# Dr Anne Skeldon

## Reader

Qualifications: MA, DPhil. (Oxon)

Email: a.skeldon@surrey.ac.uk

Phone: Work: 01483 68 2634

Room no: 11 AA 04

## Further information

## Research Interests

Details can be found on my personal web page.

## Publications

### Highlights

- 'Modelling changes in sleep timing and duration across the lifespan: changes in circadian rhythmicity or sleep homeostasis?'.
Sleep Medicine Reviews,

[ Status: Accepted ]
.
(2016) - 'Can weakly nonlinear theory explain Faraday wave patterns near onset?'.
Journal of Fluid Mechanics, 777, pp. 604-632.doi: 10.1017/jfm.2015.388
#### Abstract

© 2015 Cambridge University Press.The Faraday problem is an important pattern-forming system that provides some middle ground between systems where the initial instability involves just a single mode, and in which complexity then results from mode interactions or secondary bifurcations, and cases where a system is highly turbulent and many spatial and temporal modes are excited. It has been a rich source of novel patterns and of theoretical work aimed at understanding how and why such patterns occur. Yet it is particularly challenging to tie theory to experiment: the experiments are difficult to perform; the parameter regime of interest (large box, moderate viscosity) along with the technical difficulties of solving the free-boundary Navier-Stokes equations make numerical solution of the problem hard; and the fact that the instabilities result in an entire circle of unstable wavevectors presents considerable theoretical difficulties. In principle, weakly nonlinear theory should be able to predict which patterns are stable near pattern onset. In this paper we present the first quantitative comparison between weakly nonlinear theory of the full Navier-Stokes equations and (previously published) experimental results for the Faraday problem with multiple-frequency forcing. We confirm that three-wave interactions sit at the heart of why complex patterns are stabilised, but also highlight some discrepancies between theory and experiment. These suggest the need for further experimental and theoretical work to fully investigate the issues of pattern bistability and the role of bicritical/tricritical points in determining bifurcation structure.

.
(2015) - 'A dynamical systems analysis of the data assimilation linked ecosystem carbon (DALEC) models'.
Chaos, 25 (3) Article number 036401 doi: 10.1063/1.4897912
**Repository URL:**http://epubs.surrey.ac.uk/806896/#### Abstract

Changes in our climate and environment make it ever more important to understand the processes involved in Earth systems, such as the carbon cycle. There are many models that attempt to describe and predict the behaviour of carbon stocks and stores but, despite their complexity, significant uncertainties remain. We consider the qualitative behaviour of one of the simplest carbon cycle models, the Data Assimilation Linked Ecosystem Carbon (DALEC) model, which is a simple vegetation model of processes involved in the carbon cycle of forests, and consider in detail the dynamical structure of the model. Our analysis shows that the dynamics of both evergreen and deciduous forests in DALEC are dependent on a few key parameters and it is possible to find a limit point where there is stable sustainable behaviour on one side but unsustainable conditions on the other side. The fact that typical parameter values reside close to this limit point highlights the difficulty of predicting even the correct trend without sufficient data and has implications for the use of data assimilation methods.

.
(2014) - 'Mathematical models for sleep-wake dynamics:
comparison of the two-process model and a mutual inhibition neuronal model'.
PLoS One, 9 (8)
**Repository URL:**http://epubs.surrey.ac.uk/805623/#### Abstract

