7,600 research outputs found
On information captured by neural networks: connections with memorization and generalization
Despite the popularity and success of deep learning, there is limited
understanding of when, how, and why neural networks generalize to unseen
examples. Since learning can be seen as extracting information from data, we
formally study information captured by neural networks during training.
Specifically, we start with viewing learning in presence of noisy labels from
an information-theoretic perspective and derive a learning algorithm that
limits label noise information in weights. We then define a notion of unique
information that an individual sample provides to the training of a deep
network, shedding some light on the behavior of neural networks on examples
that are atypical, ambiguous, or belong to underrepresented subpopulations. We
relate example informativeness to generalization by deriving nonvacuous
generalization gap bounds. Finally, by studying knowledge distillation, we
highlight the important role of data and label complexity in generalization.
Overall, our findings contribute to a deeper understanding of the mechanisms
underlying neural network generalization.Comment: PhD thesi
Technology for Low Resolution Space Based RSO Detection and Characterisation
Space Situational Awareness (SSA) refers to all activities to detect, identify and track objects in Earth orbit. SSA is critical to all current and future space activities and protect space assets by providing access control, conjunction warnings, and monitoring status of active satellites. Currently SSA methods and infrastructure are not sufficient to account for the proliferations of space debris. In response to the need for better SSA there has been many different areas of research looking to improve SSA most of the requiring dedicated ground or space-based infrastructure. In this thesis, a novel approach for the characterisation of RSO’s (Resident Space Objects) from passive low-resolution space-based sensors is presented with all the background work performed to enable this novel method. Low resolution space-based sensors are common on current satellites, with many of these sensors being in space using them passively to detect RSO’s can greatly augment SSA with out expensive infrastructure or long lead times. One of the largest hurtles to overcome with research in the area has to do with the lack of publicly available labelled data to test and confirm results with. To overcome this hurtle a simulation software, ORBITALS, was created. To verify and validate the ORBITALS simulator it was compared with the Fast Auroral Imager images, which is one of the only publicly available low-resolution space-based images found with auxiliary data. During the development of the ORBITALS simulator it was found that the generation of these simulated images are computationally intensive when propagating the entire space catalog. To overcome this an upgrade of the currently used propagation method, Specialised General Perturbation Method 4th order (SGP4), was performed to allow the algorithm to run in parallel reducing the computational time required to propagate entire catalogs of RSO’s. From the results it was found that the standard facet model with a particle swarm optimisation performed the best estimating an RSO’s attitude with a 0.66 degree RMSE accuracy across a sequence, and ~1% MAPE accuracy for the optical properties. This accomplished this thesis goal of demonstrating the feasibility of low-resolution passive RSO characterisation from space-based platforms in a simulated environment
Beam scanning by liquid-crystal biasing in a modified SIW structure
A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium
Unstable Periodic Orbits: a language to interpret the complexity of chaotic systems
Unstable periodic orbits (UPOs), exact periodic solutions of the evolution equation, offer a very
powerful framework for studying chaotic dynamical systems, as they allow one to dissect their
dynamical structure. UPOs can be considered the skeleton of chaotic dynamics, its essential
building blocks. In fact, it is possible to prove that in a chaotic system, UPOs are dense in
the attractor, meaning that it is always possible to find a UPO arbitrarily near any chaotic
trajectory. We can thus think of the chaotic trajectory as being approximated by different
UPOs as it evolves in time, jumping from one UPO to another as a result of their instability.
In this thesis we provide a contribution towards the use of UPOs as a tool to understand and
distill the dynamical structure of chaotic dynamical systems. We will focus on two models,
characterised by different properties, the Lorenz-63 and Lorenz-96 model.
The process of approximation of a chaotic trajectory in terms of UPOs will play a central role
in our investigation. In fact, we will use this tool to explore the properties of the attractor of
the system under the lens of its UPOs.
In the first part of the thesis we consider the Lorenz-63 model with the classic parameters’ value.
We investigate how a chaotic trajectory can be approximated using a complete set of UPOs
up to symbolic dynamics’ period 14. At each instant in time, we rank the UPOs according to
their proximity to the position of the orbit in the phase space. We study this process from
two different perspectives. First, we find that longer period UPOs overwhelmingly provide the
best local approximation to the trajectory. Second, we construct a finite-state Markov chain
by studying the scattering of the trajectory between the neighbourhood of the various UPOs.
Each UPO and its neighbourhood are taken as a possible state of the system. Through the
analysis of the subdominant eigenvectors of the corresponding stochastic matrix we provide a
different interpretation of the mixing processes occurring in the system by taking advantage of
the concept of quasi-invariant sets.
