共查询到20条相似文献,搜索用时 62 毫秒
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对一类非自治竞争Lotka-Volterra系统,其相互作用系数为非负常数,S.Ahmad和A.C.Lazer研究了该系统并得到结果在一定条件下,系统的第n个种群趋于灭绝而前n-1个种群强持续生存.本文我们讨论一类较一般的非自治竞争Lotka-Volterra系统,其相互作用系数为一类连续函数,得到同样结果. 相似文献
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系统聚类分析在细菌全细胞脂肪酸模式识别中的应用 总被引:1,自引:1,他引:1
用欧氏距离系数和指数相关系数,结合8种常用的系统聚类算法,对用毛细管柱气相色谱祛绘制的34株莫拉氏菌(Moraxella)及其类属菌和13株嗜肺军团杆菌(Legionella pneumo-phila)的全细胞脂肪酸气相色谱图,进行了聚类分析。比较了欧氏距离系数的8种系统聚类算法所得的聚类树状谱。结果表明,奠拉氏菌与嗜肺军团杆菌可以明确区分。在奠拉氏菌中,我国分离的两个新种与目前该属的主要标准株也能明确区分。两种相似系数中,欧氏距离系数的聚类结果较好;8种系统聚类算法中,最长距离法和类平均法的聚类结果较好。 相似文献
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一类都有HollingⅡ类功能反应且具周期系数的三维顺环捕食系统的研究 总被引:17,自引:3,他引:14
本文研究一类都有HollingⅡ类功能反应且是周期系数的三维顺环捕食系统,得到了该类系统存在唯一、全局渐近稳定周期解的充分条件。 相似文献
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昆虫区系多元相似性分析方法 总被引:4,自引:0,他引:4
由植物学领域首先提出的相似性概念已广泛应用于动植物区系地理研究以及生物学、生态学等诸多自然学科乃至社会科学领域.根据Jacard提出的二元相似性系数公式SI=C/(A B-C)和Sprensen提出的二元相似性系数公式SI=2C/(A B),分别推导出2个计算多元相似性系数的数学表达式,SIJab…n=[(∑Hij)2/n (∑Hijk)3/n … Hab…n]/[∑Si-∑Hij-2∑Hijk-…-(n-1)Hab…n]和 SISab…n=[2(∑Hij)2/n (3∑Hijk)3/n … nHab…n]/ ∑Si,并用中国夜蛾广布种类在中国7个动物地理区的分布资料为例进行验证,从而可以直接从整体角度和宏观规模上简便、快捷地考量多个系统间的亲疏程度和相似关系.建议在以相似性为基础的聚类分析中,不必再先把2个系统合并成一个新系统后,再和第3个系统比较,而可直接计算多个系统的相似性系数,以避免由于合并带来的信息损失.还讨论了应该提高Sprensen公式0.5的显著性标准,以使同一组数据的两种计算结果趋向一致. 相似文献
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Coefficient of variation, standard deviation divided by mean, has some essential defects. Its density, expectation and variance are too complex to make the statistical inference for such a coefficient. The definition of stabilization coefficient is just the reciprocal of variation coefficient, mean divided by standard deviation. Such a coefficient has a simple expectation and a simple variance, and is an asymptotically unbiased estimator and a consistent estimator of its true value. Furthermore, coefficient of stabilization has an asymptotic normality. Due to its statistical advantages, coefficient of stabilization is easy to be tested statistically. In some applied fields, usually, there is an increasing standard deviation accompanying an increasing mean. Coefficient of stabilization can be practically used for some comparison studies in such fields. Illustrations about comparing microorganism strains are given in this paper. The robustness of stabilization coefficient is satisfactory. 相似文献
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降水年内分配不均匀性指标 总被引:5,自引:0,他引:5
径流量及其分配过程依赖于降雨特征,它关系到水利设施规模和水资源合理配置。本文借鉴水文界经验,应用降雨集中度、降雨不均匀系数和调节系数概念,并对后2个系数进行修订,使其值域介于0~1。利用中原地区有代表性的4个气象台站1951—2005年的日降雨资料,计算并分析比较了降雨集中度、降雨不均匀系数和调节系数在河南省的适用性。结果表明:降水集中度、降水不均匀系数和调节系数从不同侧面反映了降水量的年内分布特征。3个指标具有一定的可比性,但数值上用同样资料计算的调节系数都要大于不均匀系数;4站候、旬和月的降水不均匀系数有相同的分布规律,随着时段的延长,不均匀系数增大,其变幅也增大,而调节系数则不同,随着时段的延长,降水调节系数减小,而相应的变幅则增大;分别采用候、旬和月的降水资料计算的降水集中度、不均匀系数和调节系数之间具有很好的一致性,最小的相关系数为0.60(P<0.001),说明3个指标间可互为代替;采用候、旬和月计算的降水集中度两两之间呈显著相关,相关系数为0.98;候、旬和月不均匀系数之间的相关系数为0.75;候、旬和月的调节系数之间的相关系数与不均匀系数相当。表明这3个指标在不同时间尺度间具有明显的一致性,在河南区域只应用月降水量就能反应降水的不均匀性。 相似文献
