Dr Sadie Kaye MA PhD

• PhD, Victoria University of Manchester, 1987

Sadie Kaye studied for PhD under Jeff Paris in Manchester. She did postdoctoral work as Junior Research Fellow in Oxford from 1987 to 1994. She joined the School of Mathematics at University of Birmingham in 1994.

Her research has always been centred on the topic of non-standard models - systems of numbers behaving like normal numbers but which include infinite and infinitesimal. Study of these models is often combined with other disciplines, including complexity theory, recursion theory, set theory, algebra and analysis.

Sadie's teaching in Birmingham has ranged widely from her main interests in logic and computability theory to include most other areas of pure mathematics. She is the author of two monographs as well as co-author of a textbook on linear algebra. She is always enthusiastic in her teaching and is noted by students for some of the more unusual ways she sometimes employs to get a point across.

She has successfully supervised a number of PhD students in areas of non-standard models. As well as her research and teaching, Sadie has been engaged in administration of the School, including being Admissions Tutor and First Year Director. She is currently the School's Head of Quality Assurance and Enhancement.

Research themes

• Non-standard models of Peano Arithmetic
• Recursively saturated models of arithmetic and of other theories
• Applications of non-standard models to other areas of mathematics, especially algebra and group theory
• Combinatorial game theory and non-standard models
• Satisfaction classes and notions of truth over a non-standard model of arithmetic

Research activity

Structural properties of models of Peano arithmetic, and in particular their initial segments.

Sadie Kaye is one of the main workers in the area of models of first-order arithmetic. There are a number of themes to this research, but most structural information about models of arithmetic relates to the order structure of the model. Sadie's work includes linking this order structure to the automorphism group of models of PA, and to looking at new families of initial segments, such as generic cuts. In many cases the structural properties are best understood through a language expanding that of PA by adding other predicates and functions - one representing the cut in question for example. This leads to new ways of looking at second order theories of arithmetic (utilising coding devices for example). The recent and ongoing work with TL Wong, a PhD student of Sadie's at Birmingham illustrates these ideas very well.

Strengthenings of the notion of recursive saturation and resplendency, in particular arithmetical saturation and transplendency.

Recursive saturation is a very natural and useful property that many non-standard models of arithmetic have. In some cases (e.g. when the model has a non-standard truth definition) recursive saturation is available "for free". Recursive saturation is closely related to the idea of resplendence in second order model theory. However, recursive saturation alone is often not enough for some results. An older result by Kotlarski, Kossak and Kaye shows that a countable recursively saturated model of PA has an automorphism moving every nondefinable point if and only if the model satisfies the stronger property of being arithmetically saturated. In recent work Kaye and her PhD student Engstrom looked at a powerful extension of this to form expansions of the model simultaneously omitting a type. The resulting notion - transplendency - is very powerful and not as yet fully understood and is still the subject of current research.

Properties of non-standard algebraic structures, in particular non-standard finite symmetric groups, abelian groups and linear groups.

Algebra and logic combine very well. Kaye has instigated a study of finite algebraic objects inside non-standard models. This leads to some very attractive algebraic objects, including non-standard symmetric groups (studied for example by Kaye and her research student Allsup) and non-standard finite linear groups. Even non-standard cyclic groups have interesting structure which was investigated by another PhD student. Results in these areas show that symmetric groups are closely connected with so-called sofic groups, and non-standard groups often have natural quotients with analytic structure and often with interesting measures. The work results in interesting new examples of algebraic objects with new means of reasoning about them. Non-standard methods of this type can be applied to other areas too. Another PhD student of Kaye's investigated Conway-style combinatorial games and the sorts of non-standard games that arise from model-theoretic considerations applied to these.

• Mathematical Typesetting and MathML
• Philosophy of Mathematics