Spectral two-level preconditioners for sequences of linear systems

De nombreuses simulations numériques nécessitent la résolution d'une série de systèmes linéaires impliquant une même matrice mais des second-membres différents. Des méthodes efficaces pour ce type de problèmes cherchent à tirer bénéfice des résolutions précédentes pour accélérer les résolutions restantes. Deux grandes classes se distinguent dans la façon de procéder: la première vise à réutiliser une partie du sous-espace de Krylov, la deuxième à construire une mise à jour du préconditionneur à partir de vecteurs approximant un espace invariant. Dans cette thèse, nous nous sommes intéressés à cette dernière approche en cherchant à améliorer le préconditionneur d'origine. Dans une première partie, une seule mise à jour du préconditionneur est considérée pour tous les systèmes. Cette mise à jour consiste en une correction spectrale de rang faible qui permet de translater de un la position des plus petites valeurs propres en module de la matrice du système préconditionné de départ. Des expérimentations numériques sont réalisées en utilisant la méthode GMRES couplée à un préconditionneur de type inverse approchée. L'information spectrale est obtenue par un solveur de valeurs propres lors d'une phase préliminaire au calcul. Dans une deuxième partie, on autorise une possible mise à jour entre chaque système. Une correction spectrale incrémentale est proposée. Des expérimentations numériques sont réalisées en utilisant la méthode GMRES-DR, d'une part parce qu'elle est efficace en tant que solveur linéaire, et d'autre part parce qu'elle permet une bonne approximation des petites valeurs propres au cours de la résolution linéaire. Des stratégies sont développées afin de sélectionner l'information spectrale la plus pertinente. Ces approches ont été validées sur des problèmes de grande taille issus de simulations industrielles en électromagnétisme. Dans ce but, elles ont été implantées dans un code parallèle développé par EADS-CCR. ABSTRACT : Many numerical simulations in scientific and engineering applications require the solution of a set of large linear systems involving the same coefficient matrix but different right-hand sides. Efficient methods for tackling this problem attempt to benefit from the previously solved right-hand sides for the solution of the next ones. This goal can be achieved either by recycling Krylov subspaces or by building preconditioner updates based on near invariant subspace information. In this thesis, we focus our attention on this last approach that attempts to improve a selected preconditioner. In the first part, we consider only one update of the preconditioner for all the systems. This update consists of a spectral low-rank correction that shifts by one the smallest eigenvalues in magnitude of the matrix of the original preconditioned system. We perform experiments in the context of the GMRES method preconditioned by an approximate inverse preconditioner. The spectral information is computed by an eigensolver in a preprocessing phase. In the second part, we consider an update of the preconditioner between each system. An incremental spectral correction of the preconditioner is proposed. We perform experiments using the GMRES-DR method, thanks to its efficiency as a linear solver and its ability to recover reliable approximations of the desired eigenpairs at run time. Suitable strategies are investigated for selecting reliable eigenpairs. The efficiency of the proposed approaches is in particular assessed for the solution of large and challenging problems in electromagnetic applications. For this purpose, they have been implemented in a parallel industrial code developed by EADS-CCR

