820 research outputs found
Model bicategories and their homotopy bicategories
We give the definitions of model bicategory and -homotopy, which are
natural generalizations of the notions of model category and homotopy to the
context of bicategories. For any model bicategory , denote by
the full sub-bicategory of the fibrant-cofibrant objects. We
prove that the 2-dimensional localization of at the weak
equivalences can be computed as a bicategory \mathcal{H}\mbox{o}(\mathcal{C})
whose objects and arrows are those of and whose 2-cells are
classes of -homotopies up to an equivalence relation. The pseudofunctor
\mathcal{C} \stackrel{r}{\longrightarrow} \mathcal{H}\mbox{o}(\mathcal{C})
which yields the localization is constructed by using a notion of
fibrant-cofibrant replacement in this context. When considered for a model
category, the results in this article give in particular a bicategory whose
reflection into categories is the usual homotopy category constructed by
Quillen.Comment: 37 pages, many elevator calculus diagrams. This is a "preliminary
version
Surface Studies of Calcium Oxalate Dihydrate Single Crystals During Dissolution in the Presence of Urine
Single crystals of Calcium Oxalate Dihydrate (COD) were grown from solution under controlled release of the reacting ions. Dissolution of COD was studied at different pH levels and in different dilutions of urine. The descriptors of the contour were determined during dissolution of COD using a quantitative morphological technique. The shape parameters and surface ruggedness were determined from Fourier and fractal analysis. The results obtained give quantitative information on the dissolution kinetics and the surface geometry of COD crystals in normal and diluted urine. Dissolution inhibition and morphological changes of COD crystals during dissolution were attributed to selective adsorption of urine non-ionic macromolecules on the crystal stepped surface. Surface etching of COD was found to depend on urine dilution and time of incubation. The data obtained suggest that the geometric structure of the surface is likely to be a potential factor in understanding crystal aggregation in stone formation
Experimental determination of microwave attenuation and electrical permittivity of double-walled carbon nanotubes
The attenuation and the electrical permittivity of the double-walled carbon nanotubes (DWCNTs) were determined in the frequency range of 1â65 GHz. A micromachined coplanar waveguide transmission line supported on a Si membrane with a thickness of 1.4 ”m was filled with a mixture of DWCNTs. The propagation constants were then determined from the S parameter measurements. The DWCNTs mixture behaves like a dielectric in the range of 1â65 GHz with moderate losses and an abrupt change of the effective permittivity that is very useful for gas sensor detection. ©2006 American Institute of Physic
Exploring levels of student-athlete burnout at two Canadian universities
The purpose of the current study was to examine the levels of burnout among student-athletes at two Canadian universities and to investigate whether there were significant differences related to gender, sport, year of university sport participation, academic year, and academic program. Burnout was measured by administering Raedeke and Smithâs (2001) Athlete Burnout Questionnaire to 145 student-athletes from various sports. Overall, 1.4% of the student-athletes had elevated burnout scores on all three subscales, and 17% scored high on two of the three subscales. No significant differences emerged with regards to year of varsity sport participation, academic year, or program. However, women had significantly higher levels of emotional and physical exhaustion than men. Significant differences also emerged in relation to levels of emotional and physical exhaustion, and of sport devaluation, when comparing various sports.
