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Search Results: 1 - 10 of 32312 matches for " Nicolas Martin "
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Un sport noir ?
Nicolas Martin-Breteau
Transatlantica : Revue d'études Américaines , 2012,
Abstract: Dominé par les joueurs africains-américains, le basket-ball est aujourd’hui considéré comme un sport noir. Tout au long du xxe siècle, la communauté noire américaine a particulièrement investi ce sport jusqu’à ce que les joueurs africains-américains représentent l’immense majorité des effectifs de la National Basketball Association, la ligue professionnelle américaine. Depuis plus d’un siècle, les causes de cette domination sportive donnent lieu à des controverses virulentes recourant en général à des arguments génétiques (les noirs sont athlétiquement supérieurs) ou environnementaux (la pauvreté urbaine pousse les noirs vers le sport). Néanmoins, on peut plut t voir dans cette domination une volonté politique de lutter pour l’égalité, la dignité et la reconnaissance. Que la communauté noire américaine les célèbre comme une chance d’intégration ou les critique comme une dangereuse illusion, le sport et le basket-ball ont ainsi été un aspect essentiel des luttes politiques africaines-américaines. Basketball is today dominated by African-American players and is consequently considered as a black sport. Throughout the 20th century, vast numbers of African Americans became involved in this sport, to such an extent that African Americans have come to represent the great majority of NBA players. For more than a century, this athletic domination has given rise to heated controversies: most of the time it was explained using genetics (Blacks are athletically superior) or environmental theories (the poverty of urban Blacks leads them toward sports). However, one can see in this domination a political will to fight for equality, dignity, and recognition. Thus, whether it is celebrated by African Americans as a chance for racial uplift or criticized as a dangerous illusion, sport and basketball have been a central aspect of their political struggles.
Tachyon condensation in open-closed p-adic string theory
Nicolas Moeller,Martin Schnabl
Physics , 2003, DOI: 10.1088/1126-6708/2004/01/011
Abstract: We study a simple model of p-adic closed and open strings. It sheds some light on the dynamics of tachyon condensation for both types of strings. We calculate the effect of static and decaying D-brane configurations on the closed string background. For closed string tachyons we find lumps analogous to D-branes. By studying their fluctuation spectrum and the D-branes they admit, we argue that closed string lumps should be interpreted as spacetimes of lower dimensionality described by some noncritical p-adic string theory.
Nonnegative curvature, elliptic genus and unbounded Pontryagin numbers
Martin Herrmann,Nicolas Weisskopf
Mathematics , 2014,
Abstract: We discuss the cobordism type of spin manifolds with nonnegative sectional curvature. We show that in each dimension $4k \geq 12$, there are infinitely many cobordism types of simply connected and nonnegatively curved spin manifolds. Moreover, we raise and analyze a question about possible cobordism obstructions to nonnegative curvature.
Closures of $B$-conjugacy classes of $2$-nilpotent matrices have rational singularities
Martin Bender,Nicolas Perrin
Mathematics , 2014,
Abstract: We prove that closures of Borel conjugacy classes of $2$-nilpotent matrices have a rational resolution of singularities.
Equistarable graphs and counterexamples to three conjectures on equistable graphs
Martin Milani?,Nicolas Trotignon
Computer Science , 2014,
Abstract: Equistable graphs are graphs admitting positive weights on vertices such that a subset of vertices is a maximal stable set if and only if it is of total weight $1$. In $1994$, Mahadev et al.~introduced a subclass of equistable graphs, called strongly equistable graphs, as graphs such that for every $c \le 1$ and every non-empty subset $T$ of vertices that is not a maximal stable set, there exist positive vertex weights such that every maximal stable set is of total weight $1$ and the total weight of $T$ does not equal $c$. Mahadev et al. conjectured that every equistable graph is strongly equistable. General partition graphs are the intersection graphs of set systems over a finite ground set $U$ such that every maximal stable set of the graph corresponds to a partition of $U$. In $2009$, Orlin proved that every general partition graph is equistable, and conjectured that the converse holds as well. Orlin's conjecture, if true, would imply the conjecture due to Mahadev, Peled, and Sun. An intermediate conjecture, one that would follow from Orlin's conjecture and would imply the conjecture by Mahadev, Peled, and Sun, was posed by Miklavi\v{c} and Milani\v{c} in $2011$, and states that every equistable graph has a clique intersecting all maximal stable sets. The above conjectures have been verified for several graph classes. We introduce the notion of equistarable graphs and based on it construct counterexamples to all three conjectures within the class of complements of line graphs of triangle-free graphs.