Effect of MTU length on child-adult difference in neuromuscular fatigue

Purpose The purpose of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor muscles (KE) at different muscle-tendon unit (MTU) lengths during repeated maximal voluntary isometric contractions (MVIC) between boys and men.Methods Twenty-two pre-pubertal boys (9-11 years) and 22 men (18-30 years) performed three KE fatigue protocols at short (SHORT), optimal (OPT) and long (LONG) MTU lengths, consisting of repeating 5-s MVIC interspersed with 5-s passive recovery periods until torque reached 60% of the initial MVIC torque. The etiology of neuromuscular fatigue was identified using non-invasive methods such as surface electromyography, near-infrared spectroscopy, magnetic nerve stimulation and twitch interpolation technique.Results The number of repetitions was significantly lower in men at OPT (14.8$\pm$3.2) and LONG (15.8$\pm$5.8) than boys (39.7$\pm$18.4 and 29.5 $\pm$10.2, respectively; p<0.001), while no difference was found at SHORT between both age groups (boys: 33.7$\pm$15.4, men: 40.9$\pm$14.2). At OPT and LONG boys showed a lower reduction in the single potentiated twitch (Qtwpot) and a greater decrease in the voluntary activation level (VA) than men. At SHORT, both populations displayed a moderate Qtwpot decrement and a significant VA reduction (p<0.001). The differences in maximal torque between boys and men were almost twice greater at OPT (223.9 N.m) than at SHORT (123.3 N.m) and LONG (136.5 N.m).Conclusion The differences in neuromuscular fatigue between children and adults are dependent on MTU length. Differences in maximal torque could underpin differences in neuromuscular fatigue between children and adults at OPT and SHORT. However, at LONG these differences do not seem to be explained by differences in maximal torque. The origins of this specific effect of MTU length remain to be determined

Clustering, host halos and environment of z∼\sim2 galaxies as a function of their physical properties

Using a sample of 25683 star-forming and 2821 passive galaxies at $z\sim2$, selected in the COSMOS field following the BzK color criterion, we study the hosting halo mass and environment of galaxies as a function of their physical properties. Spitzer and Herschel provide accurate SFR estimates for starburst galaxies. We measure the auto- and cross-correlation functions of various galaxy sub-samples and infer the properties of their hosting halos using both an HOD model and the linear bias at large scale. We find that passive and star-forming galaxies obey a similarly rising relation between the halo and stellar mass. The mean host halo mass of star forming galaxies increases with the star formation rate between 30 and 200 M$_\odot$.yr$^{-1}$, but flattens for higher values, except if we select only main-sequence galaxies. This reflects the expected transition from a regime of secular co-evolution of the halos and the galaxies to a regime of episodic starburst. We find similar large scale biases for main-sequence, passive, and starburst galaxies at equal stellar mass, suggesting that these populations live in halos of the same mass. We detect an excess of clustering on small scales for passive galaxies and showed, by measuring the large-scale bias of close pairs, that this excess is caused by a small fraction ($\sim16$) of passive galaxies being hosted by massive halos ($\sim 3 \times 10^{13}$ M$_\odot$) as satellites. Finally, extrapolating the growth of halos hosting the z$\sim$2 population, we show that M$_\star \sim 10^{10}$ M$_\odot$ galaxies at z$\sim$2 will evolve, on average, into massive (M$_\star \sim 10^{11}$ M$_\odot$), field galaxies in the local Universe and M$_\star \sim 10^{11}$ M$_\odot$ galaxies at z=2 into local, massive, group galaxies. The most massive main-sequence galaxies and close pairs of massive, passive galaxies end up in today's clusters.Comment: 18 pages, 16 figures, Accepted by A&

Appropriateness of indirect markers of muscle damage following lower limbs eccentric-biased exercises: A systematic review with meta-analysis

Purpose The aim of this review was to (1) characterize the time-course of markers of exercise-induced muscle damage (EIMD) based on the level of maximal voluntary contraction torque loss at 24-48h post-exercise (MVCloss24-48h), (2) identify factors (e.g., exercise and population characteristics) affecting the level of MVCloss24-48h, and (3) evaluate the appropriateness of EIMD markers as indicators of MVCloss24-48h. Methods Magnitude of change of each EIMD markers was normalized using the standardized mean differences method to compare the results from different studies. Time-course of EIMD markers were characterized according to three levels of MVCloss24-48h based on a clustering analysis of the 141 studies included. Association between MVCloss24-48h levels and participant´s characteristics or exercise type/modalities were assessed. Meta-regressions were performed to investigate the associations between MVCloss24-48h and EIMD markers changes at \u3c6h, 24h, 48h, 72h and \u3e 96h after exercise. Results Time-course of EIMD markers recovery differs between levels of MVCloss24-48h. Training status and exercise type/modality were associated with MVCloss24-48h level (p \u3c 0.05). MVCloss24-48h was correlated to changes in myoglobin concentration ( \u3c 6h), jump height (24h) and range of motion (48h) (p \u3c 0.001). Conclusion As the exercise could differently affect markers as function of the EIMD severity (i.e., MVCloss24-48h levels), different markers should be used as function of the timing of measurement. Mb concentration should be used during the first hours after the exercise ( \u3c 6h), whereas jump height (24h) and range of motion (48h) could be used as surrogate for maximal voluntary contraction later. Moreover, training status and exercise type/modality could influence the magnitude of MVCloss24-48

