He managed to solve problems with so-called free margins. Such mathematical modelling describes, for example, the interface between different phases of a liquid, whose shape and position and possibly even their presence are initially unknown. He wrote a number of important contributions to the variational theory of elliptic and parabolic free boundary value problems. In particular, the publication on nonlinear parabolic equations written together with Stephan Luckhaus and the ground-breaking work on the beam problem received worldwide attention.

Wolfgang and I met on 12 October 1971 shortly after 11:15 in the morning in the M1 lecture hall at the University of Münster. Or rather, around 400 freshmen in the Calculus I class got to know Wolfgang, and I was among them. Wolfgang and his brother Hans Wilhelm were the assistants who supervised this lecture for Willi Jäger. In this beginner's lecture, metric spaces were introduced before real numbers, or perhaps a week later. Wolfgang and Hans Wilhelm introduced us to these abstract concepts using all possible means, for example by decorating exercise sheets and scripts with the adventures of mathematically inspired stick figures.

I would especially like to thank Prof W Jäger and Prof N Kerzman for their suggestions.

I would especially like to thank Prof Jäger and Prof Lieb for suggestions and advice in completing my dissertation, the second part of which resulted in this work.

I began my thesis in the fall of 1974, and Wolfgang was my immediate supervisor. The support was very good - he suggested directions that were promising, warned against other directions that promised difficulties or little success, and gave me a lot of leeway. Maybe too much, the work ended up being quite long.

From the spring of 1975, most of the members (assistants, doctoral students, many graduate students) of the Jäger working group moved to Heidelberg. We all spent a lot more time together than before in Münster. Hikes in the Odenwald and bike tours in the Palatinate wine regions, football games on a meadow near the institute, seminar weeks in the Black Forest at the Fehrenbacher Hof and in the Vosges, trips together to conferences, and I have very fond memories of an election party in 1976 at the house of Wolfgang and his then wife Inge on Monchhofstrasse (when Helmut Schmidt was re-elected).

Using the classical fixed point index for cones in a Banach space some criteria for the existence of periodic solutions of autonomous functional differential equations $\dot{x}(t) = F(x_t)$ are derived, regarding the asymptotic behaviour of F near zero and infinity. The general result is applied to equations with distributed time delay.

I had proofread his habilitation thesis from this time, in which his theory on biased random walk models for chemotaxis was developed in detail, and I clearly remember the impression that I had never seen anything like it before: something new under the sun.

The main theme of the conference was the mathematical representation of biolog­ical populations with an underlying spatial structure. An important feature of such populations is that they and/or their individual com­ponents may interact with each other. Such interactions may be due to external disturbances, internal regulatory factors or a combination of both. Many biological phenomena and processes including embryogenesis, cell growth, chemotaxis, cell adhesion, carcinogenesis, and the spread of an epidemic or of an advantageous gene can be studied in this context. Thus, problems of particular importance in medicine (human and veterinary), agriculture, ecology, etc. may be taken into consideration and a deeper insight gained by utilising (more) realistic mathematical models. Since the intrinsic biological mechanisms may differ considerably from each other, a great variety of mathematical approaches, theories and techniques is required. The aims of the conference were (i) To provide an overview of the most important biological aspects. (ii) To survey and analyse possible stochastic and deterministic approaches. (iii) To encourage new research by bringing together mathematicians interested in problems of a biological nature and scientists actively engaged in developing mathematical models in biology.

Stochastic models of biased random walk are discussed, which describe the behaviour of chemosensitive cells like bacteria or leukocytes in the gradient of a chemotactic factor. In particular the turning frequency and turn angle distributions are derived from certain biological hypotheses on the background of related experimental observations. Under suitable assumptions it is shown that solutions of the underlying differential-integral equation approximately satisfy the well-known Patlak-Keller-Segel diffusion equation, whose coefficients can be expressed in terms of the microscopic parameters. By an appropriate energy functional a precise error estimation of the diffusion approximation is given within the framework of singular perturbation theory.

Because of the interdisciplinary nature of your field of work, you could just as easily have been based at the Zoological Institute when you were appointed to Bonn in 1986. We are glad that this did not happen then. We owe it to this fortunate circumstance that you have stood by us as a reliable and lovable colleague in all matters relating to the Institute over the many years.

A new professor had recently moved into our rooms down on Kirschallee. We were a small group of biology graduate students and doctoral students of other professors who were struggling with all sorts of biological modelling. ... In my diploma thesis, I had been trying for some time to get my model to reproduce the plankton data in the Haltern reservoir, with moderate success. At first we were a bit sceptical about the new "office roommate", after all the new professor belonged to the exotic species of "mathematician". But we quickly realised that he didn't fit our stereotype of the distinguished professor or unworldly mathematician; he was friendly, and rather hip and alternative. Some of his hallmarks were colourful scarves and constant consumption of carrots.

My first contact with him made me I wanted to sink into the ground. He happened to be passing by the copier just as I was trying to secretly scrape the smudged film off the rollers, which had stuck there due to my improper handling. Instead of continuing without comment, he offered to help me too!

