8,243 research outputs found
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Fourth Order Dispersion in Nonlinear Media
In recent years, there has been an explosion of interest in media bearing quarticdispersion. After the experimental realization of so-called pure-quartic solitons, asignificant number of studies followed both for bright and for dark solitonic struc-tures exploring the properties of not only quartic, but also setic, octic, decic etc.dispersion, but also examining the competition between, e.g., quadratic and quarticdispersion among others.In the first chapter of this Thesis, we consider the interaction of solitary waves ina model involving the well-known φ4 Klein-Gordon theory, bearing both Laplacian and biharmonic terms with different prefactors. As a result of the competition ofthe respective linear operators, we obtain three distinct cases as we vary the modelparameters. In the first the biharmonic effect dominates, yielding an oscillatoryinter-wave interaction; in the third the harmonic effect prevails yielding exponen-tial interactions, while we find an intriguing linearly modulated exponential effectin the critical second case, separating the above two regimes. For each case, wecalculate the force between the kink and antikink when initially separated with suf-ficient distance. Being able to write the acceleration as a function of the separationdistance, and its corresponding ordinary differential equation, we test the corre-sponding predictions, finding very good agreement, where appropriate, with thecorresponding partial differential equation results. Where the two findings differ,we explain the source of disparities. Finally, we offer a first glimpse of the interplayof harmonic and biharmonic effects on the results of kink-antikink collisions andthe corresponding single- and multi-bounce windows.In the next two Chapters, we explore the competition of quadratic and quar-tic dispersion in producing kink-like solitary waves in a model of the nonlinearSchroedinger type bearing cubic nonlinearity. We present 6 families of multikink so-lutions and explore their bifurcations as a prototypical parameter is varied, namelythe strength of the quadratic dispersion. We reveal a rich bifurcation structure forthe system, connecting two-kink states with ones involving 4-, as well as 6-kinks.The stability of all of these states is explored. For each family, we discuss a “lowerbranch” adhering to the energy landscape of the 2-kink states (also discussed inthe previous Chapter). We also, however, study in detail the “upper branches”bearing higher numbers of kinks. In addition to computing the stationary statesand analyzing their stability at the PDE level, we develop an effective particle the-ory that is shown to be surprisingly efficient in capturing the kink equilibria and normal (as well as unstable) modes. Finally, the results of the bifurcation analysisare corroborated with direct numerical simulations involving the excitation of thestates in a targeted way in order to explore their instability-induced dynamics.While the previous two studies were focused on the one-dimensional problem,in the fourth and final chapter, we explore a two-dimensional realm. More specif-ically, we provide a characterization of the ground states of a higher-dimensionalquadratic-quartic model of the nonlinear Schr ̈odinger class with a combination of afocusing biharmonic operator with either an isotropic or an anisotropic defocusingLaplacian operator (at the linear level) and power-law nonlinearity. Examiningprincipally the prototypical example of dimension d = 2, we find that instabilityarises beyond a certain threshold coefficient of the Laplacian between the cubic andquintic cases, while all solutions are stable for powers below the cubic. Above thequintic, and up to a critical nonlinearity exponent p, there exists a progressivelynarrowing range of stable frequencies. Finally, above the critical p all solutionsare unstable. The picture is rather similar in the anisotropic case, with the dif-ference that even before the cubic case, the numerical computations suggest aninterval of unstable frequencies. Our analysis generalizes the relevant observationsfor arbitrary combinations of Laplacian prefactor b and nonlinearity power p.We conclude the thesis with a summary of its main findings, as well as with anoutlook towards interesting future problem
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Ensuring Access to Safe and Nutritious Food for All Through the Transformation of Food Systems
Grasping nothing: a study of minimal ontologies and the sense of music
If music were to have a proper sense – one in which it is truly given – one might reasonably place this in sound and aurality. I contend, however, that no such sense exists; rather, the sense of music takes place, and it does so with the impossible. To this end, this thesis – which is a work of philosophy and music – advances an ontology of the impossible (i.e., it thinks the being of what, properly speaking, can have no being) and considers its implications for music, articulating how ontological aporias – of the event, of thinking the absolute, and of sovereignty’s dismemberment – imply senses of music that are anterior to sound. John Cage’s Silent Prayer, a nonwork he never composed, compels a rerethinking of silence on the basis of its contradictory status of existence; Florian Hecker et al.’s Speculative Solution offers a basis for thinking absolute music anew to the precise extent that it is a discourse of meaninglessness; and Manfred Werder’s [yearn] pieces exhibit exemplarily that music’s sense depends on the possibility of its counterfeiting. Inso-much as these accounts produce musical senses that take the place of sound, they are also understood to be performances of these pieces. Here, then, thought is music’s organon and its instrument