Sleep is essential for the maintenance of human life, yet many features of sleep are poorly understood and mathematical models are an important tool for probing proposed biological mechanisms. The most well-known mathematical model of sleep regulation, the two-process model, models the sleep-wake cycle by two oscillators: a circadian oscillator and a homeostatic oscillator. An alternative, more recent, model considers the reciprocal interaction of sleep promoting neurons and the ascending arousal system regulated by homeostatic and circadian processes. Here we show there are fundamental similarities between these two models. The implications are illustrated with two important sleep-wake phenomena. Firstly, we show that in the two-process model, transitions between different numbers of daily sleep episodes can be classified as grazing bifurcations. This provides the theoretical underpinning for numerical results showing that the sleep patterns of many mammals can be explained by the reciprocal interaction model. Secondly, we show that when sleep deprivation disrupts the sleep-wake cycle, ostensibly different measures of sleepiness in the two models are closely related. The demonstration of the mathematical similarities of the two models is important because not only does it it allow some features of the two-process model to be interpreted physiologically but it also means that knowledge gained from the study of the two-process model can be used to inform understanding of the behaviour of the mutual inhibition model. This is important because the mutual inhibition model and its extensions are increasingly being used as a tool to understand a diverse range of sleep-wake phenomena sucah as the design of optimal shift-patterns, yet the values it uses for the parameters associated with the circadian and homeostatic processes are very different from those that have been experimentally measured in the context of the two-process model

.
(2014) - 'Three-wave interactions and spatio-temporal chaos'.
Physical Review Letters, 108 (7) Article number 0754504
#### Abstract

Three-wave interactions form the basis of our understanding of many pattern forming systems because they encapsulate the most basic nonlinear interactions. In problems with two comparable length scales, it is possible for two waves of the shorter wavelength to interact with one wave of the longer, as well as for two waves of the longer wavelength to interact with one wave of the shorter. Consideration of both types of three-wave interactions can generically explain the presence of complex patterns and spatio-temporal chaos. Two length scales arise naturally in the Faraday wave experiment with multi-frequency forcing, and our results enable some previously unexplained experimental observations of spatio-temporal chaos to be interpreted in a new light. Our predictions are illustrated with numerical simulations of a model partial differential equation.

.
(2012) - 'Scaling properties of weakly nonlinear coefficients in the Faraday problem'.
PHYSICAL REVIEW E, 84 (1) Article number ARTN 016209
**Repository URL:**http://epubs.surrey.ac.uk/72361/
.
(2011) - 'Spontaneous emergence of long-range shape symmetry'.
Nano Letters, United States: 11 (1), pp. 160-163.doi: 10.1021/nl1032793
#### Abstract

Self-organization of matter is essential for natural pattern formation, chemical synthesis, as well as modern material science. Here we show that isovolumetric reactions of a single organometallic precursor allow symmetry breaking events from iron nuclei to the creation of different symmetric carbon structures: microspheres, nanotubes, and mirrored spiraling microcones. A mathematical model, based on mass conservation and chemical composition, quantitatively explains the shape growth. The genesis of such could have significant implications for material design.

.
(2011)

### Journal articles

- 'The effects of self-selected light-dark cycles and social constraints on human sleep and circadian timing: a modeling approach'.
Scientific Reports,

[ Status: Accepted ]#### Abstract

Why do we go to sleep late and struggle to wake up on time? Historically, light-dark cycles were dictated by the solar day, but now humans can extend light exposure by switching on artificial lights. We use a mathematical model incorporating effects of light, circadian rhythmicity and sleep homeostasis to provide a quantitative theoretical framework to understand effects of modern patterns of light consumption on the human circadian system. The model shows that without artificial light humans wake-up at dawn. Artificial light delays circadian rhythmicity and preferred sleep timing and compromises synchronisation to the solar day when wake-times are not enforced. When wake-times are enforced by social constraints, such as work or school, artificial light induces a mismatch between sleep timing and circadian rhythmicity (‘social jet-lag’). The model implies that developmental changes in sleep homeostasis and circadian amplitude make adolescents particularly sensitive to effects of light consumption. The model predicts that ameliorating social jet-lag is more effectively achieved by reducing evening light consumption than by delaying social constraints, particularly in individuals with slow circadian clocks or when imposed wake-times occur after sunrise. These theory-informed predictions may aid design of interventions to prevent and treat circadian rhythm-sleep disorders and social jet-lag.