Le but de cette Ă©tude Ă©tait dâĂ©valuer le niveau dâĂ©puisement dâĂ©tudiants athlĂštes frĂ©quentant deux universitĂ©s canadiennes et de dĂ©terminer sâil y avait des diffĂ©rences significatives liĂ©es au sexe, au sport, au nombre dâannĂ©es de participation au sport universitaire, ainsi quâĂ lâannĂ©e universitaire et au programme dâĂ©tude. LâĂ©puisement a Ă©tĂ© mesurĂ© en distribuant le « Athlete Burnout Questionnaire » (Raedeke & Smith, 2001) Ă 145 Ă©tudiants athlĂštes universitaires de diffĂ©rentes disciplines sportives. En somme, 1,4 % des participants ont affichĂ© des scores Ă©levĂ©s sur chacune des trois sous-Ă©chelles et 17 % ont fait de mĂȘme sur deux dâentre elles. Aucune diffĂ©rence marquĂ©e nâa Ă©tĂ© identifiĂ©e quant au nombre dâannĂ©es de participation au sport universitaire, ni Ă lâannĂ©e universitaire ou au type de programme dâĂ©tude. Cependant, les femmes ont affichĂ© un niveau dâĂ©puisement Ă©motif et physique significativement plus Ă©levĂ© que celui des hommes. En outre, on a notĂ© des diffĂ©rences marquĂ©es en comparant les sports entre eux en ce qui a trait au niveau dâĂ©puisement Ă©motif et physique et au niveau de dĂ©valorisation des disciplines
A localization of bicategories via homotopies
Given a bicategory C and a family W of arrows of C, we give conditions on the pair (C, W) that allow us to construct the bicategorical localization with respect to W by dealing only with the 2-cells, that is without adding objects or arrows to C. We show that in this case, the 2-cells of the localization can be given by the homotopies with respect to W, a notion defined in this article which is closely related to Quillenâs notion of homotopy for model categories but depends only on a single family of arrows. This localization result has a natural application to the construction of the homotopy bicategory of a model bicategory, which we develop elsewhere, as the pair (Cf c, W) given by the weak equivalences between fibrant-cofibrant objects satisfies the conditions given in the present article.Fil: Descotte, MarĂa Emilia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de Investigaciones MatemĂĄticas "Luis A. SantalĂł". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones MatemĂĄticas "Luis A. SantalĂł"; ArgentinaFil: Dubuc, Eduardo Julio. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de Investigaciones MatemĂĄticas "Luis A. SantalĂł". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones MatemĂĄticas "Luis A. SantalĂł"; ArgentinaFil: Szyld, M.. Dalhousie University Halifax; Canad
Evaluation of the Surface Roughness of Cystine Stones Using a Visible Laser Diode Scattering Approach
To understand the processes of fragmentation and the chemical reactivity of solids, proper characterization of surface topography is crucial. This paper describes a non-destructive technique of quantifying the surface roughness of cystine renal stones, using visible laser diode scattering and fractal geometry. Fragments of cystine stones were mounted on microscope slides and coated by a carbon-sputtering apparatus. The slides were placed under a dynamic active-vision system, using a visible laser diode to measure three-dimensional surface coordinates. The data obtained were analyzed by fractal geometry. Surface fractal dimensions were determined by the variation method. The results showed that the surface of a compact-size sample can be evaluated quantitatively. The technique is valuable for the accurate presentation of surfaces in three dimensions
Modulations of ongoing alpha oscillations predict successful short-term visual memory encoding
Alpha-frequency band oscillations have been shown to be one of the most prominent aspects of neuronal ongoing oscillatory activity, as reflected by electroencephalography (EEG) recordings. First thought to reflect an idling state, a recent framework indicates that alpha power reflects cortical inhibition. In the present study, the role of oscillations in the upper alpha-band (12 Hz) was investigated during a change-detection test of short-term visual memory. If alpha oscillations arise from a purely inhibitory process, higher alpha power before sample stimulus presentation would be expected to correlate with poorer performance. Instead, participants with faster reaction-times showed stronger alpha power before the sample stimulus in frontal and posterior regions. Additionally, faster participants showed stronger alpha desynchronization after the stimulus in a group of right frontal and left posterior electrodes. The same pattern of electrodes showed stronger alpha with higher working-memory load, so that when more items were processed, alpha power desynchronized faster after the stimulus. During memory maintenance, alpha power was greater when more items were held in memory, likely due to a faster resynchronization. These data are consistent with the hypothesis that the level of suppression of alpha power by stimulus presentation is an important factor for successfully encoding visual stimuli. The data are also consistent with a role for alpha as actively participating in attentional processes
Solution of the Euler Unsteady Equations Using Deforming Grids. G.U. Aero Report 9704.
No abstract available
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