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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Construction of Ballistic Lunar Transfers in the Earth-Moon-Sun System

Stephen Scheuerle Jr. (10676634) 07 May 2021 (has links)
<p>An increasing interest in lunar exploration calls for low-cost techniques of reaching the Moon. Ballistic lunar transfers are long duration trajectories that leverage solar perturbations to reduce the multi-body energy of a spacecraft upon arrival into cislunar space. An investigation is conducted to explore methods of constructing ballistic lunar transfers. The techniques employ dynamical systems theory to leverage the underlying dynamical flow of the multi-body regime. Ballistic lunar transfers are governed by the gravitational influence of the Earth-Moon-Sun system; thus, multi-body gravity models are employed, i.e., the circular restricted three-body problem (CR3BP) and the bicircular restricted four-body problem (BCR4BP). The Sun-Earth CR3BP provides insight into the Sun’s effect on transfers near the Earth. The BCR4BP offers a coherent model for constructing end-to-end ballistic lunar transfers. Multiple techniques are employed to uncover ballistic transfers to conic and multi-body orbits in cislunar space. Initial conditions to deliver the spacecraft into various orbits emerge from Periapse Poincaré maps. From a chosen geometry, families of transfers from the Earth to conic orbits about the Moon are developed. Instantaneous equilibrium solutions in the BCR4BP provide an approximate for the theoretical minimum lunar orbit insertion costs, and are leveraged to create low-cost solutions. Trajectories to the <i>L</i>2 2:1 synodic resonant Lyapunov orbit, <i>L</i>2 2:1 synodic resonant Halo orbit, and the 3:1 synodic resonant Distant Retrograde Orbit (DRO) are investigated.</p>
42

DESIGN OF LUNAR TRANSFER TRAJECTORIES FOR SECONDARY PAYLOAD MISSIONS

Alexander Estes Hoffman (15354589) 27 April 2023 (has links)
<p>Secondary payloads have a rich and successful history of utilizing cheap rides to orbit to perform outstanding missions in Earth orbit, and more recently, in cislunar space and beyond. New launch vehicles, namely the Space Launch System (SLS), are increasing the science opportunity for rideshare class missions by providing regular service to the lunar vicinity. However, trajectory design in a multi-body regime brings a host of novel challenges, further exacerbated by constraints generated from the primary payload’s mission. Often, secondary payloads do not possess the fuel required to directly insert into lunar orbit and must instead perform a lunar flyby, traverse the Earth-Moon-Sun system, and later return to the lunar vicinity. This investigation develops a novel framework to construct low-cost, end-to-end lunar transfer trajectories for secondary payload missions. The proposed threephase approach provides unique insights into potential lunar transfer geometries. The phases consist of an arc from launch to initial perilune, an exterior transfer arc, and a lunar approach arc. The space of feasible transfers within each phase is determined through low-dimension grid searches and informed filtering techniques, while the problem of recombining the phases through differential corrections is kept tractable by reducing the dimensionality at each phase transition boundary. A sample mission demonstrates the trajectory design approach and example solutions are generated and discussed. Finally, alternate strategies are developed to both augment the analysis and for scenarios where the proposed three-phase technique does not deliver adequate solutions. The trajectory design methods described in this document are applicable to many upcoming secondary payload missions headed to lunar orbit, including spacecraft with only low-thrust, only high-thrust, or a combination of both. </p>
43

The effects of intermittent fasting on female athletes: A systematic review

Scarbrough, Courtnie 08 December 2023 (has links) (PDF)
The intermittent fasting diet has a historical past, but recently, it has regained popularity as a weight loss strategy for the general public. Athletes have also picked up this specialized diet in search of a competitive edge. There are ample studies testing athletes of all levels, but most do not include females or report sex-specific results. This systematic review aimed to identify current research on the effects of intermittent fasting in the understudied population of female athletes. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a comprehensive search was conducted in three databases, including PubMed, Cochrane, and SPORTDiscus, and a total of six studies with experimental or observational designs testing the various effects of intermittent fasting on physical, physiological, and psychological parameters were included. This systematic review shows that studies found significant changes in calorie and fluid intake, body composition, nutrient utilization, well-being, and athletic performances with no changes in muscle strength, ratings of perceived exertion, blood lactate levels, and cognitive function. Multiple studies in this review found mixed results on changes in heart rate and athletic performance (vertical jump height), and no studies found negative effects on athletic performance except in the survey of athletes’ perceptions. Due to conflicting results of current research, more studies are needed to determine the effects of intermittent fasting combined with continuous exercise on athletic women.
44

