Strategic arrivals to queues offering priority service

Queueing Systems - We consider strategic arrivals to a FCFS service system that starts service at a fixed time and has to serve a fixed number of customers, for example, an airplane boarding...     

Polarisabilités optiques moléculaires de dérivés benzèniques en solution déterminées par des mesures d'effet Kerr statique

Une expression théorique simplifiée de la constante de Kerr permet une comparaison cohérente des mesures d'effet Kerr statique et de diffusion Rayleigh dépolarisée. Cette comparaison ainsi que des mesures de moments dipolaires sont effectuées sur une série de composés benzèniques en solution diluée dans le tétrachlorure de carbone. La méthode d'exploitation des résultats expérimentaux conduit à la détermination du tenseur des polarisabilités optiques moléculaires α de ces composés. Un accord satisfaisant est obtenu en comparant la constante spécifique de Kerr S _k et l'anisotropie optique moléculaire γ², calculées à partir de ces polarisabilités, à leurs valeurs expérimentales.     

Load balancing via random local search in closed and open systems

In this paper, we analyze the performance of random load resampling and migration strategies in parallel server systems. Clients initially attach themselves to an arbitrary server, but may switch servers independently at random instants of time in an attempt to improve their service rate. This approach to load balancing contrasts with traditional approaches where clients make smart server selections upon arrival (e.g., Join-the-Shortest-Queue policy and variants thereof). Load resampling is particularly relevant in scenarios where clients cannot predict the load of a server before being actually attached to it. An important example is in wireless spectrum sharing where clients try to share a set of frequency bands in a distributed manner. We first analyze the natural Random Local Search (RLS) strategy. Under this strategy, after sampling a new server randomly, clients only switch to it if their service rate is improved. In closed systems, where the client population is fixed, we derive tight estimates of the time it takes under RLS strategy to balance the load across servers. We then study open systems where clients arrive according to a random process and leave the system upon service completion. In this scenario, we analyze how client migrations within the system interact with the system dynamics induced by client arrivals and departures. We compare the load-aware RLS strategy to a load-oblivious strategy in which clients just randomly switch server without accounting for the server loads. Surprisingly, we show that both load-oblivious and load-aware strategies stabilize the system whenever this is at all possible. We use large-system asymptotics to characterize system performance, and augment this with simulations, which suggest that the average client sojourn time under the load-oblivious strategy is not considerably reduced when clients apply smarter load-aware strategies.     

Molecular weights of polystyrenes in toluene solutions by light scattering. Comparison between classical values and values deduced from a new method

Light scattering measurements in toluene solutions are performed for a series of monodisperse polystyrenes with a molecular weight M_w range from 4×10³ to 8×10⁶. The scattered polarized intensities I_v and the natural depolarization ratios ρ_n are registered with different apparatus at λ=633 or 488 nm and the M_w values are deduced through different formulae. The complete Carr and Zimm formula (CL_a), from I_v and ρ_n, and the usual simplified formula (CL_b), from I_v, are considered for the classical method. An already demonstrated formula is considered for the new method (New). Values of M_w and related parameters do not depend on the experimental systems used but deviations appear when using different formulae. The deviations are generally low (about 10%) but often systematic: M_w(CL_a)<M_w(CL_b)<M_w(New). The most important difference concerns the effect of destructive interferences for M_w>5×10⁵: the new formula leads to a lower increase from θ=90° to θ→0 for M_w values (θ is the observation angle). For instance, in the 8×10⁶ sample, M_w(θ→0)/M_w(θ=90°)=3.6 instead of 6.1, which implies a revision of the usual determination of the radius of gyration, R_g.     

Redox reactions in palladium catalysis: on the accelerating and/or inhibiting effects of copper and silver salt additives in cross-coupling chemistry involving electron-rich phosphine ligands

Challenging a catalytic cycle: Pd(0) catalysts are readily oxidized by Cu and Ag salts to give dinuclear Pd(I) complexes and Cu(I) or Ag(I) cubanes (see scheme). The reactivities of the resulting Pd(I) dimers are consistent with several observations of additive effects in cross-coupling chemistry. The results indicate the possibility for alternative catalytic cycles involving dinuclear Pd(I) complexes over the currently accepted synergistic cycles involving Pd(0)/Pd(II) intermediates and Cu or Ag.     

Reactivity and stability of dinuclear Pd(I) complexes: studies on the active catalytic species, insights into precatalyst activation and deactivation, and application in highly selective cross-coupling reactions

The catalysis derived from the dinuclear Pd(I)-Pd(I) complex, {[PtBu(3)]PdBr}(2), has been studied with experimental, computational, and spectroscopic techniques. Experimental selectivity studies were performed, and the reactivity was subsequently investigated with density functional theory (B3LYP-D and M06L) to deduce information on the likely active catalytic species. The reactivity with aryl chlorides and bromides was found to be inconsistent with direct catalytic involvement of the Pd(I) dimer but consistent with mononuclear Pd(0) catalysis. Computational studies suggest that precatalyst transformation to the active catalytic species does not proceed via a direct disproportionation mechanism; a reductive pathway is the most likely scenario instead. Through (31)P NMR investigations it was identified that the combination of ArB(OH)(2), KF, and water triggers the conversion of the precatalyst to Pd(PtBu(3))(2) and, most likely, Pd-black as a competing side process, explaining the incomplete conversions of aryl chlorides in Suzuki cross-coupling reactions under Pd(I) dimer conditions. New applications in highly regio- and chemoselective transformations in short reaction times at room temperature are also demonstrated.     

Flow-level performance and capacity of wireless networks with user mobility

The performance evaluation of wireless networks is severely complicated by the specific features of radio communication, such as highly variable channel conditions, interference issues, and possible hand-offs among base stations. The latter elements have no natural counterparts in wireline scenarios, and create a need for novel performance models that account for the impact of these characteristics on the service rates of users. Motivated by the above issues, we review several models for characterizing the capacity and evaluating the flow-level performance of wireless networks carrying elastic data transfers. We first examine the flow-level performance and stability of a wide family of so-called α-fair channel-aware scheduling strategies. We establish that these disciplines provide maximum stability, and describe how the special case of the Proportional Fair policy gives rise to a Processor-Sharing model with a state-dependent service rate. Next we turn attention to a network of several base stations with inter-cell interference. We derive both necessary and sufficient stability conditions and construct lower and upper bounds for the flow-level performance measures. Lastly we investigate the impact of user mobility that occurs on a slow timescale and causes possible hand-offs of active sessions. We show that the mobility tends to increase the capacity region, both in the case of globally optimal scheduling and local α-fair scheduling. It is additionally demonstrated that the capacity and user throughput improve with lower values of the fairness index α.     

Solvent effect on palladium-catalyzed cross-coupling reactions and implications on the active catalytic species

Suzuki coupling of the bifunctional substrate 1 using [Pd(2) (dba)(3) ]/PtBu(3) gives selectivity for C?Cl in nonpolar solvents but for C?OTf in polar solvents. The results of computational and experimental studies suggest that the catalytically active species in polar solvents under conditions employing coordinating additives is inconsistent with monoligated [Pd(PtBu(3) )]. Instead, the data are consistent with an anionic palladium complex as the active species.