THE EXISTENCE OF POSITIVE PERIODIC SOLUTIONS IN A LOGISTIC DIFFERENCE MODEL WITH A FEEDBACK CONTROL

Consider the following nonautonomous delayed periodic logistic difference model with feedback control termwhich describes the evolution of a single species. The existence of a positive periodic solution is established by using the method of Mawhin's coincidence degree. This work has important significance in both theory and applications.     

Searching for counterfeit coins

The well-known counterfeit problem asks for the minimum number of weighings necessary to determine all fake coins in a given set ofn coins. We derive a new upper bound when we know that at mostd coins are defective, improving a previous result of L. Pyber.     

Measurements of total cross sections for electron scattering from helium and argon

A time-of-flight (TOF) electron spectrometer has been used to measure absolute total cross sections (TCS) scattered from helium and argon over the energy range from 1 to 50 eV. The TOF spectrometer and experimental procedure are described briefly, and experimental results are presented together with associated errors. The results are found to be in good agreement with other experimental and theoretical data.     

矩阵方程AXAT+BYBT=C的对称与反对称最小范数最小二乘解

对于任意给定的矩阵A∈Rk×m,B∈Rk×n和C∈Rk×k,利用奇异值分解和广义奇异值分解,我们给出了矩阵方程AXAT+BYBT=C的对称与反对称最小范数最小二乘解的表达式.     

Monodisperse upconversion Er3+/Yb3+:MFCl (M = Ca, Sr, Ba) nanocrystals synthesized via a seed-based chlorination route

Monodisperse Yb(3+)/Er(3+):MFCl nanocrystals were fabricated via a simple and effective seed-based chlorination route for the first time. Remarkably, their upconversion emissions are found to be greatly intensified compared with those of the well-reported Er(3+)/Yb(3+):SrF(2) with the same size and shape.     

Preparation and characterization of molecularly imprinted microspheres for selective extraction of trace melamine from milk samples

We describe molecularly imprinted microspheres (MIMs) for the selective extraction of melamine from milk. The MIMs were made from melamine as the template molecule, methacrylic acid as the functional monomer, and ethylene glycol dimethacrylate as the linking agent. The MIMs were synthesized by suspension polymerization and characterized by rebinding experiments. They displayed high adsorption capacity, fast rebinding kinetics, and highly specific rebinding of melamine. The imprinting factor is 4.1. Scatchard analysis revealed a one-type rebinding behavior, the dissociation constant and maximum rebinding capacity being 37.59 g L^?1 and 30.85 μmol g^?1, respectively. The MIMs exhibited a 25% cross-reactivity towards atrazine, but less than 3.0% towards prometryn, clenbuterol and metronidazole. In addition, a MIM-based solid phase extraction (MISPE) column for melamine was prepared by packing MIMs into a common SPE cartridge. The MISPE extraction gave recoveries of 89.8 to 100.6% of melamine, with relative standard deviations of 5.9 to 7.5%. There was no significant loss of rebinding capacity after more than 60 repeated uses, thus demonstrating the high stability of the MISPE column. The MSPE column also was applied to the extraction of melamine from spiked liquid and powdered milk with satisfying accuracy and precision.

Ge-codoped fibers for mitigating stimulated Brillouin scattering in high power fiber amplifiers

Rare-earth doped fibers for high power fiber amplifiers normally have a small refractive index difference between the core and inner cladding, but have a larger core diameter than conventional telecom fibers. We take advantage of this feature as well as the difference between the optical and acoustic waveguides and proposed several fiber designs to mitigate Stimulated Brillouin scattering (SBS) to produce higher output powers. The numerical modeling shows that an increase in SBS threshold by 3-6 dB can be achieved using Ge-codopant while maintaining diffraction limited beam quality. The key parameters that can affect the SBS performance are specified and optimal parameters for SBS suppression are obtained. The study also highlights the need to take into account the effects of minor refractive index profile variation in evaluating SBS performance.     

Enhanced mid-infrared emissions of Er3+ at 2.7 μm via Nd3+ sensitization in chalcohalide glass

Nd(3+) sensitized Er(3+):(4)I(13/2) mid-IR emissions around 2.7 μm were investigated in the transparent Ga(2)S(3)-GeS(2)-CsCl chalcohalide glasses for the first time. Remarkably, it is found that Nd(3+) greatly enhances Er(3+) 2.7 μm emission by a maximal 20 times, and depopulates the lower level of Er(3+):(4)I(13/2) for population inversion. Based on Judd-Ofelt theory, the 2.7 μm emission cross section is calculated (as high as 0.66×10(-20) cm(2)) and the gain property of the Er(3+):(4)I(11/2)→(4)I(13/2) transition is discussed. Hopefully, the materials studied here may find potential applications in the fields of fiber amplifiers and mid-IR lasers.     

含一个无外力圆柱面带转动的三维杂交应力元简

根据一种修正的余能原理,建立了一类具有一个无外力圆柱表面及结点含转动自由度的8 结点新型三维杂交应力元. 单元边界位移场选择二次位移插值函数,且与相邻元协调;单元内假定应力场满足以柱坐标表示的平衡方程及圆柱面上无外力边界条件. 数值算例表明,这种特殊杂交应力元在相当粗的网格下即能十分准确地分析圆弧形槽口附近及曲梁的三维（及二维）的孔边应力分布.     

AN APPROXIMATION METHOD FOR THE LARGE DEFLECTION OF CANTILEVER BEAMS

研究了悬臂梁自由端受集中力作用时的大挠度变形问题,对大挠度的界定方法做出了一些讨论,并从计算数据分析和理论推导两方面归纳出一种不通过复杂计算就能对大挠度变形进行定量估计的方法. 分析表明,由挠曲线近似微分方程得出的自由端挠度值与梁长度之比值的平方,可以近似表示小挠度法计算挠度值偏离精确挠度值的误差,并由此得出大挠度变形的估计值. 该方法避免了复杂的微分方程求解和数值计算,有一定的工程实际意义.