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Criterion for the accumulation of v-walls
Get PDFIn this thesis we consider walls in the slice of geometric stability conditions for Chern characters on a principally abelian polarised surface. It is known that there is a vertical line of stability conditions in our choice of parametrisation, determined entirely numerically, which must intersect every wall, apart from a single vertical one. Furthermore if this line has rational horizontal coordinate β_ , then there can only be finitely many walls. In principal, when this horizontal coordinate is not rational, then there could be infinitely many walls. This has also been shown for certain examples and classes of Chern characters, but not exhaustively for all Chern characters where the aforementioned vertical line of stability conditions has irrational horizontal coordinate.
The main conclusion of this thesis is that all other classes of Chern characters not considered also have infinitely many walls. That is, a Chern character v on a principally polarised abelian surface with NS() = ℤℓ has finitely many walls if and only if β_(v) is rational. The other aspect of this thesis is developing methods to computing possibilities for walls more efficiently
Growing Dryland Futures: Notes from a breakfast discussion held in Nairobi on 10 April 2025
Get PDFThese notes capture the ‘raw’ discussions and serve as an aide-memoire to take forward various actions through local, national and regional channels. For each of seven themes – Peace and security, land and natural resources, pastoralist policies, youth and inclusion, livestock markets, climate change and drought, and livestock production – mixed tables explored critical issues, identified priority investment and policy changes and actions to advance these.On 10 April 2025, the Jameel Observatory for Food Security Early Action, the Drylands Learning and Capacity Building Initiative and the Pastoralist Parliamentary Group convened an informal dialogue among policymakers, politicians, researchers, development partners and young scholars to explore 'grounded' narratives on drylands in the Horn of Africa, their importance, and opportunities to enhance the prosperity and resilience of dryland communities and pastoralists through early action and targeted investment
Neural computation through geometry and dynamics
No full textThe brain is a dynamical system made of billions of interconnected neurons, and its function is to transform sensory input into behavioural output. Yet, the mechanisms behind the computations performed by large networks of neurons — whether biological or artificial — remain poorly understood. To tackle this challenge, neuroscience has experienced a recent push towards large-scale neural data. These increasingly high-dimensional neural recordings have created a need for new mathematical and artificial intelligence (AI) tools for data analysis and modelling. This has led to the emergence of two lines of research which have evolved in parallel: i) the design of new theoretical tools to model the dynamics of networks of neurons, and ii) the development of new data analysis methods to uncover low-dimensional geometry in neural recordings. Yet, network models often do not work well with the noisy high-dimensional data yielded by experiments, while geometric methods can be fitted to data but lack ways to test specific hypotheses about the computations performed by the recorded neurons.
My doctoral work developed new methods for the study of neural computations which integrate dynamical systems modelling and data-driven geometric tools to provide a means to generate and systematically test neuroscientific hypotheses against high-dimensional data. I start by reviewing the linear algebra and dynamical systems theory relevant to the neural subspace hypothesis. Then, to extend this view, I introduce novel methods based on tensors, differential geometry and dynamical systems. I show that these methods can be used to study how task and behavioural variables are non-linearly represented in the activity of populations of neurons. They also have practical applications, for example by improving the decoding of the kinematics of the arm from neural activity. Going one step further, I probe how biological and artificial neural networks reshape their connectivity to change these representations to learn new tasks. I derive mathematically, in models, and in data, the learning mechanisms underlying the widely observed phenomenon of low-rank neural connectivity. Finally, I provide a new theoretical framework grounded in differential geometry to characterise the manifolds of recurrent neural networks. Together, these results provide a means to study neural computations in biological and artificial neural networks by linking the geometric and dynamical systems perspectives on neural activity
Switching techniques for quasi-square-wave modulated modular multilevel converters in DC-DC conversion and motor drive systems
No full textThe Modular Multilevel DC Transformer (MMDCT), originating from the Modular Multilevel Converter (MMC), offers a promising solution for high-power, high-gain DC-DC conversion, effectively addressing challenges such as high power semiconductor voltage ratings and dv/dt stress in conventional two-level Dual-Active-Bridge (DAB) topologies. This thesis investigates the design, optimization, and experimental validation of MMDCT, aiming to enhance its performance and efficiency.