.
(2017) - 'Nonsmooth maps and the fast-slow dynamics of sleep-wake regulation: Part 1'.
Research Perspectives CRM Barcelona: Nonsmooth Dynamics,

[ Status: Accepted ]**Repository URL:**http://epubs.surrey.ac.uk/811093/#### Abstract

Sleep-wake regulation is an example of a system with multiple timescales, with switching between sleep and wake states occurring in minutes but the states of wake or sleep usually existing for some hours. Here we discuss some general features of models of sleep-wake regulation. We show that some typical models of sleep-wake regulation can be reduced to one-dimensional maps with discontinuities, and show that this reduction is useful in understanding some of the dynamical behaviour seen in sleep-wake models.

.
(2017) - 'Nonsmooth maps and the fast-slow dynamics of sleep-wake regulation: Part 2'.
Research Perspectives CRM Barcelona: Nonsmooth Dynamics,

[ Status: Accepted ]#### Abstract

In part I, the Two-Process model for sleep-wake regulation was discussed and it was shown that it could usefully be represented as a one-dimensional map with discontinuities. Here we discuss some recent, more physiological, models of sleep wake dynamics. We describe how their fast-slow structure means that one can expect them to inherit many of the dynamical features of the Two-Process model.

.
(2017) - 'Modelling changes in sleep timing and duration across the lifespan: changes in circadian rhythmicity or sleep homeostasis?'.
Sleep Medicine Reviews,

[ Status: Accepted ]
.
(2016) - 'Equation-free analysis of agent-based models and systematic parameter determination'.
PHYSICA A-STATISTICAL MECHANICS AND ITS APPLICATIONS, 464, pp. 27-53.

[ Status: Accepted ]
.
(2016) - 'BIOLOGICAL RHYTHMS Human sleep before the industrial era'. NATURE, 527 (7577), pp. 176-177. . (2015)
- 'Can weakly nonlinear theory explain Faraday wave patterns near onset?'.
Journal of Fluid Mechanics, 777, pp. 604-632.doi: 10.1017/jfm.2015.388
#### Abstract

© 2015 Cambridge University Press.The Faraday problem is an important pattern-forming system that provides some middle ground between systems where the initial instability involves just a single mode, and in which complexity then results from mode interactions or secondary bifurcations, and cases where a system is highly turbulent and many spatial and temporal modes are excited. It has been a rich source of novel patterns and of theoretical work aimed at understanding how and why such patterns occur. Yet it is particularly challenging to tie theory to experiment: the experiments are difficult to perform; the parameter regime of interest (large box, moderate viscosity) along with the technical difficulties of solving the free-boundary Navier-Stokes equations make numerical solution of the problem hard; and the fact that the instabilities result in an entire circle of unstable wavevectors presents considerable theoretical difficulties. In principle, weakly nonlinear theory should be able to predict which patterns are stable near pattern onset. In this paper we present the first quantitative comparison between weakly nonlinear theory of the full Navier-Stokes equations and (previously published) experimental results for the Faraday problem with multiple-frequency forcing. We confirm that three-wave interactions sit at the heart of why complex patterns are stabilised, but also highlight some discrepancies between theory and experiment. These suggest the need for further experimental and theoretical work to fully investigate the issues of pattern bistability and the role of bicritical/tricritical points in determining bifurcation structure.

.
(2015) - 'A dynamical systems analysis of the data assimilation linked ecosystem carbon (DALEC) models'.
Chaos, 25 (3) Article number 036401 doi: 10.1063/1.4897912
**Repository URL:**http://epubs.surrey.ac.uk/806896/#### Abstract

Changes in our climate and environment make it ever more important to understand the processes involved in Earth systems, such as the carbon cycle. There are many models that attempt to describe and predict the behaviour of carbon stocks and stores but, despite their complexity, significant uncertainties remain. We consider the qualitative behaviour of one of the simplest carbon cycle models, the Data Assimilation Linked Ecosystem Carbon (DALEC) model, which is a simple vegetation model of processes involved in the carbon cycle of forests, and consider in detail the dynamical structure of the model. Our analysis shows that the dynamics of both evergreen and deciduous forests in DALEC are dependent on a few key parameters and it is possible to find a limit point where there is stable sustainable behaviour on one side but unsustainable conditions on the other side. The fact that typical parameter values reside close to this limit point highlights the difficulty of predicting even the correct trend without sufficient data and has implications for the use of data assimilation methods.

