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1.
A stochastic two-prey-one-predator model with Beddington-DeAngelis functional response and L\''{e}vy jumps is proposed and investigated in this paper. First of all, we prove the existence and uniqueness of the global positive solution, and stochastic ultimate boundedness of the solution. Next, under a simple assumption, by using It\^{o} formula and other important inequalities, some sufficient conditions are established to ensure the extinction and persistence in the mean of the system. The results show that neither strong white noise nor L\''{e}vy noise is conducive to the persistence of the population. Finally, the theoretical results are verified by numerical simulations.  相似文献   
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色彩是民族服饰的核心要素,也是民族文化元素的重要组成部分,构建一套兼具科学性与实用性的色彩体系极具挑战性。在传统色彩地理学方法基础上,通过提取主题色和改进的关联规则挖掘方法,获取色彩数据、寻找色彩规则,并对其进行自然色彩体系(NCS)编谱分析,构建了一套苗族服饰色彩体系(Miao’s costume color system,MCCS)。该体系有助于进一步挖掘苗族服饰的配色规律,实现对苗族服饰色彩的数字化保护,为民族服饰色彩传承机理的探索与研究提供新的思路。  相似文献   
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The cycloaddition of heptafulvalene ( 1 ) with tetracyanoethylene (TCNE) was previously described as an example of an antarafacial cycloaddition, a [π14a2s] process that afforded only the trans cycloadduct by virtue of the edge-to-face approach of TCNE, facilitated by the S shape of 1 . The reaction has been investigated in depth and found not to be a concerted antarafacial process. At low temperature, the reaction is observed to give a mixture of cis and trans cycloadducts as well as a [4+2] cycloadduct. The mixture of products is converted to the trans cycloadduct by equilibration upon warming to room temperature. Studies with diethyl 2,3-dicyanofumarate and -maleate confirmed the formation of cis cycloadducts. DFT studies at the M06-2X/6-311+G(2d,p) SCRF=acetone level of theory show that the originally proposed edge-to-face approach of TCNE to 1 is highly disfavored, whereas a stepwise mechanism involving the addition of TCNE at C2 to form a zwitterion followed by collapse at either C2′ or C7′ is energetically accessible. The Diels-Alder adduct is also formed in a stepwise reaction by competitive addition of TCNE at C4 of 1 . These studies suggest that edge-to-face interactions are prohibitive in even the most favorable cases.  相似文献   
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共焦布里渊光谱技术因其具有无创、无标记、高空间分辨等优点,被广泛应用在物理化学、材料科学、矿物学等领域。但自发布里渊散射强度弱,在探测系统消光比不足的情况下,布里渊信号光谱容易与弹性背景光发生交叠甚至是被湮没,因而无法实现对布里渊频移的精确测量。尤其在生物医学等前沿领域,浑浊介质粘弹性探测需求的日益增加对布里渊光谱探测系统的抗弹性背景光性能提出了更高的要求,解决共焦布里渊光谱探测系统消光比不足这一问题刻不容缓。为了提高共焦布里渊光谱探测系统的消光比,本文构建了一种暗场共焦布里渊光谱探测系统,将暗场照明应用于共焦布里渊探测中,利用光阑实现了中心遮挡的环形照明、中心通光的圆形收集的光路配置。照明光路与收集光路非交叉的特殊配置,保证了激发光强的同时避免了系统对镜面反射光的收集,因而使得弹性背景光被削弱,布里渊信号光谱显露,提高了系统的消光比。实验表明:相较于传统明场照明配置,暗场共焦布里渊探测系统的抗弹性背景光性能提升,消光比提高了20 dB;0.001%浓度的脂肪乳溶液在暗场配置下背景光得到明显压制,布里渊信号光谱显露,实现了对浑浊介质的布里渊频移数据的精确测量;选取蒸馏水、聚甲基丙烯酸甲酯、二氧化硅玻璃三个标准样品验证暗场配置下的非严格背向散射角,理论分析与实验相吻合,保证了后续轴向声速、纵向弹性模量等参数的计算结果的准确有效。暗场共焦布里渊光谱探测系统综合暗场照明与共焦探测的优点,既拥有共焦探测的高空间分辨率,又借助暗场光路配置提升了系统的抗弹性背景光性能,实现了高消光比、高空间分辨的布里渊光谱探测,为生物医学、材料科学等前沿领域实现对物质机械性能的实时无损探测提供了新的思路。  相似文献   
7.
