Introduction: the mathematics of energy systems

The urgent need to decarbonize energy systems gives rise to many challenging areas of interdisciplinary research, bringing together mathematicians, physicists, engineers and economists. Renewable generation, especially wind and solar, is inherently highly variable and difficult to predict. The need to keep power and energy systems balanced on a second-by-second basis gives rise to problems of control and optimization, together with those of the management of liberalized energy markets. On the longer time scales of planning and investment, there are problems of physical and economic design. The papers in the present issue are written by some of the participants in a programme on the mathematics of energy systems which took place at the Isaac Newton Institute for Mathematical Sciences in Cambridge from January to May 2019—see http://www.newton.ac.uk/event/mes . This article is part of the theme issue ‘The mathematics of energy systems’.

Reinvigorating the Wiener-Hopf technique in the pursuit to understand processes and materials

This perspective originated during the Isaac Newton Institute for Mathematical Sciences research programme “Bringing pure and applied analysis together via the Wiener-Hopf technique, its generalisations and applications (WHT)”. It fuelled intensive discussions on current pure and applied mathematical challenges of the Wiener-Hopf technique, driven by its impact on real-world problems.

Sir Michael Atiyah OM. 22 April 1929–11 January 2019

Michael Atiyah was the dominant figure in UK mathematics in the latter half of the twentieth century. He made outstanding contributions to geometry, topology, global analysis and, particularly over the last 30 years, to theoretical physics. Not only was he held in high esteem at a worldwide level, winning a Fields Medal in 1966, the Abel Prize in 2004 and innumerable other international awards, but his irrepressible energy and broad interests led him to take on many national roles too, including the presidency of the Royal Society, the mastership of Trinity College, Cambridge, and the founding directorship of the Isaac Newton Institute for Mathematical Sciences. His most notable mathematical achievement, with Isadore Singer, is the index theorem, which occupied him for over 20 years, generating results in topology, geometry and number theory using the analysis of elliptic differential operators. Then, in mid life, he learned that theoretical physicists also needed the theorem and this opened the door to an interaction between the two disciplines that he pursued energetically until the end of his life. It led him not only to mathematical results on the Yang--Mills equations that the physicists were seeking, but also to encouraging the importation of concepts from quantum field theory into pure mathematics.

Key questions for modelling COVID-19 exit strategies

Combinations of intense non-pharmaceutical interventions (lockdowns) were introduced worldwide to reduce SARS-CoV-2 transmission. Many governments have begun to implement exit strategies that relax restrictions while attempting to control the risk of a surge in cases. Mathematical modelling has played a central role in guiding interventions, but the challenge of designing optimal exit strategies in the face of ongoing transmission is unprecedented. Here, we report discussions from the Isaac Newton Institute ‘Models for an exit strategy’ workshop (11–15 May 2020). A diverse community of modellers who are providing evidence to governments worldwide were asked to identify the main questions that, if answered, would allow for more accurate predictions of the effects of different exit strategies. Based on these questions, we propose a roadmap to facilitate the development of reliable models to guide exit strategies. This roadmap requires a global collaborative effort from the scientific community and policymakers, and has three parts: (i) improve estimation of key epidemiological parameters; (ii) understand sources of heterogeneity in populations; and (iii) focus on requirements for data collection, particularly in low-to-middle-income countries. This will provide important information for planning exit strategies that balance socio-economic benefits with public health.

Editorial

The special lead article in this issue of EJAM, by Profs. John Ockendon and Brian Sleeman, is dedicated to Prof. Joseph B. Keller (referred to as Joe by his friends), Emeritus professor of Stanford University, who was one of the greatest applied mathematicians of our time. Joe died in September 2016. A workshop in honour of Joe was held at the Isaac Newton Institute for Mathematical Sciences, Cambridge, UK, in March 2017, focussing on some of the astonishingly wide range of topics in the mathematical sciences and continuum mechanics that Joe made substantial contributions to over his long career. This article discusses some of these pioneering contributions, describes some modern developments as discussed by the workshop speakers, and provides a more personal tribute to Joe, his history, and to his large influence on the international applied mathematics community. May the spirit of Joe, and his flavour of applied mathematics, continue to be reflected in EJAM.

Algebraic Properties of Chromatic Roots

A chromatic root is a root of the chromatic polynomial of a graph. Any chromatic root is an algebraic integer. Much is known about the location of chromatic roots in the real and complex numbers, but rather less about their properties as algebraic numbers. This question was the subject of a seminar at the Isaac Newton Institute in late 2008. The purpose of this paper is to report on the seminar and subsequent developments.We conjecture that, for every algebraic integer $\alpha$, there is a natural number n such that $\alpha+n$ is a chromatic root. This is proved for quadratic integers; an extension to cubic integers has been found by Adam Bohn. The idea is to consider certain special classes of graphs for which the chromatic polynomial is a product of linear factors and one "interesting" factor of larger degree. We also report computational results on the Galois groups of irreducible factors of the chromatic polynomial for some special graphs. Finally, extensions to the Tutte polynomial are mentioned briefly.