Helical Dielectrophoretic Particle Separator Fabricated by Conformal Spindle Printing  [PDF]
Nicolas Guérin, Martin Lévesque, Daniel Therriault
Journal of Biomedical Science and Engineering (JBiSE) , 2014, DOI: 10.4236/jbise.2014.79064
Abstract: This paper reports the fabrication and testing of a helical cell separator that uses insulator-based dielectrophoresis as the driving force of its separation. The helical channel shape’s main advantage is its constant curvature radius which generates a constant electric field gradient. The presented separator was fabricated by extruding a sacrificial ink on rotating spindles using a computer-controlled robot. After being assembled, connected to the reservoir and encapsulated in epoxy resin, the ink was removed to create a helical microchannel. The resulting device was tested by circulating polystyrene microbeads of 4 and 10 μm diameter through its channel using a voltage of 900 VDC. The particles were separated with efficiencies of 94.0% and 92.5%, respectively. However, roughness in some parts of the channel and connections that had larger diameters compared to the channel created local electric field gradients which, doubtless, hindered separation. It is a promising device that could lead the way toward portable and affordable medical devices.
Physical Conditions in the Low-Ionization Component of Starburst Outflows: The Shape of Near-Ultraviolet and Optical Absorption-Line Troughs in Keck Spectra of ULIRGs
Crystal L. Martin,Nicolas Bouche
Physics , 2009, DOI: 10.1088/0004-637X/703/2/1394
Abstract: We analyze the physical conditions in the low-ionization component of starburst outflows (in contrast to the high-ionization wind fluid observed in X-rays), based on new Keck/LRIS spectroscopy of partially resolved absorption troughs in near-ultraviolet and optical spectra of Ultraluminous Infrared Galaxies. The large velocity width and blueshift present in seven, atomic transitions indicate a macroscopic velocity gradient within the outflowing gas. The \mgII 2796, 2803 (and \feII 2587, 2600) doublet lines in these data constrains the gas kinematics better than the heavily blended \nad 5892, 98 doublet. The identical shape of the \mgII 2796 absorption troughs to that of the normally weaker transition at 2803\AA requires both transitions be optically thick at all outflow velocities. The fraction of the galactic continuum covered by the outflow at each velocity therefore dictates the shape of these absorption troughs. We suggest that the velocity offset of the deepest part of the troughs, where the covering factor of low-ionization gas is near unity, reflects the speed of a shell of swept-up, interstellar gas at the time of blowout. In a spherical outflow, we show that the fragments of this shell expand slowly relative to the geometrical dilution; and the covering fraction of low-ionization gas decreases with increasing radius. Our measurement of a covering factor that decreases with increasing velocity can therefore be interpreted as evidence that the low-ionization outflow is accelerating. We also present measurements of C_f(v) in 4 species, place an upper limit of 3000 cm3 on the density of the outflowing gas, and discuss lower limits on the mass outflow rate.
Inferring the dynamics of stellar streams via distance gradients
Shoko Jin,Nicolas F. Martin
Physics , 2009, DOI: 10.1111/j.1745-3933.2009.00759.x
Abstract: We present a simple result in which the distance gradient along a stream can be used to derive the transverse velocity (i.e. proper motion) along it, if the line-of-sight velocity is also known. We show its application to a mock orbit to illustrate its validity and usage. For less extended objects, such as globular clusters and satellite galaxies being tidally disrupted, the same result can be applied in its small-angle approximation. The procedure does not rely on energy or angular momentum conservation and hence does not require a Galactic model in order to deduce the local velocity vector of the stream.
The Hercules satellite: a stellar stream in the Milky Way halo?
Nicolas F. Martin,Shoko Jin
Physics , 2010, DOI: 10.1088/0004-637X/721/2/1333
Abstract: We investigate the possibility that the recently discovered Hercules Milky Way satellite is in fact a stellar stream in formation, thereby explaining its very elongated shape with an axis ratio of 3 to 1. Under the assumption that Hercules is a stellar stream and that its stars are flowing along the orbit of its progenitor, we find an orbit that would have recently brought the system close enough to the Milky Way to induce its disruption and transformation from a bound dwarf galaxy into a stellar stream. The application of simple analytical techniques to the tentative radial velocity gradient observed in the satellite provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s in the Galactic Standard of Rest). Combined with its large receding velocity, the determined tangential velocity yields an orbit with a small pericentric distance (R_peri = 6^{+9}_{-2} kpc). Tidal disruption is therefore a valid scenario for explaining the extreme shape of Hercules. The increase in the mean flattening of dwarf galaxies as one considers fainter systems could therefore be the impact of a few of these satellites not being bound stellar systems dominated by dark matter but, in fact, stellar streams in formation, shedding their stars in the Milky Way's stellar halo.
Could the Hercules satellite be a stellar stream in the Milky Way halo?
Shoko Jin,Nicolas F. Martin
Physics , 2010, DOI: 10.1051/eas/1148082
Abstract: We investigate the possibility that Hercules, a recently discovered Milky Way (MW) satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules' highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body. The application of simple analytical techniques on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km/s) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6^{+9}_{-2} kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the MW's gravitational potential, inducing its transformation into a stellar stream.
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