In the second part of the thesis we provide an extensive numerical investigation of the variability
of the dynamical properties across the attractor of the much studied Lorenz ’96 dynamical
system. By combining the Lyapunov analysis of the tangent space with the study of the
shadowing of the chaotic trajectory performed by a very large set of unstable periodic orbits,
we show that the observed variability in the number of unstable dimensions, which shows a
serious breakdown of hyperbolicity, is associated with the presence of a substantial number of
finite-time Lyapunov exponents that fluctuate about zero also when very long averaging times
are considered
Utilitarianism and the Social Nature of Persons
This thesis defends utilitarianism: the view that as far as morality goes, one ought to choose the option which will result in the most overall well-being. Utilitarianism is widely rejected by philosophers today, largely because of a number of influential objections. In this thesis I deal with three of them. Each is found in Bernard Williams’s ‘A Critique of Utilitarianism’ (1973). The first is the Integrity Objection, an intervention that has been influential whilst being subject to a wide variety of interpretations. In Chapter Two I give my interpretation of Williams’s Integrity objection; in Chapter Three I discuss one common response to it, and in Chapters Four and Five I give my own defence of utilitarianism against it. In Chapter Six I discuss a second objection: the problem of pre-emption. This problem is also found in Williams, but has received greater attention through the work of other authors in recent years. It suggests that utilitarianism is unable to deal with some of the modern world’s most pressing moral problems, and raises an internal tension between the twin utilitarian aims of making a difference and achieving the best outcomes. In Chapter Seven I discuss a third objection: that utilitarianism is insufficiently egalitarian. I find this claim to be unwarranted, in light of recent social science and philosophy. My responses to Williams’s objections draw upon resources from the socialist tradition – in particular, that tradition’s emphasis on the importance of social connections between individuals. Socialists have often been hostile to utilitarianism, in part for socialist-inflected versions of Williams’s objections. Thus, in responding to these objections I aim to demonstrate that socialist thought contains the means to defuse not only mainstream philosophy’s rejection of utilitarianism but also its own, and thus to re-open the possibilities for a productive engagement between the two traditions
Electromagnetic scattering by lossy plasmonic and non-plasmonic half-spaces from vertically polarized incident waves
In this research, approximate analytical solutions for the scattered electromagnetic
(EM) fields radiated by a vertical electric dipole (VED) antenna in the presence
of a lossy half-space for ordinary and plasmonic media are investigated. First, an
approximate analytical solution for the wave scattering above a lossy half-space with
a smooth interface is proposed for frequencies below the very high frequency (VHF)
band. The solution to the problem is given in terms of two-dimensional Fourier
transforms, which leads to Sommerfeld-type integrals. The solution is decomposed
into three terms. Two terms are expressed with hyperbolic functions and the third
term is presented using the Gauss error function. A numerical evaluation of the
integrals validates the accuracy and efficiency of the proposed solution at various
frequencies and distances from the source. Second, an approximate analytical solution
of the problem with a smooth interface is proposed for frequencies below 10 GHz.
The solution for the intermediate Hertz potential is decomposed into two integrals
and a rigorous approximate closed-form solution in the near and far field regions
is presented for each term. Then, the scattered electric field (E-field) components
are calculated from the intermediate Hertz potential. A numerical evaluation of the
solution for different lossy half-spaces, i.e., seawater, wet earth, dry earth and lake
water, validates the accuracy of the proposed solution at various frequencies and
distances from the antenna. Following this work, a new asymptotic solution for the
scattered EM fields above a lossy half-space with a smooth interface for ordinary
and plasmonic media is proposed using the modified saddle point method. The new
formulations are applied to calculate radiation patterns of different impedance half-
planes for both ordinary media (e.g., seawater, silty clay soil, silty loam soil and lake
water) and plasmonic media (e.g., silver and gold). A numerical evaluation of the
proposed solution at various frequencies and comparisons with two alternative state-
of-the-art solutions show that the proposed solution has higher accuracy for plasmonic
and non-plasmonic structures. Lastly, random roughness is added to the interface, and
a solution for EM scattering over a two-dimensional random rough surface with large
roughness height using the generalized functions approach is proposed. The EM field
derivation incorporates an arbitrary rough surface profile with small slope, a radiation
source and involves all scattering orders of the scattered E-field for high and moderate
contrast media. Subsequently, the first-order scattered E-field is calculated using the
Neumann series solution for transverse magnetic (TM) polarization. By considering a
pulsed dipole antenna and a two-dimensional Gaussian rough surface distribution with
different root mean square heights and correlation lengths, the scattered E-field along
with the radar cross-section is calculated. Using the result of the method of moments
(MoM) as reference, a numerical evaluation of the solution for different roughness
heights and contrast media demonstrates that the proposed solution is better than
those of the small perturbation method (SPM), Kirchhoff approximation (KA) and
small-slope approximation (SSA)
Specificity of the innate immune responses to different classes of non-tuberculous mycobacteria
Mycobacterium avium is the most common nontuberculous mycobacterium (NTM) species causing infectious disease. Here, we characterized a M. avium infection model in zebrafish larvae, and compared it to M. marinum infection, a model of tuberculosis. M. avium bacteria are efficiently phagocytosed and frequently induce granuloma-like structures in zebrafish larvae. Although macrophages can respond to both mycobacterial infections, their migration speed is faster in infections caused by M. marinum. Tlr2 is conservatively involved in most aspects of the defense against both mycobacterial infections. However, Tlr2 has a function in the migration speed of macrophages and neutrophils to infection sites with M. marinum that is not observed with M. avium. Using RNAseq analysis, we found a distinct transcriptome response in cytokine-cytokine receptor interaction for M. avium and M. marinum infection. In addition, we found differences in gene expression in metabolic pathways, phagosome formation, matrix remodeling, and apoptosis in response to these mycobacterial infections. In conclusion, we characterized a new M. avium infection model in zebrafish that can be further used in studying pathological mechanisms for NTM-caused diseases
Diagnostics of the thermodynamic properties of solar prominences
Solar prominences have been observed for years, with observations dating back to the 12th century. Only in recent history however, have we had the spectroscopic tools required to probe and understand their structure and dynamics. Even then, with ground bases observations we have been unable to observe the hottest parts of solar prominences due to the atmosphere which cruelly hides this ultraviolet radiation from us. Several observational satellites have been launched over the years so that we may observe these high temperatures. With the launch of the Interface Region and Imaging Telescope (IRIS), we were able to observe the Mg II h&k lines. Which gives us a unique view into the atmosphere surrounding solar prominences.
