The abundance threshold for plague as a critical percolation phenomenon

Percolation theory is most commonly associated with the slow flow of liquid through a porous medium, with applications to the physical sciences. Epidemiological applications have been anticipated for disease systems where the host is a plant or volume of soil, and hence is fixed in space. However, no natural examples have been reported. The central question of interest in percolation theory, the possibility of an infinite connected cluster, corresponds in infectious disease to a positive probability of an epidemic. Archived records of plague (infection with Yersinia pestis) in populations of great gerbils (Rhombomys opimus) in Kazakhstan have been used to show that epizootics only occur when more than about 0.33 of the burrow systems built by the host are occupied by family groups. The underlying mechanism for this abundance threshold is unknown. Here we present evidence that it is a percolation threshold, which arises from the difference in scale between the movements that transport infectious fleas between family groups and the vast size of contiguous landscapes colonized by gerbils. Conventional theory predicts that abundance thresholds for the spread of infectious disease arise when transmission between hosts is density dependent such that the basic reproduction number (R(0)) increases with abundance, attaining 1 at the threshold. Percolation thresholds, however, are separate, spatially explicit thresholds that indicate long-range connectivity in a system and do not coincide with R(0) = 1. Abundance thresholds are the theoretical basis for attempts to manage infectious disease by reducing the abundance of susceptibles, including vaccination and the culling of wildlife. This first natural example of a percolation threshold in a disease system invites a re-appraisal of other invasion thresholds, such as those for epidemic viral infections in African lions (Panthera leo), and of other disease systems such as bovine tuberculosis (caused by Mycobacterium bovis) in badgers (Meles meles).     

The virophage as a unique parasite of the giant mimivirus

Viruses are obligate parasites of Eukarya, Archaea and Bacteria. Acanthamoeba polyphaga mimivirus (APMV) is the largest known virus; it grows only in amoeba and is visible under the optical microscope. Mimivirus possesses a 1,185-kilobase double-stranded linear chromosome whose coding capacity is greater than that of numerous bacteria and archaea1, 2, 3. Here we describe an icosahedral small virus, Sputnik, 50 nm in size, found associated with a new strain of APMV. Sputnik cannot multiply in Acanthamoeba castellanii but grows rapidly, after an eclipse phase, in the giant virus factory found in amoebae co-infected with APMV4. Sputnik growth is deleterious to APMV and results in the production of abortive forms and abnormal capsid assembly of the host virus. The Sputnik genome is an 18.343-kilobase circular double-stranded DNA and contains genes that are linked to viruses infecting each of the three domains of life Eukarya, Archaea and Bacteria. Of the 21 predicted protein-coding genes, eight encode proteins with detectable homologues, including three proteins apparently derived from APMV, a homologue of an archaeal virus integrase, a predicted primase-helicase, a packaging ATPase with homologues in bacteriophages and eukaryotic viruses, a distant homologue of bacterial insertion sequence transposase DNA-binding subunit, and a Zn-ribbon protein. The closest homologues of the last four of these proteins were detected in the Global Ocean Survey environmental data set5, suggesting that Sputnik represents a currently unknown family of viruses. Considering its functional analogy with bacteriophages, we classify this virus as a virophage. The virophage could be a vehicle mediating lateral gene transfer between giant viruses.     

The origin of giant ferroelectric distortion and the nature of pressure-induced spin-state transition in BiCoO_3

Multiferroic materials, which have coexisting magnetism and ferroelectricity, are of great technological and fundamental importance. Researchers from the Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China have     

Polarization rotation mechanism for ultrahigh electromechanical response in single-crystal piezoelectrics

Piezoelectric materials, which convert mechanical to electrical energy (and vice versa), are crucial in medical imaging, telecommunication and ultrasonic devices. A new generation of single-crystal materials, such as Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), exhibit a piezoelectric effect that is ten times larger than conventional ceramics, and may revolutionize these applications. However, the mechanism underlying the ultrahigh performance of these new materials-and consequently the possibilities for further improvements-are not at present clear. Here we report a first-principles study of the ferroelectric perovskite, BaTiO3, which is similar to single-crystal PZN-PT but is a simpler system to analyse. We show that a large piezoelectric response can be driven by polarization rotation induced by an external electric field. Our computations suggest how to design materials with better performance, and may stimulate further interest in the fundamental theory of dielectric systems in finite electric fields.     