Spitzer Observations of Cold Dust Galaxies

We combine new Spitzer Space Telescope observations in the mid- and far-infrared with SCUBA 850 micron observations to improve the measurement of dust temperatures, masses and luminosities for 11 galaxies of the SCUBA Local Universe Galaxy Survey (SLUGS). By fitting dust models we measure typical dust masses of 10E7.9 M_sol and dust luminosities of ~ 10E10 L_sol, for galaxies with modest star formation rates. The data presented in this paper combined with previous observations show that cold dust is present in all types of spiral galaxies and is a major contributor to their total luminosity. Because of the lower dust temperature of the SCUBA sources measured in this paper, they have flatter Far-IR nu F_nu(160um)/nu F_nu(850um) slopes than the larger Spitzer Nearby Galaxies Survey (SINGS), the sample that provides the best measurements of the dust properties of galaxies in the nearby universe. The new data presented here added to SINGS extend the parameter space that is well covered by local galaxies, providing a comprehensive set of templates that can be used to interpret the observations of nearby and distant galaxies.Comment: Accepted by A.J. 16 pages, 10 figures, 7 tables. High resolution version at http://mips.as.arizona.edu/~cnaw/slugs_hires.pd

The Origin of the 24-micron Excess in Red Galaxies

Observations with the Spitzer Space Telescope have revealed a population of red-sequence galaxies with a significant excess in their 24-micron emission compared to what is expected from an old stellar population. We identify 900 red galaxies with 0.15<z<0.3 from the AGN and Galaxy Evolution Survey (AGES) selected from the NOAO Deep Wide-Field Survey Bootes field. Using Spitzer/MIPS, we classify 89 (~10%) with 24-micron infrared excess (f24>0.3 mJy). We determine the prevalence of AGN and star-formation activity in all the AGES galaxies using optical line diagnostics and mid-IR color-color criteria. Using the IRAC color-color diagram from the IRAC Shallow Survey, we find that 64% of the 24-micron excess red galaxies are likely to have strong PAH emission features in the 8-micron IRAC band. This fraction is significantly larger than the 5% of red galaxies with f24<0.3 mJy that are estimated to have strong PAH emission, suggesting that the infrared emission is largely due to star-formation processes. Only 15% of the 24-micron excess red galaxies have optical line diagnostics characteristic of star-formation (64% are classified as AGN and 21% are unclassifiable). The difference between the optical and infrared results suggest that both AGN and star-formation activity is occurring simultaneously in many of the 24-micron excess red galaxies. These results should serve as a warning to studies that exclusively use optical line diagnostics to determine the dominant emission mechanism in the infrared and other bands. We find that ~40% of the 24-micron excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24-micron emission comes from a combination of obscured AGN and star-formation activity.Comment: ApJ, accepted; 11 pages, 7 figures; corrected reference to IRAC Shallow Survey in abstrac

Neglecting the Importance of the Decision Making and Care Regimes of Personal Support Workers: A Critique of Standardization of Care Planning Through the RAI/MDS