A kinetic model which describes Ostwald's rule of stages, during the process of crystal growth from solution, is reported here. Reaction equations for stages are given where the stages convert from one to another. The final stage reacts to release a portion of solute back into solution, while the remainder converts to the final equilibrium form. Additionally, a remnant of the solute that was not consumed by any of the transitional stages, ultimately is converted into the final product. This particular model was motivated by a recent report for Boc-diphenylalanine self-assembly where the dissolved peptide was observed to go through two polymorphic stages before reaching the equilibrium supramolecular assembly [A. Levin et al., Nat. Commun. 5, 5219, (2014)]. Kinetic data for the concentration of solute present during the process are listed in the above-mentioned report. We show here how the model, for , describes the time-dependent behavior of the solute decay during the growth process. After comparing the model to the experimental data, we are able to report values for all of the rate constants and propose a rule whereby the relative magnitudes of these constants can be used to predict whether a supersaturated substance will noticeably pass through transitional stages or simply convert from solute to the equilibrium solid form.  相似文献   
8.
Transparent and nearly colorless single crystals of r‐LiB13C2 were obtained by reaction of boron with Li2CO3 in a Cu melt at 1250–1300 °C. The structure analysis [R3 m, a = 5.6535(1), c = 12.5320(2) Å, 421 independent reflections, 22 parameters, R1 = 0.034, wR2 = 0.093] revealed a crystal structure that can be described as a filling variant of rhombohedral B13C2. Li+ is located in a void above or below the linear CBC unit. The site occupation is close to 50 % resulting in an electron‐precise composition according to Wade's rules if a positive charge is given to the CBC entity: Li+(B12)2–(CBC)+. The displacement parameters of the CBC unit indicate disorder in the [001] direction, that relates to the short Li–C distance and the partial occupation of the Li+ site. The composition is confirmed by EELS measurements of single crystals. Band gap calculations give a value of 2.94 eV, which is in agreement to the crystals being colorless. The evaluation of the electron density by application of the QTAIM formalism as proposed by Bader modifies the assignment pictured above according to Wade's rules. In agreement to the electronegativities the carbon atoms carry a negative charge (–2.31/–2.42) and the effective charges are: Li+0.81(B12)+2.02(CBC)–2.83.  相似文献   
9.
火星大气气溶胶的地基探测对研究火星大气环境具有重要意义,为了能够在节约火星巡视器/着陆器体积、重量的条件下进行气溶胶探测,论证了基于巡视器车载激光诱导击穿光谱仪系统设计米散射激光雷达方案的可行性。所设计的米散射激光雷达系统使用巡视器车载激光诱导击穿光谱仪的既有硬件资源,加入分光元件和探测器模块,构成与激光诱导击穿光谱仪系统集成的米散射激光雷达,米散射激光雷达与激光诱导击穿光谱仪在火星地表分时工作,互不影响。为了论证所设计的米散射激光雷达的探测性能,对Phoenix火星探测任务中独立的米散射激光雷达得到的一组原始回波信号数据进行了处理,反演得到一组典型火星大气消光系数廓线,结合消光系数廓线与系统硬件参数计算了所设计的米散射激光雷达的回波信噪比,结果表明该系统在所用原始数据被记录的当日在火星大气边界层顶4 km高度处信噪比达到26 dB,在10 km高度附近下降到0 dB,说明基于火星巡视器车载激光诱导击穿光谱仪系统设计米散射激光雷达进行气溶胶探测具有现实可行性。对比Phoenix的独立米散射激光雷达设计方案,基于激光诱导击穿光谱仪的米散射激光雷达不但能够节省巡视器体积、重量,而且发射能量更高,回波接收方案更为简单,数据反演步骤更为简洁。  相似文献   
10.
In this article, we present a novel method to obtain both improved estimates and reliable stopping rules for stochastic optimization algorithms such as the Monte Carlo EM (MCEM) algorithm. By characterizing a stationary point, θ*, of the algorithm as the solution to a fixed point equation, we provide a parameter estimation procedure by solving for the fixed point of the update mapping. We investigate various ways to model the update mapping, including the use of a local linear (regression) smoother. This simple approach allows increased stability in estimating the value of θ* as well as providing a natural quantification of the estimation uncertainty. These uncertainty measures can then also be used to construct convergence criteria that reflect the inherent randomness in the algorithm. We establish convergence properties of our modified estimator. In contrast to existing literature, our convergence results do not require the Monte Carlo sample size to go to infinity. Simulation studies are provided to illustrate the improved stability and reliability of our estimator.  相似文献   
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