.
(2014) - 'Mathematical models for sleep-wake dynamics:
comparison of the two-process model and a mutual inhibition neuronal model'.
PLoS One, 9 (8)
**Repository URL:**http://epubs.surrey.ac.uk/805623/#### Abstract

Sleep is essential for the maintenance of human life, yet many features of sleep are poorly understood and mathematical models are an important tool for probing proposed biological mechanisms. The most well-known mathematical model of sleep regulation, the two-process model, models the sleep-wake cycle by two oscillators: a circadian oscillator and a homeostatic oscillator. An alternative, more recent, model considers the reciprocal interaction of sleep promoting neurons and the ascending arousal system regulated by homeostatic and circadian processes. Here we show there are fundamental similarities between these two models. The implications are illustrated with two important sleep-wake phenomena. Firstly, we show that in the two-process model, transitions between different numbers of daily sleep episodes can be classified as grazing bifurcations. This provides the theoretical underpinning for numerical results showing that the sleep patterns of many mammals can be explained by the reciprocal interaction model. Secondly, we show that when sleep deprivation disrupts the sleep-wake cycle, ostensibly different measures of sleepiness in the two models are closely related. The demonstration of the mathematical similarities of the two models is important because not only does it it allow some features of the two-process model to be interpreted physiologically but it also means that knowledge gained from the study of the two-process model can be used to inform understanding of the behaviour of the mutual inhibition model. This is important because the mutual inhibition model and its extensions are increasingly being used as a tool to understand a diverse range of sleep-wake phenomena sucah as the design of optimal shift-patterns, yet the values it uses for the parameters associated with the circadian and homeostatic processes are very different from those that have been experimentally measured in the context of the two-process model

.
(2014) - 'The effect of the G1 - S transition checkpoint on an age structured cell cycle model'. PLoS One, 9 (1) Article number 83477 . (2014)
- 'The effect of the G1-S transition checkpoint on an age structured cell cycle model.'.
PLoS One, United States: 9 (1)
**Repository URL:**http://epubs.surrey.ac.uk/805502/#### Abstract

Knowledge of how a population of cancerous cells progress through the cell cycle is vital if the population is to be treated effectively, as treatment outcome is dependent on the phase distributions of the population. Estimates on the phase distribution may be obtained experimentally however the errors present in these estimates may effect treatment efficacy and planning. If mathematical models are to be used to make accurate, quantitative predictions concerning treatments, whose efficacy is phase dependent, knowledge of the phase distribution is crucial. In this paper it is shown that two different transition rates at the G1-S checkpoint provide a good fit to a growth curve obtained experimentally. However, the different transition functions predict a different phase distribution for the population, but both lying within the bounds of experimental error. Since treatment outcome is effected by the phase distribution of the population this difference may be critical in treatment planning. Using an age-structured population balance approach the cell cycle is modelled with particular emphasis on the G1-S checkpoint. By considering the probability of cells transitioning at the G1-S checkpoint, different transition functions are obtained. A suitable finite difference scheme for the numerical simulation of the model is derived and shown to be stable. The model is then fitted using the different probability transition functions to experimental data and the effects of the different probability transition functions on the model's results are discussed.

.
(2014) - 'Three-wave interactions and spatio-temporal chaos'.
Physical Review Letters, 108 (7) Article number 0754504
#### Abstract

Three-wave interactions form the basis of our understanding of many pattern forming systems because they encapsulate the most basic nonlinear interactions. In problems with two comparable length scales, it is possible for two waves of the shorter wavelength to interact with one wave of the longer, as well as for two waves of the longer wavelength to interact with one wave of the shorter. Consideration of both types of three-wave interactions can generically explain the presence of complex patterns and spatio-temporal chaos. Two length scales arise naturally in the Faraday wave experiment with multi-frequency forcing, and our results enable some previously unexplained experimental observations of spatio-temporal chaos to be interpreted in a new light. Our predictions are illustrated with numerical simulations of a model partial differential equation.