Purpose: The Resident Assessment Instrument–Minimum Data Set (RAI/MDS) is an interdisciplinary standardized process that informs care plan development in nursing homes. This standardized process has failed to consistently result in individualized care planning, which may suggest problems with content and planning integrity. We examined the decision making and care practices of personal support workers (PSWs) in relation to the RAI/MDS standardized process. Design and Methods: This qualitative study utilized focus groups and semi-structured interviews with PSWs (n = 26) and supervisors (n = 9) in two nursing homes in central Canada. Results: PSWs evidenced unique occupational contributions to assessment via proximal familiarity and biographical information as well as to individualizing care by empathetically linking their own bodily experiences and forging bonds of fictive kinship with residents. These contributions were neither captured by RAI/MDS categories nor relayed to the interdisciplinary team. Causal factors for PSW exclusion included computerized records, low status, and poor interprofessional collaboration. Intraprofessional collaboration by PSWs aimed to compensate for exclusion and to individualize care. Implications: Exclusive institutional reliance on the RAI/MDS undermines quality care because it fails to capture residents’ preferences and excludes input by PSWs. Recommendations include incorporating PSW knowledge in care planning and documentation and examining PSWs’ nascent occupational identity and their role as interprofessional brokers in long-term care

Influence of history on saccade countermanding performance in humans and macaque monkeys

AbstractThe stop-signal or countermanding task probes the ability to control action by requiring subjects to withhold a planned movement in response to an infrequent stop signal which they do with variable success depending on the delay of the stop signal. We investigated whether performance of humans and macaque monkeys in a saccade countermanding task was influenced by stimulus and performance history. In spite of idiosyncrasies across subjects several trends were evident in both humans and monkeys. Response time decreased after successive trials with no stop signal. Response time increased after successive trials with a stop signal. However, post-error slowing was not observed. Increased response time was observed mainly or only after cancelled (signal inhibit) trials and not after noncancelled (signal respond) trials. These global trends were based on rapid adjustments of response time in response to momentary fluctuations in the fraction of stop signal trials. The effects of trial sequence on the probability of responding were weaker and more idiosyncratic across subjects when stop signal fraction was fixed. However, both response time and probability of responding were influenced strongly by variations in the fraction of stop signal trials. These results indicate that the race model of countermanding performance requires extension to account for these sequential dependencies and provide a basis for physiological studies of executive control of countermanding saccade performance

Children Exhibit a More Comparable Neuromuscular Fatigue Profile to Endurance Athletes Than Untrained Adults

The present study compared neuromuscular fatigue profiles between children, untrained adults and adult endurance athletes during repeated maximal muscle contractions. Eighteen prepubertal boys, 19 untrained men and 13 endurance male athletes performed 5-s maximal voluntary isometric knee extensor contractions (MVICs) interspersed with 5-s recovery until MVIC reached 60% of its initial value. Single and doublet magnetic stimulations were delivered to the femoral nerve to quantify the time course of potentiated twitch amplitude (Ttw,pot), high-frequency torque (T100Hz) and the low-to-high frequency torque ratio (T10Hz/T100Hz), i.e., indicators of peripheral fatigue. M-wave-normalized EMG amplitudes (EMG/M) and the maximal voluntary activation level (VA) were calculated to quantify central fatigue. Adults (15.9 ± 3.9 repetitions) performed fewer MVICs than children (40.4 ± 19.7) and endurance athletes (51.7 ± 19.6), however, no difference was observed between children and athletes (P = 0.13). Ttw,pot (∼52%, P &lt; 0.001), T100Hz (∼39%, P &lt; 0.001) and T10Hz/T100Hz (∼23%, P &lt; 0.001) decreased only in adults. Similar decrements in vastus medialis and vastus lateralis EMG/M were observed in children and endurance athletes (range: 40–50%), and these were greater than in adults (∼15%). Whilst VA decreased more in children (-38.4 ± 22.5%, P &lt; 0.001) than endurance athletes (-20.3 ± 10.1%, P &lt; 0.001), it did not change in adults. Thus, children fatigued more slowly than adults and as much as endurance athletes. They developed less peripheral and more central fatigue than adults and, although central fatigue appeared somewhat higher in children than endurance athletes, both children and endurance athletes experienced greater decrements than adults. Therefore, children exhibit a more comparable neuromuscular fatigue profile to endurance athletes than adults