Obituary of Professor Mogens Flensted-Jensen

Mogens was born in 1942 and got his high school diploma from Virum Gymnasium in 1961. After a one-year worldwide trip with his father’s gymnastics team, he began his mathematics studies at the University of Copenhagen and finished with the cand. scient. degree in 1968. Immediately after graduation, he was employed at the Department of Mathematics. He became an associate professor in 1973, and in 1979 he moved on to a professorship at the Royal Veterinary and Agricultural University (KVL). In connection with the merger of KVL and the university I 2007, he returned to the University of Copenhagen, and shortly before he retired, he was again affiliated with the Department of Mathematical Sciences, from 2012 as Professor Emeritus.

During his 44-years of employment at our department and the sister department at KVL, Mogens was influential both as an internationally highly recognized researcher and as an efficient and well-liked university and research administrator. His fame as a researcher is primarily based on results published in the Annals of Mathematics in 1980, which caused a breakthrough within his field, the analysis of symmetric spaces.

Symmetric spaces

Mogens became interested in this area during a one-year stay at the Mittag-Leffler Institute in Stockholm in 1970-71. Here he met the mathematician who, according to Mogens himself, influenced him more than anyone else, namely Sigurdur Helgason from MIT. During his stay in Stockholm, Mogens wrote two articles, one of them with Tom Koornwinder from Amsterdam, which used special functions for harmonic analysis on symmetric spaces of rank 1.

He continued this strand of research in the following years, and after a guest visit to MIT in 1976-77, this research culminated in his main contribution in the Annals of Mathematics. Along with the articles with Koornwinder, these results were included in Mogens' Sc.D. dissertation, which he defended at the University of Copenhagen in 1980 with Christian Berg and Sigurdur Helgason as official opponents.

The Annals paper is concerned with the discrete series of symmetric spaces. These are the irreducible representations of a semi-simple Lie group G, which can be realized within the square integrable functions on a symmetric space of G. The special case, where the symmetric space is G itself, was at that time a hot research topic, prompted by the fundamental works of Harish-Chandra. Mogens found a generalization to symmetric spaces of Harish-Chandra's condition for the existence of discrete series, and showed that this, moreover, led to a new and simpler proof for the sufficiency of the original condition.

This discovery was a breakthrough for research in symmetric spaces, and Mogens' work was subsequently used and extended by mathematicians from all over the world - including himself in a paper in Acta Mathematica with Patrick Delorme from 1991. In 1995-96 he organized a programme at the Mittag-Leffler Institute in Stockholm, which was mainly focused on research in symmetric spaces. This programme contributed to the further development of research in the field, all based on Mogens' result on the discrete series.

The university politician

After he moved to KVL (Royal Veterinary and Agricultural University), Mogens became more involved in university politics and management. He was a member of Konsistorium (the governing body of KVL) for more than 10 years. His work in relation to the PhD programme at KVL and the research training programme in the agricultural sector generally was of fundamental significance. In 1997, Mogens joined the Research Council for the Agricultural and Veterinary Sciences, where he became the chairman after a few months - a post he held for 6 years.

In 2001, Mogens became Deputy Chairman of the Research Forum (Forskningsforum), a body that acted as a board for the Danish research councils, and which played an important role in relation to reorganisation plans for the research councils proposed by the government at the time.

In connection with the Danish EU Presidency in 2002, Mogens was the initiator of a joint European conference on the establishment of a European research council. Subsequently, he was the Danish member of the committee that laid the foundations for the European Research Council, ERC.

In the period 2006 - 2013, Mogens was a member of Singapore Academic Research Council, a government advisory body concerning universities in Singapore, which was also involved in the awarding very large research grants.

In the years 2007-2009, after the merger of KVL and the University of Copenhagen, Mogens was a member of the management team of the new LIFE-faculty. He contributed to starting and chaired the University of Copenhagen's Research Training Council, KUFUR.

Towards his retirement and in the time thereafter, Mogens found time to resume his research activities. This was a great satisfaction to him and led to several international publications, the most recent published in 2018.

Great influence

Mogens had great influence both in abstract mathematics and in research politics at the national as well as the international level. In both worlds, he contributed decisively to opening new directions for research. He was chairman of the Danish Mathematical Society 1982-86, and since 1992 an elected member of the Royal Danish Academy of Sciences and Letters, where he was an influential member of the Research Policy Committee for a number of years.

Mogens was characterized by great humility towards his own research, and he was always happy to acknowledge the contributions of others. He was never afraid to ask questions when there was something he did not understand, but when doing so he always avoided appearing intimidating. As a head of section at KVL, Mogens was characterised by his openness and appreciative management approach. He worked diligently and enterprisingly with the many duties he undertook, and at his retirement, he said that he had always been happy with his job. We will remember and miss Mogens as a highly esteemed colleague and friend.

Mogens leaves behind Inger, his wife for more than 50 years, the children and children-in-law Mads and Michelle, Mikala and Søren, as well as the grandchildren Albert, Carl, Elias, Max and Ronja. Our thoughts are with all of them.

Henrik Schlichtkrull and Henrik Laurberg

Appendix:

Not long before Mogens Flensted died, Sigurdur Helgason sent him a letter. The following is an excerpt quoted with Helgason’s permission.

“Dear Mogens: Mads sent to Artie the sad report on your health problem which has hit you after 78 years. It brings to mind our long friendship and many memorable days we have shared through the years. Last time it was at my 90th birthday …
But I think we met already during the summer 1970 (Montecatini perhaps) and I remember you telling me you were very much looking forward to a coming year at Mittag Leffler. And there you succeded very skillfully at the problems of rank one analysis which I had thought would suit your background. This also led to a productive collaboration with Koornwinder and further influence in Holland. I also remember a meeting in Oberwolfach where you surprised everyone with your discovery of a relation of some subtle connection between spherical analysis on real real G and similar analysis on complex G. The idea I explained in detail in my 1984 book, pp. 489 - 491. It was very original. And here at MIT you were led to inspiring work on discrete series on real G/H. I was very proud of being an opponent at your Danish Thesis with a large party at your house afterwards…
With my profound greetings to you and Inger”