.
(2012) - 'Modelling and detecting tumour oxygenation levels.'.
PLoS One, United States: 7 (6)
**Repository URL:**http://epubs.surrey.ac.uk/802560/#### Abstract

Tumours that are low in oxygen (hypoxic) tend to be more aggressive and respond less well to treatment. Knowing the spatial distribution of oxygen within a tumour could therefore play an important role in treatment planning, enabling treatment to be targeted in such a way that higher doses of radiation are given to the more radioresistant tissue. Mapping the spatial distribution of oxygen in vivo is difficult. Radioactive tracers that are sensitive to different levels of oxygen are under development and in the early stages of clinical use. The concentration of these tracer chemicals can be detected via positron emission tomography resulting in a time dependent concentration profile known as a tissue activity curve (TAC). Pharmaco-kinetic models have then been used to deduce oxygen concentration from TACs. Some such models have included the fact that the spatial distribution of oxygen is often highly inhomogeneous and some have not. We show that the oxygen distribution has little impact on the form of a TAC; it is only the mean oxygen concentration that matters. This has significant consequences both in terms of the computational power needed, and in the amount of information that can be deduced from TACs.

.
(2012) - 'Scaling properties of weakly nonlinear coefficients in the Faraday problem'.
PHYSICAL REVIEW E, 84 (1) Article number ARTN 016209
**Repository URL:**http://epubs.surrey.ac.uk/72361/
.
(2011) - 'Spontaneous emergence of long-range shape symmetry'.
Nano Letters, United States: 11 (1), pp. 160-163.doi: 10.1021/nl1032793
#### Abstract

Self-organization of matter is essential for natural pattern formation, chemical synthesis, as well as modern material science. Here we show that isovolumetric reactions of a single organometallic precursor allow symmetry breaking events from iron nuclei to the creation of different symmetric carbon structures: microspheres, nanotubes, and mirrored spiraling microcones. A mathematical model, based on mass conservation and chemical composition, quantitatively explains the shape growth. The genesis of such could have significant implications for material design.

.
(2011) - 'Wavelet analysis of DEM simulations of samples under biaxial compression'.
GRANULAR MATTER, 10 (5), pp. 389-398.
**Repository URL:**http://epubs.surrey.ac.uk/293558/
.
(2008) - 'Non-linear wavelength selection for pattern-forming systems in channels'.
IMA JOURNAL OF APPLIED MATHEMATICS, 73 (6), pp. 873-901.
**Repository URL:**http://epubs.surrey.ac.uk/72366/
.
(2008) - 'Pattern selection for Faraday waves in an incompressible viscous fluid'.
SIAM Journal on Applied Mathematics, 67 (4), pp. 1064-1100.doi: 10.1137/050639223
**Repository URL:**http://epubs.surrey.ac.uk/72369/#### Abstract

When a layer of fluid is oscillated up and down with a sufficiently large amplitude, patterns form on the surface, a phenomenon first observed by Faraday. A wide variety of such patterns have been observed from regular squares and hexagons to superlattice and quasipatterns and more exotic patterns such as oscillons. Previous work has investigated the mechanisms of pattern selection using the tools of symmetry and bifurcation theory. The hypotheses produced by these generic arguments have been tested against an equation derived by Zhang and Viñals in the weakly viscous and large depth limit. However, in contrast, many of the experiments use shallow viscous layers of fluid to counteract the presence of high frequency weakly damped modes that can make patterns hard to observe, Here we develop a weakly nonlinear analysis of the full Navier-Stokes equations for the two-frequency excitation Faraday experiment. The problem is formulated for general depth, although results are presented only for the infinite depth limit. We focus on a few particular cases where detailed experimental results exist and compare our analytical results with the experimental observations. Good agreement with the experimental results is found. © 2007 Society for Industrial and Applied Mathematics.

