### Professor Anne Skeldon

### Biography

### Research interests

Details can be found on my personal web page.

### Teaching

In 2015/16 I am teaching the following modules

- MAT3043 Graphs and Networks
- MAT1005 Vector Calculus

### My publications

### Publications

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.

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.

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.

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.

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.

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.

In each study I model the tension between the individual and the group using the public goods game. This game is played on a structured population defined by a multilayered network. Each layer represents a different sphere of influence on the player?s decision to cooperate or defect.

The first model studies the effect of a player choosing whether to cooperate or defect on either all layers simultaneously (synchronously) or on one layer at a time (asynchronously). Updating asynchronously leads to increased cooperation across a number of different parameter regimes. This demonstrates a new way in which cooperation can be increased in a system with multiple influences, and also helps to understand exactly why cooperation is increased in multilayered systems.

Inspired by empirical examples, the second model adds to the standard model of the public goods game on networks in two ways. The first is to include conditional cooperators, and the second is the addition of a layer of social influence. This combination of economic and social influence has not been considered in previous models of the public goods game, and I find that this additional layer of influence results in high levels of cooperation. In the final chapter, I study these dynamics on more realistic network structures, with results echoing empirical findings under certain parameters.

standing of sleep/wake regulation. This ostensibly simple model is an interesting example

of a nonsmooth dynamical system whose rich dynamical structure has been relatively un-

explored. The two process model can be framed as a one-dimensional map of the circle

which, for some parameter regimes, has gaps. We show how border collision bifurcations

that arise naturally in maps with gaps extend and supplement the Arnold tongue saddle-

node bifurcation set that is a feature of continuous circle maps. The novel picture that

results shows how the periodic solutions that are created by saddle-node bifurcations in

continuous maps transition to periodic solutions created by period-adding bifurcations as

seen in maps with gaps.

Optimising policy choices to steer social/economic systems efficiently

towards desirable outcomes is challenging. The inter-dependent nature of

many elements of society and the economy means that policies designed to

promote one particular aspect often have secondary, unintended, effects.

In order to make rational decisions, methodologies and tools to assist

the development of intuition in this complex world are needed. One

approach is the use of agent-based models. These have the ability to

capture essential features and interactions and predict outcomes in a way

that is not readily achievable through either equations or words alone.

In this paper we illustrate how agent-based models can be used in

a policy setting by using an example drawn from the biowaste industry.

This example describes the growth of in-vessel composting and anaerobic

digestion to reduce food waste going to landfill in response to policies in

the form of taxes and financial incentives. The fundamentally dynamic

nature of an agent-based modelling approach is used to demonstrate that policy outcomes depend not just on current policy levels but also on the

historical path taken.

the laboratory they often cooperate conditionally, and the frequency of conditional

cooperators differs between communities. However, this has not yet been fully explained

by social dilemma models in structured populations. Here we model a

population as a two-layer multiplex network, where the two layers represent economic

and social interactions respectively. Players play a conditional public goods game on

the economic layer, their donations to the public good dependent on the donations

of their neighbours, and player strategies evolve through a combination of payoff

comparison and social influence. We find that both conditional cooperation and

social influence lead to increased cooperation in the public goods game, with social

influence being the dominant factor. Cooperation is more prevalent both because

conditional cooperators are less easily exploited by free-riders than unconditional

cooperators, and also because social influence tends to preserve strategies over time.

Interestingly the choice of social imitation rule does not appear to be important: it

is rather the separation of strategy imitation from payoff comparison that matters.

Our results highlight the importance of social influence in maintaining cooperative

behaviour across populations, and suggest that social behaviour is more important

than economic incentives for the maintenance of cooperation.