Multi-Attribute Utility Preference Robust Optimization: A Continuous Piecewise Linear Approximation Approach
In this paper, we consider a multi-attribute decision making problem where
the decision maker's (DM's) objective is to maximize the expected utility of
outcomes but the true utility function which captures the DM's risk preference
is ambiguous. We propose a maximin multi-attribute utility preference robust
optimization (UPRO) model where the optimal decision is based on the worst-case
utility function in an ambiguity set of plausible utility functions constructed
using partially available information such as the DM's specific preferences
between some lotteries. Specifically, we consider a UPRO model with two
attributes, where the DM's risk attitude is multivariate risk-averse and the
ambiguity set is defined by a linear system of inequalities represented by the
Lebesgue-Stieltjes (LS) integrals of the DM's utility functions. To solve the
maximin problem, we propose an explicit piecewise linear approximation (EPLA)
scheme to approximate the DM's true unknown utility so that the inner
minimization problem reduces to a linear program, and we solve the approximate
maximin problem by a derivative-free (Dfree) method. Moreover, by introducing
binary variables to locate the position of the reward function in a family of
simplices, we propose an implicit piecewise linear approximation (IPLA)
representation of the approximate UPRO and solve it using the Dfree method.
Such IPLA technique prompts us to reformulate the approximate UPRO as a single
mixed-integer program (MIP) and extend the tractability of the approximate UPRO
to the multi-attribute case. Furthermore, we extend the model to the expected
utility maximization problem with expected utility constraints where the
worst-case utility functions in the objective and constraints are considered
simultaneously. Finally, we report the numerical results about performances of
the proposed models.Comment: 50 pages,18 figure
On the Capacity of Communication Channels with Memory and Sampled Additive Cyclostationary Gaussian Noise: Full Version with Detailed Proofs
In this work we study the capacity of interference-limited channels with
memory. These channels model non-orthogonal communications scenarios, such as
the non-orthogonal multiple access (NOMA) scenario and underlay cognitive
communications, in which the interference from other communications signals is
much stronger than the thermal noise. Interference-limited communications is
expected to become a very common scenario in future wireless communications
systems, such as 5G, WiFi6, and beyond. As communications signals are
inherently cyclostationary in continuous time (CT), then after sampling at the
receiver, the discrete-time (DT) received signal model contains the sampled
desired information signal with additive sampled CT cyclostationary noise. The
sampled noise can be modeled as either a DT cyclostationary process or a DT
almost-cyclostationary process, where in the latter case the resulting channel
is not information-stable. In a previous work we characterized the capacity of
this model for the case in which the DT noise is memoryless. In the current
work we come closer to practical scenarios by modelling the resulting DT noise
as a finite-memory random process. The presence of memory requires the
development of a new set of tools for analyzing the capacity of channels with
additive non-stationary noise which has memory. Our results show, for the first
time, the relationship between memory, sampling frequency synchronization and
capacity, for interference-limited communications. The insights from our work
provide a link between the analog and the digital time domains, which has been
missing in most previous works on capacity analysis. Thus, our results can help
improving spectral efficiency and suggest optimal transceiver designs for
future communications paradigms.Comment: accepted to the IEEE Transactions on Information Theor
Binaural virtual auditory display for music discovery and recommendation
Emerging patterns in audio consumption present renewed opportunity for searching or navigating music via spatial audio interfaces. This thesis examines the potential benefits and considerations for using binaural audio as the sole or principal output interface in a music browsing system. Three areas of enquiry are addressed. Specific advantages and constraints in spatial display of music tracks are explored in preliminary work. A voice-led binaural music discovery prototype is shown to offer a contrasting interactive experience compared to a mono smartspeaker. Results suggest that touch or gestural interaction may be more conducive input modes in the former case. The limit of three binaurally spatialised streams is identified from separate data as a usability threshold for simultaneous presentation of tracks, with no evident advantages derived from visual prompts to aid source discrimination or localisation. The challenge of implementing personalised binaural rendering for end-users of a mobile system is addressed in detail. A custom framework for assessing head-related transfer function (HRTF) selection is applied to data from an approach using 2D rendering on a personal computer. That HRTF selection method is developed to encompass 3D rendering on a mobile device. Evaluation against the same criteria shows encouraging results in reliability, validity, usability and efficiency. Computational analysis of a novel approach for low-cost, real-time, head-tracked binaural rendering demonstrates measurable advantages compared to first order virtual Ambisonics. Further perceptual evaluation establishes working parameters for interactive auditory display use cases. In summation, the renderer and identified tolerances are deployed with a method for synthesised, parametric 3D reverberation (developed through related research) in a final prototype for mobile immersive playlist editing. Task-oriented comparison with a graphical interface reveals high levels of usability and engagement, plus some evidence of enhanced flow state when using the eyes-free binaural system
