In this paper, we consider a generalized two-demand queueing model, the same model studied in Wright (Adv. Appl. Prob., 24, 986–1007, 1992). Using this model, we show how the kernel method can be applied to a two-dimensional queueing system for exact tail asymptotics in the stationary joint distribution and also in the two marginal distributions. We demonstrate in detail how to locate the dominant singularity and how to determine the detailed behavior of the unknown generating function around the dominant singularity for a bivariate kernel, which is much more challenging than the analysis for a one-dimensional kernel. This information is the key for characterizing exact tail asymptotics in terms of asymptotic analysis theory. This approach does not require a determination or presentation of the unknown generating function(s). 相似文献
The crystal structure and photoluminescent properties of europium doped silicate Sr2Y8(SiO4)6O2:Eu3+ are reported. The Sr2Y8−xEux(SiO4)6O2 compounds have typical apatite crystal structures with the P63/m space group. The distributions of Eu3+ between the two crystallographic sites 4f and 6h in the apatite structure are investigated by the powder X-ray diffraction and Rietveld refinement. Results show that Eu3+ ions only occupy the 4f sites when the Eu doping concentration is low (x=0-0.5 in Sr2Y8−xEux(SiO4)6O2). However, in higher concentrations, Eu3+ ions begin to enter the 6h sites as well. The distributions of the Eu3+ are also reflected in photoluminescent spectra. The CIE coordinates for Sr2Y6Eu2(SiO4)6O2 are (0.63, 0.37), which is close to the pure red color. 相似文献
Single-crystalline CrSi(2) nanostructures with a unique hexagonal nanoweb morphology have been successfully synthesized for the first time. These nanowebs span 150-200 nm and are composed of <112?0> nanowire segments with a thickness of 10-30 nm. It is proposed that surface charges on the {101?0} sidewalls and the minimization of electrostatic energy induce the nanoweb formation. Calculations of the electrostatic energies were used to predict the transitions between different modes of bending, which agreed well with the experimental observations. 相似文献
Microparticle HZSM-5 zeolite (MPZ) has been prepared without employing any organic templates, and used as a catalyst for the hydration of cyclohexene to synthesize cyclohexanol. MPZ exhibits better catalytic performance and superior settlement separation property than those of commercial HZSM-5 prepared by the traditional method using an organic template. The stability of MPZ has been investigated for a 1200-h test, and the regenerated performance of MPZ has also been investigated. The results show that although MPZ was reused for five recycles, the high cyclohexene conversion of 9.6 % and the high cyclohexanol selectivity of 96.8 % are still attained after the fifth regeneration. FT-IR, XRD and N2 adsorption–desorption characterizations show that coke deposit on the surface and in the channels of MPZ is the main reason for the deactivation. ICP-AES, SEM–EDS and NH3-TPD characterizations indicate that hydrothermal dealumination reduces the strong acidity and accelerates the catalyst deactivation. The spent catalyst by regeneration with H2O2 could be recovered to its initial high catalytic activity, due to the restored appropriate channels and exposed active sites.
A new type of narrowly dispersed fluorescent crosslinked polystyrene (PS) nanoparticles (20-50 nm) was synthesized via a modified microemulsion copolymerization of styrene, crosslinker divinyl benzene (DVB) and a hydrophilic comonomer amino ethyl methacrylate hydrochloride (AEMH), in the presence of pyrene. Characterized by steady-state fluorescence spectra, these nanoparticles show high luminescent intensity and the embedded pyrene has a negligible desorption from the nanoparticles. The emission intensity I1 of the pyrene in the crosslinked nanoparticles is 40 times higher than that of pyrene in toluene or styrene solution with the same concentration. The fluorescence emission intensity can be varied by the amount of the monomer, crosslinker and pyrene, but is influenced little by the amount of AEMH in the range of investigation. The surface of the nanoparticles is modified by amino and amidino functional groups introduced by the comonomer and the initiator 2, 2'-azobis(2-amidinopropane) dihydrochloride (V50), which controls the zeta potential on the particle surface. 相似文献
Flexible composite films were produced by impregnating aqueous phenol formaldehyde (PF) resin into water-swollen cellulose nanofibril (CNF) films. CNF films were prepared using a pressurized filtration method in combination with freeze drying. The freeze-dried films were swollen with water then impregnated with PF resin by soaking in aqueous resin solutions of varying concentrations. Small amounts of PF slightly enhanced the tensile properties of CNF films. The formulation with the best mechanical properties was CNF/PF films with 8 wt % resin exhibiting tensile stress and toughness of 248 MPa and 26 MJ/m3, respectively. Resin concentrations higher than about 8 % resulted in composites with decreased tensile properties as compared to neat CNF films. The wet strength of the composite films was significantly higher than that of the neat CNF films. The resulting composites showed greater resistance to moisture absorption accompanied by reduced thickness swelling when soaked in water as compared to neat CNF films. The composites also showed decreased oxygen permeability at low humidity compared to neat films, but the composites did not show improved barrier properties at high humidity. 相似文献
A new approach for time-delay identification is proposed in linear controlled systems. The delay is derived from the control loop in the system. The frequency-response function of the system is presented in terms of the impedance matrix. It is proved that the inverse form of the function may be expressed in the harmonic function, which is used to fit those data from the experiment. As an example, an isolator with the delayed feedback control is schemed to acquire such data. Using least square algorithm yields that the identified delay can reach any required accuracy. 相似文献
Level-expanding quasi-birth-and-death (QBD) processes have been shown to be an efficient modeling tool for studying multi-dimensional
systems, especially two-dimensional ones. Computationally, it changes the more challenging problem of dealing with algorithms
for two-dimensional systems to a less challenging one for block-structured transition matrices of QBD type with varying finite
block sizes. In this paper, we focus on tail asymptotics in the stationary distribution of a level-expanding QBD process.
Specifically, we provide sufficient conditions for geometric tail asymptotics for the level-expanding QBD process, and then
apply the result to an interesting two-dimensional system, an inventory queue model. 相似文献