## Stephen Siklos, 1950-2019

Dr Stephen Siklos, who sadly passed away on the 17th August, was educated at Collyer's School in Horsham, then a state grammar school, now a sixth form college. He came up to Cambridge to read Mathematics at Pembroke College. After taking Part III Mathematics with Distinction and receiving the Tyson Medal (awarded to the top candidate each year in Astronomy), he spent a year as a teacher at Dulwich College in London before returning to DAMTP in 1973 to become a research student of Stephen Hawking. His PhD, awarded in 1976, was on the properties of solutions of Einstein's equations representing spatially homogeneous and anisotropic cosmological models. He then worked as an SERC postdoctoral researcher in Dennis Sciama's group in the Department of Astrophysics in Oxford from 1976-8. His time in Oxford was followed by a second postdoctoral position, this time at Queen Mary University of London with Malcolm MacCallum. He returned to Cambridge in 1980 to become a College Lecturer and Director of Studies at Newnham College, and then an unestablished University Lecturer in DAMTP. He joined Jesus College as a Fellow in 1999, the same year that he received a Pilkington Teaching Prize from the University for the outstanding quality of his teaching. He served the College first as Senior Tutor for ten years and then as President, work he continued until just a few months before his death.

Stephen's PhD work in General Relativity was to be an enduring research interest. Notably, he established that universes containing gentle 'whimper' singularities were less general than those with 'big bang' singularities. This stimulated his long interest in schemes for assessing the relative generality of solutions of Einstein's equations. He developed a classification of universes containing gravitational waves, and found some new solutions of Einstein's equations. This work led to the study of what are now known as Siklos invariants, Siklos universes and Siklos waves, which appear regularly in the titles of papers in the research literature. His classification of plane gravitational waves propagating in anti-de Sitter space times has proved to be of importance for string theorists and students of supergravity. Another of his influential contributions to the progress of modern cosmology was through the organization and editing of the proceedings of the important Nuffield Workshop on the Very Early Universe which took place in DAMTP in the summer of 1982, and which was published by Cambridge University Press in 1983. It was at that meeting that much of our current understanding of the consequences of Alan Guth's idea of a possible primordial inflationary epoch first emerged.

At the same time as completing what to many would have seemed liked a full-time job in Jesus College, Stephen continued to provide inspiration and leadership of the teaching and admissions in the Mathematics Faculty. This he did not only by example (as already noted his own lecturing was superb) and through characteristically friendly and supportive advice after observing a colleague's lectures, but also by sheer hard work. He held an astonishingly large number of roles in the Faculty (Chair of the Teaching Committee, Faculty Admissions Officer, Mathematics Subject Convenor for the Admissions Forum, leading on the Faculty's outreach work, responsibility for documentation, to name but a few from a list stretching to two tightly-typed pages). Indeed, when Stephen decided to retire from his nominally 50% position in DAMTP in 2016 his duties were so numerous and so broad that they had to be reassigned to three different colleagues. Stephen's administrative abilities and eye for detail were indispensable on many occasions, as demonstrated by his successfully guiding the Faculty through the (mercifully not-since-repeated) national Teaching Assessment Exercise and by implementation (and subsequent improvement) of major changes to the undergraduate Tripos. His understanding of, and sympathy for, the student perspective was equally important – feedback on lectures was scrutinised and acted upon, and new ways of improving the student learning experience were introduced, with induction meetings at the start of each year and exam briefing sessions towards the end.

Of all Stephen's educational activities, however, it is his leadership of STEP, the Sixth Term Examination Papers in Mathematics, which has made the greatest impact beyond Cambridge. Stephen became Senior Examiner of STEP Mathematics on its inception in 1987, a position he occupied for the next 32 years and during which time he steered STEP through major challenges and significant changes in the school curriculum. He played a pivotal role at every stage of the examination process, from rewriting skeleton draft questions (he said that 'setting questions, and knocking other peoples' questions into shape, has been a great source of interest and entertainment over very many years'), to detailed scrutiny of the finalised papers, to directing the huge marking effort and final grading. Such was the extent of his involvement that, since his death, Cambridge Assessment has (a common theme here) had to reassign his duties to three different people. Stephen was very much the public face of STEP – his talks about the papers at the Faculty open days were received in hushed awe by the potential applicants, and his book of model problems and answers, *Advanced Problems in Mathematics*, reaches a large audience. The aims of this text, to equip students to take STEP even if their school cannot provide support and to guide them through the solutions in a way that the broadest lessons can be learned, exemplify the Siklos approach. There can be no doubt that he has inspired and equipped generations of young people to study mathematics at a high level. He also wrote a piece of mathematical and social history in the form of a short book entitled *Philippa Fawcett and the Mathematical Tripos*, published by Newnham College in 1990.

Stephen will be greatly missed, and is remembered with huge gratitude and affection by his many colleagues and friends in Cambridge and in the wider mathematics community. He is survived by his wife, Professor Marian Holness, their two sons Arthur and Edward, and by his children from his first marriage to Catherine (née Marshall), Tabitha and Jonathan.