.
(2007) - 'The effect of different forms for the delay in a model of the nephron'.
Mathematical Biosciences and Engineering, 2 (1), pp. 97-109.
**Repository URL:**http://epubs.surrey.ac.uk/72372/#### Abstract

We investigate how the dynamics of a mathematical model of a nephron depend on the precise form of the delay in the tubuloglomerular feed- back loop. Although qualitative behavioral similarities emerge for di®erent orders of delay, we ¯nd that signi¯cant quantitative di®erences occur. With- out more knowledge of the form of the delay, this places restrictions on how reasonable it is to expect close quantitative agreement between the mathemat- ical model and experimental data.

.
(2005) - 'Solutions for nonlinear convection in the presence of a lateral boundary'.
PHYSICA D-NONLINEAR PHENOMENA, 178 (1-2), pp. 83-102.
**Repository URL:**http://epubs.surrey.ac.uk/72367/
.
(2003) - 'Two-frequency forced Faraday waves: Weakly damped modes and pattern selection'.
Physica D: Nonlinear Phenomena, 143 (1-4), pp. 205-225.
**Repository URL:**http://epubs.surrey.ac.uk/72368/#### Abstract

Recent experiments (Kudrolli, Pier and Gollub, 1998) on two-frequency parametrically excited surface waves exhibit an intriguing "superlattice" wave pattern near a codimension-two bifurcation point where both subharmonic and harmonic waves onset simultaneously, but with different spatial wavenumbers. The superlattice pattern is synchronous with the forcing, spatially periodic on a large hexagonal lattice, and exhibits small-scale triangular structure. Similar patterns have been shown to exist as primary solution branches of a generic 12-dimensional $D_6\dot{+}T^2$-equivariant bifurcation problem, and may be stable if the nonlinear coefficients of the bifurcation problem satisfy certain inequalities (Silber and Proctor, 1998). Here we use the spatial and temporal symmetries of the problem to argue that weakly damped harmonic waves may be critical to understanding the stabilization of this pattern in the Faraday system. We illustrate this mechanism by considering the equations developed by Zhang and Vinals (1997, J. Fluid Mech. 336) for small amplitude, weakly damped surface waves on a semi-infinite fluid layer. We compute the relevant nonlinear coefficients in the bifurcation equations describing the onset of patterns for excitation frequency ratios of 2/3 and 6/7. For the 2/3 case, we show that there is a fundamental difference in the pattern selection problems for subharmonic and harmonic instabilities near the codimension-two point. Also, we find that the 6/7 case is significantly different from the 2/3 case due to the presence of additional weakly damped harmonic modes. These additional harmonic modes can result in a stabilization of the superpatterns.

.
(2000) - 'Parametrically excited surface waves: Two-frequency forcing, normal form symmetries, and pattern selection'.
Physical Review E, 59 (5), pp. 5446-5456.
**Repository URL:**http://epubs.surrey.ac.uk/72370/#### Abstract

Motivated by experimental observations of exotic standing wave patterns in the two-frequency Faraday experiment, we investigate the role of normal form symmetries in the pattern selection problem. With forcing frequency components in ratio m/n, where m and n are co-prime integers, there is the possibility that both harmonic and subharmonic waves may lose stability simultaneously, each with a different wavenumber. We focus on this situation and compare the case where the harmonic waves have a longer wavelength than the subharmonic waves with the case where the harmonic waves have a shorter wavelength. We show that in the former case a normal form transformation can be used to remove all quadratic terms from the amplitude equations governing the relevant resonant triad interactions. Thus the role of resonant triads in the pattern selection problem is greatly diminished in this situation. We verify our general results within the example of one-dimensional surface wave solutions of the Zhang-Vinals model of the two-frequency Faraday problem. In one-dimension, a 1:2 spatial resonance takes the place of a resonant triad in our investigation. We find that when the bifurcating modes are in this spatial resonance, it dramatically effects the bifurcation to subharmonic waves in the case of forcing frequencies are in ratio 1/2; this is consistent with the results of Zhang and Vinals. In sharp contrast, we find that when the forcing frequencies are in ratio 2/3, the bifurcation to (sub)harmonic waves is insensitive to the presence of another spatially-resonant bifurcating mode.