A comprehensive analysis is conducted on the transient behavior of MMDCT under Quasi-Square-Wave (QSW) modulation, providing insights into potential voltage mismatches, component sizing, and voltage estimation. However, the investigation reveals that the implementation of MMDCT introduces significant frequency-related losses due to the energy stored in arm inductors. To address this, an alternative switching sequence is proposed to recover the inductor energy and enable MMDCT operation at high switching frequencies. Additionally, this thesis presents the utilization of sub-module capacitance as snubber components to achieve zero-voltage switching (ZVS) and suppress voltage overshoot during high-current operations. The coordinated use of these switching techniques improves conversion efficiency and optimizes MMDCT performance.
Moreover, this thesis incorporates the proposed inductor energy recovery switching technology into the QSW-modulated MMC inverter. This method enhances efficiency and eliminates the need for high-frequency current control, thereby simplifying the control requirements of the QSW-modulated MMC inverter.
The design and implementation of a scaled experimental platform are presented, enabling the validation of theoretical findings and proposed control strategies under practical conditions. Experimental results demonstrate significant improvements in converter efficiency, voltage control, and overall performance
Debating the future of Scottish imprisonment, 2008-2020
No full textThis thesis explores political struggle to change imprisonment in a post-devolution, nation-building Scotland. It focuses on key moments within which conflicts over the role of the prison in the ‘new’ Scotland were crystalised: in the emergence of ‘decarceration consensus’ in 2008, and during the project of trying to transform the Scottish Prison Service (‘SPS’) between 2012 and 2020. Both trajectories are often described as having failed to substantially change the role of imprisonment or the high prison population, and this project recovers some of the key boundaries and opportunities, hopes and disappointments, bound up in this sense of failure.
The research consisted of a review of key texts connected to these events, in addition to in-depth oral history or ‘career story’ interviews with actors central to Scottish imprisonment debates. Both the fieldwork and analysis were orientated towards narrative inquiry – the narratives encoded in texts, in the recollections of interviewees, and in what emerged as some fairly fixed narratives around the Scottish nation. Drawing from developments in the sociology of punishment, this project questions the degree to which any place can claim to have a single, intrinsic, penal (or national) identity.
Exploring the early post-devolution years leading up to 2008, I identify key areas of path-paving for a sense of sudden change, as well as certain explicit and implicit boundaries to what change could be pursued. The construction and then mobilization of decarceration consensus revealed a set of constrained sensibilities and discourses around the prison not immediately apparent in the story of national and penal change. I argue that the decarceration consensus that emerged in 2008 was limited in important ways, and that the place of the prison in Scottish society remained conflicted. This shaped subsequent efforts to transform the prison estate, during which SPS embodied and reflected imprisonment’s contested and contradictory role. With significant conflicts around 'Scottishness’, the prison, and those imprisoned unresolved – even suppressed – efforts for change have been firmly bounded. The thesis concludes that certain longstanding narratives around Scottish society, and even the narrative of consensus itself, created a somewhat ‘imaginary’ space for imprisonment change in this period
Intestines to radio signals: Informing locally led anticipatory action in drylands of the Horn of Africa
Get PDFPresented at Building the Resilience and Prosperity of Pastoralists and Dryland Communities, Nairobi, Kenya, 1-2 October 202
Towards Lattice QCD calculation of 2 → 3 and 3 → 3 scattering
Get PDFThe strong force, responsible for binding quarks and gluons inside hadrons, is described by the mathematical framework of quantum chromodynamics (QCD). One approach to studying the non-perturbative region of this theory is lattice QCD. This numerical method extracts the hadron spectrum and other physical observables from first principles by formulating QCD on a discretized Euclidean spacetime lattice with finite volume.
Lattice QCD combined with the finite-volume formalism, enables the extraction of infinite-volume scattering observables from the discrete energy spectrum obtained in lattice simulations. This formalism is derived from relativistic-field-theoretic analysis of the poles of the finite-volume scattering amplitude and realized via a quantization condition, whose solutions give the finite spectrum. Over the last decades, the formalism has matured from treating two-particle to three-particle systems with sub-resonant channels, including odd-legged vertices that allow for two-to-three scattering.
This thesis derives and implements novel theoretical results concerning two key aspects of lattice QCD scattering calculations.
