Paul Moritz Cohn, mathematician: born Hamburg 8 January 1924; Chargé de Recherches, University of Nancy 1951-52; Lecturer in Mathematics, Manchester University 1952-62; Reader in Mathematics, Queen Mary College, London 1962-67; Professor of Mathematics, Bedford College, London 1967-84; FRS 1980; Professor of Mathematics, UCL 1984-86, Astor Professor of Mathematics 1986-89 (Emeritus), Honorary Research Fellow 1989-2006; married 1958 Deirdre Sharon (two daughters); died London 20 April 2006.
Paul Cohn, Emeritus Professor of Mathematics of London University, was an algebraist above all other things. He was devoted to research which was profound, original and lonely, choosing to work on what he felt to be important rather than to follow fashionable trends in mathematics. In the early Sixties he set himself a particular task and, 20 years later, he had completed it. In doing so, he uncovered mathematical structures whose importance is beginning to be recognised in areas apparently far removed from the algebras and skew fields with which he worked.
He was born in Hamburg in 1924, the only child of James Cohn, owner of an import business, and his wife Julia, a schoolteacher. After a contented childhood, the darkening situation in Germany as the Nazis came to power led to the confiscation of the business, the dismissal of his mother, and the arrest and imprisonment of his father in Sachsenhausen concentration camp. Paul's schooling was interrupted a number of times as the Nazis closed various schools he was at. It was after his father had been arrested for a second time that his mother looked for ways for the family to leave Germany.
At this time, Britain announced that it would allow Jewish children to immigrate and so Paul was sent to England in the spring of 1939 on the Kindertransport; he was never to see his parents again. He worked on a chicken farm for two years until the farm could no longer be run for lack of feed and then in a factory for a further two years as a fitter. Although his work at the farm took up 70 hours each week, he continued to study in the evenings, although at times the ink froze in the inkwell. To better his English, he would watch the same film over and over again at the cinema.
His love of learning and obvious intelligence made the Committee for Refugees encourage him to take the scholarship examination at Cambridge although he had had no formal education since the age of 15. He taught himself what he had missed by correspondence course, which included learning Latin from scratch as it was then a requirement for entry to Cambridge. He duly won an Exhibition to Trinity College in 1944.
Despite being prevented from attending for some of his first year, he was able to take his exams and do so well that there was no further obstruction to his studies and he obtained his BA in 1948 and his PhD, working with Philip Hall, at Cambridge in 1951. From Paul Cohn's subsequent success, one could have no idea of the sadness that had framed his early life, although, in later years, he returned often to Germany to find out about his origins.
After a year at the University of Nancy, in 1952 Cohn came back to a lectureship in Manchester, where he remained for 10 years. He came to London University in 1962 as a Reader at Queen Mary College and subsequently became Professor of Mathematics at Bedford College in 1967. He was awarded the Senior Berwick Prize in 1974. He was elected a Fellow of the Royal Society in 1980 and served as President of the London Mathematical Society in 1982-84.
When the colleges of London were reorganised in 1984, he moved to a Professorship at University College London and became the Astor Professor of Mathematics there in 1986 until his retirement in 1989. He then became an Honorary Research Fellow at UCL where he continued to write papers and books until his death. Throughout his life, he held visiting professorships around the world at, amongst others, Yale, Berkeley, Chicago, Rutgers, Paris, the Haifa Technion, Bar Ilan, the Hebrew University of Jerusalem, Delhi, Heidelberg and Hamburg. He also wrote textbooks to expound his and others' work; these range in level from undergraduate texts through to expositions of research recently completed.
In 1980 I became Paul Cohn's graduate student at Bedford College in Regent's Park. His books had been an inspiration and a delight for me and he himself fulfilled all I had been led to expect from them. The books reflected his scholarly and his gentle approach. He was always willing to make time to talk about mathematics and he had a strong desire to explain what he had seen to those that had not. He also wanted to hear what others had to say and took pleasure in the successes of those around him.