日历中一种周期现象的解析

指出日历中的国庆日与中秋节重合这种现象发生的周期性.从公历和农历的历法依据——回归年和朔望月出发,利用辗转相除法以及连分数等数学方法对农历的置闰月规律加以推演,进而得出两种历法运行的会合周期.同时对其会合周期的差异性加以分析.     

离体培养成骨细胞的有限元分析

将成骨细胞看作是一种均匀、各向同性、不可压缩的粘弹性体,利用标准的线性粘弹性固体模型推导了成骨细胞的应力应变关系的矩阵形式,再利用Lagrange方程推导得出了系统的动力学有限元平衡方程。通过算例,计算得到多孔介质基板上骨细胞的位移、速度和应力强度历程。通过自编程序对单向拉伸离体培养成骨细胞装置进行了数值分析和模拟,计算了细胞体的变形、速度以及应力等,为进一步进行骨组织工程研究提供了理论基础。     

基于模拟器的驾驶员应急状态下刹车反应时间的研究

本文研究了驾驶时次级任务产生的脑负荷,在应急状况下对反应时间的影响。在驾驶模拟器上测试了五种条件下驾驶员的应急刹车反应时间,通过对实验结果进行分析,获得了外界因素对对应急刹车反应时间的影响。     

Vapor corridor phenomenon and response of vegetation NPP in Longitudinal Range-Gorge Region

To study the causation and ecological effect of ＂Corridor＂ phenomena in the Longitudinal Range-Gorge Region （LRGR） in South China and South-East Asian. Supported by the geographic information system （GIS）, vegetation, climate and topography database of the LRGR are founded, The northernmost limit of the tropical rain forest is 28ο-22, which has been promoted 3 or 4 latitude by Vapor ＂Corridor Phenomenon＂. Through ANN analysis, landform proves to be a very important factor affecting NPP distribution. Affected by longitudinal valley landform and monsoon, moisture and quantity of heat has a longitudinal belt distribution, and NPP distribution has seemly distribution. Take Lyonia ovalifolia, Myrica nana scrub as an example, high NPP valuescrub distributes on a wet-tongue area in the valleys. The minimum temperature sensitive value to NPP is 1.588, and altitude is 1.053, precipitation is 1.006, aspect is 1.003. Corridor phenomenon in the LRGR formed special moisture and heat patterns and brings about a series of ecological effects, such as influencing space distribution of vegetation form and NPP spatial patterns of both vegetations and plants.     

语言夸张中的偏离现象

通过分析夸张中的逻辑和语义偏离，来考察夸张辞格的语用功能，其主要功能不在于表事而在于表情。因此，夸张虽然在表述上言过其实，违反质准则，但是在本质上却是符合事实的，不违反质准则。这就保证了会话合作。同时，语境制约着夸张中的偏离。     

Dissymmetric flow phenomenon in a multistrand tundish

The dissymmetric flow phenomenon exists in a symmetric multistrand tundish. It was studied by the physical simulation experiment. The fundamental flow characteristic of dissymmetry was analyzed. The asymmetry of the flow field, the temperature field, and the inclusions distribution without flow-control devices (FCDs) were compared with those with FCDs. It is proved that the asymmetry of the flow and temperature field along the outlets at the long range is more obvious. The symmetric FCDs installation has a slight effect on the dissymmetric temperature field, simultaneously, the symmetry of the average residence time and the fluid flow pattern has improved, and the fluid flow in the tundish has been more reasonable. In case of a symmetric multistrand tundish having a large volume, the influence of the dissymmetric phenomenon should be considered and the flow behaviors in the whole tundish should be studied completely.     

一类二维差分方程中的混沌现象

主要研究了一个二维差分方程——宿主-寄生物模型的混沌现象,通过分岔图、Lyapunov指数图、时间序列图和相图分析了该方程由周期运动到混沌运动的变化过程,发现该二维差分方程随参数变化表现出丰富的动力学行为,其混沌现象具有遍历性、非周期运动性、初值敏感性的特征。