First, we implement the formalism for the three-pion system in all allowed non-maximal isospin channels to numerically compute the finite-volume spectrum using Python called ampyl. This is performed across multiple moving frames for various irreducible representations of the finite-volume symmetry group. The system includes two-pion resonant subchannels associated with the rho and sigma resonances, with vanishing three-body interactions. These results serve as a benchmark and database for future lattice calculations aimed at extracting three-pion resonances.
Second, we implement the coupled quantization condition, i.e., including 2-to-3 scattering, for identical scalar particles in a rest frame to extract the energy spectrum.
Then, we focus on the avoided-level crossing that occurs when a two- and three-particle level intersect in the spectrum and show that the spectrum in this region follows the form of a rotated hyperbola, leading to a simple analytic expression that can be fitted to lattice data in weakly interacting systems. This offers a practical alternative to using the full quantization condition
Measuring adverse childhood experiences (ACEs) and deprivation during early childhood and its link to self-reported victimisation during adolescence among young adults in the UK
Get PDFOBJECTIVE:
This study aims to apply data-driven techniques to measure early Adverse Childhood Experiences (ACEs) in a contemporary cohort of children in the UK and examine their association with adolescent victimisation, stratified by socio-economic positions.
METHODS:
This study draws on data from the Millennium Cohort Study (MCS), a nationally representative longitudinal survey following 18,818 children born between 2000 and 2002 from infancy through age 17. A complete case analysis (N = 9,804) was conducted, guided by a socio-ecological life course framework. ACEs are operationalised through variables from the second and third sweeps, with Exploratory Factor Analysis and Item Response Theory used to assess their psychometric properties. The primary ACEs indicators include household dysfunction as reported by mothers and fathers, along with harsh discipline practices reported by mothers only. Cumulative scores and Item Response Theory derived scores are estimated and monotonically transformed for comparison. Quintile-quintile plots are used to compare between methods of operationalisation and respondent type (mothers vs. fathers). Socio-economic positions are measured during early childhood (at age three) and include OECD household income quintiles, in addition to parental: educational attainment, employment status, ethnicity and age. Linear regression models are used to estimate associations between socially stratified early ACE scores by respondent type and method of operationalisation. Early ACEs and subsequent adversities during middle childhood (7-14 years) are explored through a descriptive analysis of the data. Victimisation outcomes at ages 14 and 17 are measured through five and nine self-reported questions, respectively. The types of victimisations covered include emotional, physical, and sexual victimisation, as well as cyberbullying and theft. Logistic regression models are employed to examine associations between ACEs and victimisation, adjusting for socio-economic positions to account for potential confounding effects. Data management and preliminary and final analysis was conducted in Stata 18, while Item Response Theory analysis is performed in R using the MIRT package.
RESULTS:
Findings highlight the inherent complexities of measuring latent constructs such as ACEs using observational panel data, while also underscoring the importance of rigorous operationalisation to ensure relevance for policy and practice within specific generational and local contexts. Despite the widespread use of ACE frameworks in public health and social policy, significant gaps in evidence persist regarding their measurement in panel data, raising concerns about potential misapplication and misallocation of resources.
The findings demonstrate clear gender differences in exposure to victimisation during adolescence. Theft was the only victimisation type reported at similar rates by both boys and girls. Overall, victimisation increased with age; by age 17, fewer adolescents reported no victimisation experiences compared to age 14. Notably, girls showed a greater increase in victimisation over time, shifting from single incidents at age 14 to higher levels of victimisation at age 17, indicating a shift in gendered patterns of risk.
High exposure to early ACEs as reported by fathers was significantly associated with all forms of victimisation at age 14, while high exposure reported by mothers was significantly linked to three of the five types assessed at this stage. By age 17, the pattern reversed: high maternal ACE exposure was significantly associated with eight of the nine types of victimisation measured, whereas paternal ACE exposure was associated with only four. Although maternal and paternal ACE reports share some common associations, they also captured distinct patterns; maternal ACEs were more closely linked to emotional victimisation and cyberbullying, while paternal ACEs were associated with physical victimisation, a form of sexual victimisation, and theft at age 17.
