A stochastic model for the HIV epidemic in homosexual populations involving age and race

In this paper, we derive a stochastic model for the HIV epidemic in homosexual populations involving age and race. To account for effects of different mixing patterns, a low risk selection rule is introduced. The model is then formulated in terms of chain multinomial distributions by means of which the means are derived. Some simulation studies by computer indicate clearly that age and race have a significant impact on the HIV epidemic.     

Existence and Uniqueness of the Solution of Nonlinear Population Evolution Equations

ＥｘｉｓｔｅｎｃｅａｎｄＵｎｉｑｕｅｎｅｓｓｏｆｔｈｅＳｏｌｕｔｉｏｎｏｆＮｏｎｌｉｎｅａｒＰｏｐｕｌａｔｉｏｎＥｖｏｌｕｔｉｏｎＥｑｕａｔｉｏｎｓＬｉＨｏｎｇｙｉ（李红裔）（Ｄｅｐｔ．ｏｆＢａｓｉｃＳｃｉｅｎｃｅ，ＮｏｒｔｈＣｈｉｎａＩｎｓｔｉｔｕ...     

象山港毛蚶的种群生物学及其繁殖保护措施

象山港毛蚶群体组成以２龄贝为主，占６６．１０％。体重与壳长之间关系为Ｗ＝１０－４×１．１．１７５Ｌ．其生长规律可用Ｌｏｇｉｔｉｃ自然生长方程Ｌ１＝３９．２７３９／１（１＋（为月份）描述。壳长生长曲线的拐点为３０月龄，体重生长曲线的拐点为６６月龄，合理采捕时间应在生长转折点以后，主要采捕２８～３４ｍｍ大规格的４龄贝，黄墩港和铁港两支港毛蚶的资源实际结算量为９２９．２５ｔ，理论估算量为９２４．７８ｔ，合理捕捞量估算为２７０～３７０ｔ采捕季节规定为１２月至翌年３月为宜。     

Formation of Si-nanoparticles in a microwave reactor: Comparison between experiments and modelling

The formation and growth of silicon-nanoparticles from silane in a microwave reactor was investigated. Experiments were performed for the following conditions: precursor concentration 380–2530 ppm, pressures of 20–30 mbar, microwave powers 120–300 W. The formed particles were examined in-situ with a particle mass spectrometer. Additionally, particles were collected on grids and analyzed by transmission electron microscopy, X-ray diffraction, and by determining the specific surface area by BET. The particle size was found to be in the range of 5–8 nm in diameter. A simple model was used to simulate the particle formation processes taking place inside the reactor. The microwave energy coupled into the reactor flow was treated as a spatially distributed energy source resulting in a local temperature increase. The particles were assumed to have a monodisperse size distribution. To allow an approximation of their shape they were characterized by their volume and surface area. The model takes nucleation, convection, coagulation, and coalescence into account. The fluid flow inside the microwave reactor was simulated with the commercial CFD-code Fluent.     

The Distributions of the Frequency of Occurrence of Nucleotide Subsequences

The objective is to derive the probability distribution of the frequency of occurrence of a subsequence within a nucleotide sequence under the hypothesis that the four nucleotides occur at random and with equal probability. We also consider the Compound Poisson approximation for the same distribution. The exact probability distribution can be obtained by the finite Markov chain imbedding technique introduced by Fu and Koutras (1994), however we can manage the case as well if the probabilities are not all equal. The compound Poisson approximation by Stein-Chen's method can be used to develop an approximate probability distribution with proper setting of the definition of the sets of dependence. Such structure gives a bound on the total variation distance, which tends to get relatively larger as the frequency goes up. AMS 2000 Subject Classification: Primary: 60E05; Secondary: 60J10     

Temperature Dependence of the Gibbs State in the Random Energy Model

We consider the problem of temperature dependence of the Gibbs states in two spin-glass models: Derrida's Random Energy Model and its analogue, where the random variables in the Hamiltonian are replaced by independent standard Brownian motions. For both of them we compute in the thermodynamic limit the overlap distribution ^N _{i=1 i i} /N[–1,1] of two spin configurations , under the product of two Gibbs measures, which are taken at temperatures T,T respectively. If TT are fixed, then at low temperature phase the results are different for these models: for the first one this distribution is D _{0 0}+D _{1 1}, with random weights D ₀, D ₁, while for the second one it is ₀. We compute consequently the overlap distribution for the second model whenever T–T0 at different speeds as N.     

An environmental pseudopotential approach to molecular interactions: Implementation in MOLPRO

We present the implementation into the MOLPRO package of a model for the interaction of a central system with its surrounding environment. The properties of a target system enclosed by a noncovalently bound environment or solvent are modeled as those of a system embedded into the effective pseudopotential arising from the exact electrostatic Coulomb potential and the approximated exchange-repulsion potential. For the latter we use the charge-density overlap model, which relates the exchange-repulsion interaction energy between two species with the overlap of their ground-state electron charge densities. The solutions of the modified Hartree-Fock equations for the target system are obtained self-consistently. This way the exchange-induction effects arising from the converged electron-charge density of the embedded system are implicitly included. Inclusion of the correlation effects is provided by the use of post-Hartree-Fock and density-functional techniques available in the MOLPRO package. The computational and conceptual advantages provided by this approach are shown in the calculation of the dipole polarizabilities of halide and chalcogenide anions in different environments.     

Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems

We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in various pulse schemes. These sweeps consist of two sidebands generated by an amplitude modulation of the RF carrier. Using a time-dependent amplitude modulation the sidebands can be swept through a certain frequency range. Inspired by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is used to manipulate +/-(m - 1) <--> +/-m (3/2 < or = m < or = N/2) satellite transitions in half-integer spin systems simultaneously. For (23)Na (I = 3/2) and (27)Al (I = 5/2) spins in single crystals it proved possible to transfer the populations of the outer +/-m spin levels to the inner +/-1/2 spin levels. A detailed analysis shows that the efficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these sweep parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and experimentally. Using a DFS as a preparation period leads to significantly enhanced central transition powder spectra under both static and MAS conditions, compared to single pulse excitation. DFSs prove to be very efficient tools not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS experiments. It is demonstrated, theoretically and experimentally, that DFSs are capable of transferring both quintuple-quantum and triple-quantum coherence into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the RF power requirement compared to pulsed MQMAS experiments, thus extending their applicability. This is demonstrated by (27)Al 3QMAS experiments on 9Al(2)O(3). 2B(2)O(3) and the mineral andalusite. In the latter compound, Al experiences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3). 2B(2)O(3) demonstrates the sensitivity enhancement of this experiment using a double frequency sweep.     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.