Probabilistic models for melodic sequences

Spiliopoulou, Athina January 2013 (has links)
Structure is one of the fundamentals of music, yet the complexity arising from the vast number of possible variations of musical elements such as rhythm, melody, harmony, key, texture and form, along with their combinations, makes music modelling a particularly challenging task for machine learning. The research presented in this thesis focuses on the problem of learning a generative model for melody directly from musical sequences belonging to the same genre. Our goal is to develop probabilistic models that can automatically capture the complex statistical dependencies evident in music without the need to incorporate significant domain-specifc knowledge. At all stages we avoid making assumptions explicit to music and consider models that can can be readily applied in different music genres and can easily be adapted for other sequential data domains. We develop the Dirichlet Variable-Length Markov Model (Dirichlet-VMM), a Bayesian formulation of the Variable-Length Markov Model (VMM), where smoothing is performed in a systematic probabilistic manner. The model is a general-purpose, dictionary-based predictor with a formal smoothing technique and is shown to perform significantly better than the standard VMM in melody modelling. Motivated by the ability of the Restricted Boltzmann Machine (RBM) to extract high quality latent features in an unsupervised manner, we next develop the Time-Convolutional Restricted Boltzmann Machine (TC-RBM), a novel adaptation of the Convolutional RBM for modelling sequential data. We show that the TC-RBM learns descriptive musical features such as chords, octaves and typical melody movement patterns. To deal with the non-stationarity of music, we develop the Variable-gram Topic model, which employs the Dirichlet-VMM for the parametrisation of the topic distributions. The Dirichlet-VMM models the local temporal structure, while the latent topics represent di erent music regimes. The model does not make any assumptions explicit to music, but it is particularly suitable in this context, as it couples the latent topic formalism with an expressive model of contextual information.
45

Using Explicit Social Skills Instruction Combined with a Restricted Interest Group to Increase the Frequency of Social Skills in Students with Autism

Cummings, Krista Shea 01 April 2017 (has links)
The literature suggests that adolescents with Autism Spectrum Disorder (ASD) are characterized by restricted interests and deficits in social skills (White, Keonig, & Scahill, 2007). Due to the deficits associated with ASD, adolescents with ASD do not typically engage in social interactions at a rate similar to their same age typicallydeveloping peers (Wagner, Cadwallader, Garza, & Cameto, 2004). This study assessed whether explicit social skills instruction in a restricted interest group in an afterschool club setting increased the frequency of specific social skills in two students identified with ASD. A multiple baseline experimental design was implemented across behaviors. During baseline measures, the participants showed minimal social interactions. During intervention, results showed increases in the social skills being measured. These results have implications for understanding factors related to developing social skills in adolescents with ASD.
46

Dietary Treatment of Hyperactive Children

Rogers, Gary S. 08 1900 (has links)
This study investigated whether a salicylate-restricted diet (eliminating foods containing artificial additives and natural salicylates) could effectively reduce hyperactivity in children more so than a diet not restricting salicylates (ostensibly restricting foods containing refined sugar). Ten hyperactive children, nine boys and one girl, were matched on their pre-treatment activity rates and assigned to either a salicylate-restricted diet (Group I) or a diet not restricting salicylates (Group II). After approximately nine weeks, post-treatment activity rates were obtained, and a significant difference in favor of the salicylate-restricted diet group was found with this diet group exhibiting a significantly lower mean post-treatment activity rate in comparison to the group placed on a diet not restricting salicylates (p<.05). Implications for diagnosis and treatment of hyperactivity in children were discussed.
47

Choosing the Cut Point for a Restricted Mean in Survival Analysis, a Data Driven Method

Sheldon, Emily H 25 April 2013 (has links)
Survival Analysis generally uses the median survival time as a common summary statistic. While the median possesses the desirable characteristic of being unbiased, there are times when it is not the best statistic to describe the data at hand. Royston and Parmar (2011) provide an argument that the restricted mean survival time should be the summary statistic used when the proportional hazards assumption is in doubt. Work in Restricted Means dates back to 1949 when J.O. Irwin developed a calculation for the standard error of the restricted mean using Greenwood’s formula. Since then the development of the restricted mean has been thorough in the literature, but its use in practical analyses is still limited. One area that is not well developed in the literature is the choice of the time point to which the mean is restricted. The aim of this dissertation is to develop a data driven method that allows the user to find a cut-point to use to restrict the mean. Three methods are developed. The first is a simple method that locates the time at which the maximum distance between two curves exists. The second is a method adapted from a Renyi-type test, typically used when proportional hazards assumptions are not met, where the Renyi statistics are plotted and piecewise regression model is fit. The join point of the two pieces is where the meant will be restricted. Third is a method that applies a nonlinear model fit to the hazard estimates at each event time, the model allows for the hazards between the two groups to be different up until a certain time, after which the groups hazards are the same. The time point where the two groups’ hazards become the same is the time to which the mean is restricted. The methods are evaluated using MSE and bias calculations, and bootstrap techniques to estimate the variance.
48

