wolff and A.G.M. Janner) and in 2004 the Ewald prize of the International Union of Crystallography (with Janner), the most prestigious prizes in crystallography. These achievements were merit of his unique talent, combining a deep knowledge of physics with a rigorous mathematical approach. Their theoretical description of the structure and symmetry of incommensurate crystals using higher dimensional superspace groups also included the quasi- crystals that were discovered in 1982 by Schechtman, who received the Chemistry Nobel prize for this in 2011. The Swedish Academy of Sciences explicitly mentioned their work at this occasion. With his two famous papers “Crystallography of quasi-crystals” (T. Janssen, Acta Cryst. A42, 261-271, 1986) and “Aperiodic crystals: a contradictio in terminis?” (T. Janssen, Phys. Rep. 168, 55-113, 1988) Ted made a major mathematical contribution to this field. However, apart from being an excellent mathematical physicist, Ted was known as a very modest and pleasant colleague and friend. Already from his early years Ted also developed a deep appreciation for literature and music, in addition to visual arts, ballet and architecture. The enjoyment of the arts was essential to him, he called it vital components of life. He started playing the piano, harpsichord and cello in his early twenties, which brought him great joy during his whole life. Ted’s work on crystallographic symmetry was a beautiful combination of his passion for art as well as mathematics. Ted has also actively contributed to education with many beautiful courses of quantum mechanics and mathematical physics and with his great talent in explaining complex problems in simple words to students as well as colleagues. With his research he has strengthened the international scientific reputation of physics in Nijmegen, for which the university is very grateful. He will be missed. Theo Rasing, Radboud University

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