.
(1999) - 'New stability results for patterns in a model of long-wavelength convection'.
Physica D-Nonlinear Phenomena, 122 (1-4), pp. 117-133.
**Repository URL:**http://epubs.surrey.ac.uk/72365/#### Abstract

We consider the transition from a spatially uniform state to a steady, spatially- periodic pattern in a partial differential equation describing long-wavelength convec- tion [1]. This both extends existing work on the study of rolls, squares and hexagons and demonstrates how recent generic results for the stability of spatially-periodic patterns may be applied in practice. We find that squares, even if stable to roll perturbations, are often unstable when a wider class of perturbations is considered. We also find scenarios where transitions from hexagons to rectangles can occur. In some cases we find that, near onset, more exotic spatially-periodic planforms are preferred over the usual rolls, squares and hexagons.

.
(1998) - 'On a codimension-three bifurcation arising in a simple dynamo model'.
Physica D-Nonlinear Phenomena, 117 (1-4), pp. 117-127.
**Repository URL:**http://epubs.surrey.ac.uk/72363/#### Abstract

In this paper we investigate the dynamics associated with a degenerate codimension-two Takens-Bogdanov bifurcation which arises in a recently derived model for self-exciting dynamo action introduced by Hide et al. [R. Hide, A.C. Skeldon, D.J. Acheson, A study of two novel self-exciting single-disk homopolar dynamos: theory, Proc. R. Soc. Lond. A 452 (1996) 1369-1395]. The general unfolding of such a codimension-three bifurcation has already been discussed in an abstract setting by Li and Rousseau [Codimension-2 symmetric homoclinic bifurcations and application to 1:2 resonance, Can J. Math. 42 (1990) 191-212].Here we describe the unfolding scenario in the context of the dynamo problem. In particular we compare the behaviour predicted by the normal form analysis with a bifurcation study of the full dynamo equations in the neighbourhood of the codimension-three point.

.
(1998) - 'Stability Results for Steady, Spatially--Periodic Planforms'.
Nonlinearity, 10 (2) Article number 321
**Repository URL:**http://epubs.surrey.ac.uk/72371/#### Abstract

We consider the symmetry-breaking steady state bifurcation of a spatially-uniform equilibrium solution of E(2)-equivariant PDEs. We restrict the space of solutions to those that are doubly-periodic with respect to a square or hexagonal lattice, and consider the bifurcation problem restricted to a finite-dimensional center manifold. For the square lattice we assume that the kernel of the linear operator, at the bifurcation point, consists of 4 complex Fourier modes, with wave vectors K_1=(a,b), K_2=(-b,a), K_3=(b,a), and K_4=(-a,b), where a>b>0 are integers. For the hexagonal lattice, we assume that the kernel of the linear operator consists of 6 complex Fourier modes, also parameterized by an integer pair (a,b). We derive normal forms for the bifurcation problems, which we use to compute the linear, orbital stability of those solution branches guaranteed to exist by the equivariant branching lemma. These solutions consist of rolls, squares, hexagons, a countable set of rhombs, and a countable set of planforms that are superpositions of all of the Fourier modes in the kernel. Since rolls and squares (hexagons) are common to all of the bifurcation problems posed on square (hexagonal) lattices, this framework can be used to determine their stability relative to a countable set of perturbations by varying a and b. For the hexagonal lattice, we analyze the degenerate bifurcation problem obtained by setting the coefficient of the quadratic term to zero. The unfolding of the degenerate bifurcation problem reveals a new class of secondary bifurcations on the hexagons and rhombs solution branches.