Einstein's Equations in Vacuum Spacetimes with Two Spacelinke Killing Vectors Using Affine Projection Tensor Geometry

Lawrence, Miles D. 01 January 1994 (has links)
Einstein's equations in vacuum spacetimes with two spacelike killing vectors are explored using affine projection tensor geometry. By doing a semi-conformal transformation on the metric, a new "fiducial" geometry is constructed using a projection tensor fields. This fiducial geometry provides coordinate independent information about the underlying structure of the spacetime without the use of an explicit form of the metric tensor.
49

Invertibilité restreinte, distance au cube et covariance de matrices aléatoires / Restricted invertibilité, distance to the cube and the covariance of random matrices

Youssef, Pierre 21 May 2013 (has links)
Dans cette thèse, on aborde trois thèmes : problème de sélection de colonnes dans une matrice, distance de Banach-Mazur au cube et estimation de la covariance de matrices aléatoires. Bien que les trois thèmes paraissent éloignés, les techniques utilisées se ressemblent tout au long de la thèse. Dans un premier lieu, nous généralisons le principe d'invertibilité restreinte de Bourgain-Tzafriri. Ce résultat permet d'extraire un "grand" bloc de colonnes linéairement indépendantes dans une matrice et d'estimer la plus petite valeur singulière de la matrice extraite. Nous proposons ensuite un algorithme déterministe pour extraire d'une matrice un bloc presque isométrique c’est à dire une sous-matrice dont les valeurs singulières sont proches de 1. Ce résultat nous permet de retrouver le meilleur résultat connu sur la célèbre conjecture de Kadison-Singer. Des applications à la théorie locale des espaces de Banach ainsi qu'à l'analyse harmonique sont déduites. Nous donnons une estimation de la distance de Banach-Mazur d'un corps convexe de Rn au cube de dimension n. Nous proposons une démarche plus élémentaire, basée sur le principe d'invertibilité restreinte, pour améliorer et simplifier les résultats précédents concernant ce problème. Plusieurs travaux ont été consacrés pour approcher la matrice de covariance d'un vecteur aléatoire par la matrice de covariance empirique. Nous étendons ce problème à un cadre matriciel et on répond à la question. Notre résultat peut être interprété comme une quantification de la loi des grands nombres pour des matrices aléatoires symétriques semi-définies positives. L'estimation obtenue s'applique à une large classe de matrices aléatoires / In this thesis, we address three themes : columns subset selection in a matrix, the Banach-Mazur distance to the cube and the estimation of the covariance of random matrices. Although the three themes seem distant, the techniques used are similar throughout the thesis. In the first place, we generalize the restricted invertibility principle of Bougain-Tzafriri. This result allows us to extract a "large" block of linearly independent columns inside a matrix and estimate the smallest singular value of the restricted matrix. We also propose a deterministic algorithm in order to extract an almost isometric block inside a matrix i.e a submatrix whose singular values are close to 1. This result allows us to recover the best known result on the Kadison-Singer conjecture. Applications to the local theory of Banach spaces as well as to harmonic analysis are deduced. We give an estimate of the Banach-Mazur distance between a symmetric convex body in Rn and the cube of dimension n. We propose an elementary approach, based on the restricted invertibility principle, in order to improve and simplify the previous results dealing with this problem. Several studies have been devoted to approximate the covariance matrix of a random vector by its sample covariance matrix. We extend this problem to a matrix setting and we answer the question. Our result can be interpreted as a quantified law of large numbers for positive semidefinite random matrices. The estimate we obtain, applies to a large class of random matrices
50

Průběh užšího zadávacího řízení na veřejné zakázky / The course of restricted bidding procedure in public procurement

Čechová, Štěpánka January 2017 (has links)
The topic of this diploma thesis is The course of restricted bidding procedure in public procurement. It is one of nine types of award procedures which are allowed by act on public procurement. The thesis is compiled with respect to short effectiveness of new Act. Nr. 134/2016 Coll., on public procurement, its main legislative source, which is however on many places compared to foregoing Act. Nr. 137/2006 Sb., on public procurement, as amended. In the first section of the thesis there is short excursion into history of public procurement. Second part is dealing with legislation regulating public procurement, namely European legislation, local laws and decrees. Third section is defining major terms relating to public procurement. In fourth section the discussed issues are being narrowed down to above-the-threshold public contracts, by which the usage of the restricted procedure is not limited by any conditions as other types of award procedures usually are. All other types of award procedures which can the contracting entity chose for above-the-threshold public contracts are shortly discussed in this section. Fifth section is discussing initiation of award procedure, submission of requests to participate, tenders, their submission and opening. Sixth part is dedicated to assessment of tenders and...

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