Donor-anion interactions in quarter-filled low-dimensional organic conductors
Anions have often been considered to act essentially as electron donors or acceptors in molecular conductors. However there is now growing evidence that they play an essential role in directing the structural and hence electronic properties of many of these systems. After reviewing the basic interactions and different ground states occurring in molecular conductors we consider in detail how anions influence the structure of donor stacks and often guide them toward different types of transitions. Consideration of the Bechgaard and Fabre salts illustrates how anions play a crucial role in directing these salts through complex phase diagrams where different conducting and localized states are in competition. We also emphasize the important role of hydrogen bonding and conformational flexibility of donors related to BEDT-TTF and we discuss how anions have frequently a strong control of the electronic landscape of these materials. Charge ordering, metal to metal and metal to insulator transitions occurring in these salts are considered
CITIES: Energetic Efficiency, Sustainability; Infrastructures, Energy and the Environment; Mobility and IoT; Governance and Citizenship
This book collects important contributions on smart cities. This book was created in collaboration with the ICSC-CITIES2020, held in San José (Costa Rica) in 2020. This book collects articles on: energetic efficiency and sustainability; infrastructures, energy and the environment; mobility and IoT; governance and citizenship
Time- and value-continuous explainable affect estimation in-the-wild
Today, the relevance of Affective Computing, i.e., of making computers recognise and simulate human emotions, cannot be overstated. All technology giants (from manufacturers of laptops to mobile phones to smart speakers) are in a fierce competition to make their devices understand not only what is being said, but also how it is being said to recognise user’s emotions. The goals have evolved from predicting the basic emotions (e.g., happy, sad) to now the more nuanced affective states (e.g., relaxed, bored) real-time. The databases used in such research too have evolved, from earlier featuring the acted behaviours to now spontaneous behaviours. There is a more powerful shift lately, called in-the-wild affect recognition, i.e., taking the research out of the laboratory, into the uncontrolled real-world.
This thesis discusses, for the very first time, affect recognition for two unique in-the-wild audiovisual databases, GRAS2 and SEWA. The GRAS2 is the only database till date with time- and value-continuous affect annotations for Labov effect-free affective behaviours, i.e., without the participant’s awareness of being recorded (which otherwise is known to affect the naturalness of one’s affective behaviour). The SEWA features participants from six different cultural backgrounds, conversing using a video-calling platform. Thus, SEWA features in-the-wild recordings further corrupted by unpredictable artifacts, such as the network-induced delays, frame-freezing and echoes. The two databases present a unique opportunity to study time- and value-continuous affect estimation that is truly in-the-wild.
A novel ‘Evaluator Weighted Estimation’ formulation is proposed to generate a gold standard sequence from several annotations. An illustration is presented demonstrating that the moving bag-of-words (BoW) representation better preserves the temporal context of the features, yet remaining more robust against the outliers compared to other statistical summaries, e.g., moving average. A novel, data-independent randomised codebook is proposed for the BoW representation; especially useful for cross-corpus model generalisation testing when the feature-spaces of the databases differ drastically. Various deep learning models and support vector regressors are used to predict affect dimensions time- and value-continuously. Better generalisability of the models trained on GRAS2 , despite the smaller training size, makes a strong case for the collection and use of Labov effect-free data.
A further foundational contribution is the discovery of the missing many-to-many mapping between the mean square error (MSE) and the concordance correlation coefficient (CCC), i.e., between two of the most popular utility functions till date. The newly invented cost function |MSE_{XY}/σ_{XY}| has been evaluated in the experiments aimed at demystifying the inner workings of a well-performing, simple, low-cost neural network effectively utilising the BoW text features. Also proposed herein is the shallowest-possible convolutional neural network (CNN) that uses the facial action unit (FAU) features. The CNN exploits sequential context, but unlike RNNs, also inherently allows data- and process-parallelism. Interestingly, for the most part, these white-box AI models have shown to utilise the provided features consistent with the human perception of emotion expression
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