.
(1998) - 'New stability results for long-wavelength convection patterns'.
#### Abstract

We consider the transition from a spatially uniform state to a steady, spatially-periodic pattern in a partial differential equation describing long-wavelength convection. This both extends existing work on the study of rolls, squares and hexagons and demonstrates how recent generic results for the stability of spatially-periodic patterns may be applied in practice. We find that squares, even if stable to roll perturbations, are often unstable when a wider class of perturbations is considered. We also find scenarios where transitions from hexagons to rectangles can occur. In some cases we find that, near onset, more exotic spatially-periodic planforms are preferred over the usual rolls, squares and hexagons.

.
(1997) - 'Free convection in liquid gallium'. JOURNAL OF FLUID MECHANICS, 342, pp. 295-314. . (1997)
- 'Grid design for the computation of a hexagon-roll interaction using a finite element method'. JOURNAL OF COMPUTATIONAL PHYSICS, 133 (1), pp. 18-26. . (1997)
- 'A study of two novel self-exciting single-disk homopolar dynamos: Theory'. PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, 452 (1949), pp. 1369-1395. . (1996)
- 'Convection in a low Prandtl number fluid'. JOURNAL OF CRYSTAL GROWTH, 162 (1-2), pp. 95-106. . (1996)
- 'On long-wave morphological instabilities in directional solidification'. European Journal of Applied Mathematics, 6, pp. 639-652. . (1995)
- 'DYNAMICS OF A PARAMETRICALLY EXCITED DOUBLE PENDULUM'. PHYSICA D, 75 (4), pp. 541-558. . (1994)
- 'MODE INTERACTION IN A DOUBLE PENDULUM'. PHYSICS LETTERS A, 166 (3-4), pp. 224-229. . (1992)
- 'PHASE PORTRAITS FOR PARAMETRICALLY EXCITED PENDULA - AN EXERCISE IN MULTIDIMENSIONAL DATA VISUALIZATION'. COMPUTERS & GRAPHICS, 16 (3), pp. 331-337. . (1992)
- 'THE 5-DIMENSIONAL PENDULUM PICTURE SHOW'. NEW SCIENTIST, 124 (1689), pp. 46-47. . (1989)

### Conference papers

- 'Changes in sleep across the lifespan: using mathematical models to explore hypotheses to explain sleep timing'. WILEY-BLACKWELL JOURNAL OF SLEEP RESEARCH, Tallinn, ESTONIA: 22nd Congress of the European-Sleep-Research-Society 23, pp. 169-170. . (2014)
- 'Defining Relevance and Finding Rules: An Agent-Based Model of Biomass Use in the Humber Area'. SPRINGER-VERLAG BERLIN
ADVANCES IN SOCIAL SIMULATION, Warsaw Sch Econ, Warsaw, POLAND: 9th Conference of the European-Social-Simulation-Association (ESSA) 229, pp. 373-384.
**Repository URL:**http://epubs.surrey.ac.uk/806477/#### Abstract

The field of industrial ecology applies ecosystem theory to industrial production, human consumption and societies. This article presents a case study of the development of the bio-based economy in the area surrounding the Humber estuary in the North-East of England. The study developed an agent-based model to simulate the evolution of the industrial system. We explain how the qualitative research process led to the development of a toy model that has successively been specified

.
(2014)

### Software

- EMBER: Emergent and Macroscopic Behavioural ExtRaction.
#### Abstract

Java based analysis tool that is able to extract systematic behaviour of complex and stochastic systems directly from 'black box' simulations. Performing rigorous analysis such as; parameter dependence, sensitivity analysis, bifurcations or regime shifts, tipping or limit point identification, statistical properties (variance and underlying distributions), path dependence and stability analysis. This tool extracts insight directly from a simulator, e.g. micro-level or agent-based model, without the need to understand any of the underlying algorithms involved.

.
(2016)

## Teaching

In 2015/16 I am teaching the following modules

- MAT3043 Graphs and Networks
- MAT1005 Vector Calculus