CONCLUSION:
This study addresses methodological complexities in measuring ACEs using observational panel data. By integrating data-driven psychometric assessments with established theoretical frameworks, it provides crucial insights into the validity and reliability of ACEs measurement within the MCS. The findings will be instrumental for policymakers and researchers, informing decisions on survey design in longitudinal cohort studies and ensuring methodological rigour in ACEs measurement. Additionally, this study contributes to the broader understanding of whether different adversities genuinely constitute a single latent construct (ACEs), enhancing the accuracy of ACE-related policy and intervention strategies
Security, Fragmentation, and Civic Futures: Rethinking Reform in Sudan’s Militarised Political Economy
No full textSudan’s security sector has long functioned not merely as a military institution but as a mechanism of authoritarian governance and economic extraction. Since independence, successive regimes have maintained power through coercive force and the dispersal of fiscal authority to unaccountable military, paramilitary, and commercial actors.
This brief contends that any serious approach to security sector reform (SSR) in Sudan must reckon with the political economy that sustains militarised rule. It focuses on Sudan’s colonial military legacies, the political economy of security financing, and the emergence of civic resistance as both a political and fiscal counter-power. Drawing from archival, civic, and interview-based sources, the brief provides a grounded account of how popular actors have theorised and contested militarised governance. We argue that SSR has repeatedly failed not because of technical deficiencies, but because of exclusionary settlements, donor complicity, and a refusal to confront the core structures of military-fiscal rule.
A future reform agenda must centre civic actors, address fiscal fragmentation, and challenge the transnational architecture that has enabled elite impunity. The insights presented here aim to inform humanitarian and development interventions by the United Kingdom government and other partners who are seeking to forge an enduring peace once the opportunity arises
Plural fricative lenisisation in English: testing phonological intuitions and analysing weak suppletion
Get PDFThis thesis studies the phenomenon of irregular plural forms in fricative-final count nouns in
English, whereby stem-final fricatives are lenis in plurals, despite the fact that they are fortis
in corresponding singulars (as in wolf ~ wolves). This phenomenon has not been analysed in
depth before and its current state is unclear. The aim of this thesis is to explore the synchronic
state of this laryngeal alternation, which I call Plural Fricative Lenisisation (PFL), and to
propose a phonological analysis of it.
In Chapter 1, I introduce the phenomenon of PFL and present the research questions and aims.
Chapter 2 presents a discussion of laryngeal representations, as part of which I adopt the
analysis known as Laryngeal Realism. Previous studies that have touched upon PFL are
considered, among which are analyses which either treat this laryngeal alternation as a
suppletion which is stored lexically, or as a minor rule targeting a specific group of forms, or
as a problem of morphophonemes, or as a possibly productive rule which is able to apply to
new items.
Chapter 3 introduces the methodology of a survey-based investigation which aims to explore
the current state of PFL by testing the phonological intuitions of participants. The survey
consists of 2 experiments. Experiment 1 is an introspection task which examines how
participants pronounce the plural forms of fricative-final nouns. Experiment 2 is recording-based,
where participants are provided with separate recordings of lenis and fortis
pronunciations of the plural forms, from which they choose the one they deem acceptable.
Chapter 4 and Chapter 5 present the results of Experiment 1 and Experiment 2 respectively.
The results show that words which undergo PFL behave significantly differently from words
that do not. The behaviours of /f/-final words are significantly different compared to /s/-final
and /θ/-final words. I also explore whether there are gender differences and the differences in
the responses between Experiment 1 and Experiment 2.
In Chapter 6 I discuss further the results presented in Chapters 4 and 5, and compare the
differences between the results of Experiment 1 and Experiment 2. The results suggest that
PFL is still alive in a specific group of words. This shows that PFL is not just a collection of
random exceptional plural forms – I argue that there is a distinct pattern behind it.
The theoretical analysis is presented in Chapter 7. I consider a number of possible approaches
to PFL, including the potential to analyse it using an Exemplar-Theoretic approach (which I
show to be untenable), and as (strong) suppletion (which is simply stored lexically) or as a
standard rule through which lenisisation is realised. I also consider another approach which
stands in-between these latter two approaches: that PFL involves weak suppletion – the
lenisised fricative-final plurals match a pattern of allomorphy which is encoded in a via-rule.
I argue for this latter analysis: PFL involves weak suppletion
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