A few days later I wanted to make a pot of tea for all of us. The kettle that we used for our immense tea consumption was in his office. I noticed he wasn't there right then. So I turned on the stove, emptied the tea strainer with the old, dried-out tea, and briefly went back to my terminal. Shortly afterwards, a rather irritated Wolfgang stood in our room and asked: "Do any of you know where my special (XY) tea blend has gone? I was just about to make myself a nice cup and had already filled the tea strainer?" My second sinking into the ground ...

What is astonishing is that despite this somewhat unfortunate start, Wolfgang then set the course for my career, as he found me the doctoral position in Heidelberg (of all things in mathematics), supervised my dissertation from a distance, and also had a hand in arranging a postdoc position for me in Zurich.

What I have always liked a lot of Wolfgang, has been his genuine interest for biology, though with a deep competence in advanced methods of mathematics. Different from many biomathematicians, he has always been method independent, since at the centre of his scientific concern has been the biological problems.

His communication skills being excellent, once, for a lecture of his on chemotaxis, he impressed me and the whole audience by carrying in the lecture hall a basin with two real insects swimming in water that he had just taken from a pond!

He has been always a source of enthusiasm for science at large, with a spirit of curiosity that many time professional academicians loose too early, moved by career and competition to fast publishing of about anything.
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I like to mention that we also share a great interest in supporting young scientists; Wolfgang is very special as a teacher putting himself all the time aside if required, to emphasise the importance of the contribution by the young members of his team.

The University of Bonn is very lucky to have him among its faculty members, and all of us are grateful to Wolfgang for his strong and enthusiastic support to the interdisciplinary field of mathematical and theoretical biology.

Working with you is always a pleasure; regardless of whether it is about the mathematics education of biology students or the history of the university.

When I gave the mathematics lecture "Mathematics in Biology" for the first time in the winter semester of 2006/07, you made it incredibly easy for me to get started. Your concept, which includes the interlinking of theoretical knowledge in mathematics and biology with applied project work, including on computers in small groups, was convincing from the start and has survived the not insignificant changes, which brought with it the change to the bachelor's and master's degree programmes.

In March 1989, the workshop 'Biological motion' was held in Königswinter, Germany and the Proceedings were published in 1990 with Wolfgang Alt and Gerhard Hoffmann as editors. The editors wrote the Preface and we give an extract (see [17]):-

During the 70's, when Biomathematics (beyond Biomedical Statistics and Computing) became more popular at universities and research institutes, the problems dealt with came mainly from the general fields of 'Population Biology' and 'Complex Systems Analysis' such as epidemics, ecosystems analysis, morphogenesis, genetics, immunology and neurology. Since then, the picture has not considerably changed, and it seems that "a thorough analysis of behaviour" of single organisms and, moreover, of their mutual interactions, is far from being understood. On the contrary, mathematical modellers and analysts have been well-advised to restrict their investigations to specific aspects of 'biological behaviour', one of which is 'biological motion'.
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The idea for this Workshop, the Proceedings of which we now present, arose between 1987 and 1988, when Gerhard Hoffmann from Würzburg and Hans Scharstein from Koln had reported on their work on Animal Physiology at the Biomathematical Colloquium organised by three colleagues in Bonn. In response, one of these, Wolfgang Alt, visited the Zoological Institute at Köln (group of Gernot Wendler) and was fascinated by a colloquium talk on ciliary motion presented by Hans Machemer from the Cell Physiology Department in Bochum, in particular by the demonstration of his famous plastic flagella model. Finally, coming into mind as participant of an interesting meeting on chemotaxis of leukocytes, Hans Gruler from the Biophysics Department in Ulm joint us and completed the group of five organisers. We soon developed the concept of a four-day Workshop on "Modelling, Analysis and Simulation of Biological Motion" and requested various scientists to assemble for a discussion of our main topic: "Mechanisms and Control of Locomotion". The selection of motile species taken into consideration was clearly influenced by the organisers' own research fields: predominantly unicellular organisms, blood and tissue cells, but also insects, crustaceans and even fish were placed 'on the table'.

... the atmosphere during the Workshop was exciting, and it stimulated a continuous work from Thursday evening, 16 March, until Sunday morning, 19 March, 1989, in the comfortable 'Stegerwaldhaus' of the Jakob-Kaiser-Stiftung placed at the little wine-growing village Königswinter on the Rhine, opposite to Bonn and at the foot of the 'Drachenfels'. Those who never can finish a discussion concluded the Workshop by a walk through the forest around this pleasant hill.
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... the complex dynamical systems inducing the 'miracles' of naturally evolved 'biological motors' (on the individual side) and resulting cooperative movements of organisms (on the population side) require adequate mathematical theories and techniques to give insight into the underlying highly non-linear world of spatio-temporal phenomena. Therefore, it has been very fortunate for the Workshop to obtain generous financial, material and personnel support by the Research Program SFB 256 on "Non-linear Partial Differential Equations" sponsored by the German Research Fond (DFG).

The work that you and the Theoretical Biology group at Bonn have done has been outstanding and an inspiration to all of us in mathematical biology. It has always been exciting to visit Bonn and to talk with the group there and at conferences and meetings. In particular, talking with you about science or other things is always a pleasure.