In Chap 1, we give a brief overview of the solar atmosphere and what we know about the life and evolution of solar prominences. We cover their morphology, typical thermodynamic parameters, velocity distributions, and surrounding magnetic field strength. This is followed by a brief aside into radiative transfer where the basic equations are outlined with the LTE solution for radiative transfer through some emitting material. The principle ion in this work, Mg II h&k, is also presented with mention of its key features. The instrumentation used in the study are then introduced; IRIS, the X-Ray Telescope (XRT) onboard Hinode, Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), and the Sun-Earth Connection Coronal and Heliospheric Imager (SECCHI) onboard the Solar Terrestrial Relations Observatory Ahead (STEREO-A).
In Chap 2, we cover the coordinated observation of a solar prominence on the 19 April 2018 by IRIS, AIA, and XRT.We show the distribution of the following statistical measures of the prominence, the integrated line intensity, Doppler velocity, line widths, and asymmetries using the quantile method. We also present the distribution of line profile types, whether they be singe, double, or complexly peaked. A method for filtering out coronal pixels using the line widths, peak intensities, and pixel connectivity is presented and shown to work to effectively isolate the prominence. We draw conclusions surrounding its observed dynamics and the relationships between its statistical measures.
Chap 3 sees the introduction of a new method used to ascertain the properties of solar prominences observed in Mg II h&k. This is achieved by the point-for-point comparison of 1007 line profiles generated by the 1D non-local thermodynamic equilibrium (NLTE) radiative transfer code, PROM, with that of the observations. PROM treats its resonance lines in PRD and allows us to generate the Mg II h&k line profiles, with the three Mg II triplet lines and the principle Hydrogen lines, which are not used in this method. This matching method is named rRMS. Not only do we find the best fitting profiles in our bank, but we also find some measure of the ‘goodness’ of these fits. Therefore we can set a cut-off for what is considered a good or a bad fit. This is the first time a prominence has been inverted in this way. An updated version of rRMS called xRMS is then presented. It demonstrates a significant computational improvement over rRMS, allowing us to use a larger grid of 23940 line profiles. We present both of these results and compare what is found for the smaller and bigger grids. While both achieve similar levels of successful fits, the parameters which they recover are not exactly the same.
Chap 4 introduces the radiative transfer code RTCY. It is a 2D cylindrical radiative transfer code. With which we are able to simulate an array of geometric configurations and velocity fields. As such, we explore the effect that these velocities have on line formation through the use of plots similar to that of Figs 4 through 7 of (Carlsson & Stein 1997). The most interesting result is that of the expanding velocity field where the prominence moves radially away from the axis of rotational symmetry of the cylinder. Following this, we explore multithread simulations where we stack these cylinders behind each other. We see how this affects the observed line profiles and how adding random line-of-sight velocities produces interesting asymmetries. In addition to the latter, the spectral and spatial point spread function (PSF) of the IRIS spectrograph is convolved with our final profiles demonstrating what IRIS would see if presented with these profiles. Finally to close the chapter, we perform a manual multithread forward fit.
In Chap 5, we explore the formation of a coronal bright point (CBP) due to flux emergence and the associated filament channel of this emergence. Using Fourier Local Correlation tracking on HMI intensity images, we are able to recover the overall global velocity vectors of magnetic patches predict their global movement. The flows of these patches are seen to influence the stability of a minifilaments which form in the filament channel. These minifilaments are both seen to erupt when certain magnetic phenomena, such as flux cancellation occur.
Chap 6 offers our conclusions where we outline the main results found and present our plans for future work
Barbarian Europe
European culture has been greatly influenced by the Christian Church and Greek and Roman culture. However, the peoples of Europe’s remote past, whom the Greeks, Romans, and their medieval heirs called the «barbarians», also left their mark. Closely examining ancient and medieval narratives and the codifications of laws, this thoughtfully conducted comparative study sheds light on the illiterate societies of the early Germanic and Slavic peoples. The picture that emerges is one of communities built on kinship, neighborly, and tribal relations, where decision making, judgement, and punishment were carried out collectively, and the distinction between the sacred and profane was unknown
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