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White spot syndrome virus (WSSV) is devastating shrimp aquaculture throughout the world, but despite its economic importance no work has been done on modeling epidemics of this pathogen. Therefore we developed a Reed-Frost epidemic model for WSSV in Litopenaeus vannamei. The model includes uninfected susceptible, latently infected, acutely infected, and dead infected shrimp. The source of new infections during an outbreak is considered to be dead infected shrimp. The transmission coefficient, patency coefficient, virulence coefficient, and removal coefficient (disappearance of dead infected shrimp) control the dynamics of the model. In addition, an explicit area parameter is included to help to clarify the distinction between density and absolute shrimp population size. An analysis of the model finds that as number of shrimp, initial dose, transmission coefficient, patency coefficient, virulence coefficient, or removal coefficient changes, the speed of the epidemic changes. The model predicts that a threshold density of susceptible shrimp exists below which an outbreak of WSSV will not occur. Only initial dose, transmission coefficient, removal coefficient, and area coefficient affect the predicted threshold density. Increases in the transmission coefficient reduce the threshold value, whereas increases in the other factors cause the threshold value to increase. Epidemic models may prove useful to the shrimp aquaculture industry by suggesting testable hypotheses, some of which may contribute to the eventual control of WSSV outbreaks. 相似文献
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Analysis of the inbreeding coefficient, its structure and dynamics in rural and urbanized populations of the Kostroma province was performed. The coefficient of inbreeding was estimated for the "old" and "new" villages via migration and isonymy, the values being 0.001185; 0.000786 and 0.001341; 0.000682, respectively. It follows from these data that there is a good agreement between the values of the inbreeding coefficient estimated by two different methods and that this coefficient is diminished doubly in "new" villages. The coefficient of inbreeding in small towns was estimated via isonymy. It is 0.000704 in ancient and 0,000229 in modern towns. The decrease in the inbreeding coefficient was more pronounced in towns, as compared to villages. 相似文献
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水文过程演变存在时间相依性,含有相依成分的水文序列无法满足水文计算中的一致性假设,给水问题研究带来诸多困难.针对水文序列相依性变异这一现象,以自相关模型为例,提出基于相关系数的水文相依性变异分级方法.该方法通过计算相依成分与原序列之间的相关系数,并选取合理的相关系数阈值,将相依性变异程度分为无变异、弱变异、中变异、强变异、巨变异.通过推导相关系数与序列各阶自相关系数之间的公式,说明相关系数主要取决于1阶到p阶自相关系数的大小,从而阐明分级方法的理论基础.以一阶和二阶自回归模型为例,利用统计试验验证了公式的合理性,并说明了相关系数和自相关系数的联系.将所提分级方法应用于3个实测径流序列进行分析,结果显示水文过程常常存在相依性与随机性并存的现象. 相似文献
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在文献基础上梳理了街道安全感影响因素,并采用上海样本检验了“街道眼”等西方街道安全理论。邀请30位学生和30位市民对上海5个不同发展时期社区的300张百度街景图片进行安全感评定。实验发现绿视率、管理程度、车道数等都对安全感起着显著作用,并分别建立了单双车道和多车道街道空间的安全感回归模型。其中发现绿视率(单双车道相关系数R=0.728,p<0.01;多车道相关系数R=0.471,p<0.01)、管理程度(单双车道相关系数R=0.766,p<0.01;多车道相关系数R=0.450,p<0.01)、车道数量因素(相关系数R=0.502,p<0.01)对安全感均有显著的积极作用,界面透明度(单双车道相关系数R=0.222,p<0.01)、独立自行车道(相关系数R=0.309,p<0.01)及设计美感(相关系数R=0.432,p<0.01)等因素在单双车道空间中具有积极影响,而助动车与自行车(单双车道相关系数R=-0.327,p<0.01;多车道相关系数R=-0.281,p<0.01)在对安全感知评价具有消极影响,机动车(单双车道相关系数R=0.251,p<0.01;多车道相关系数R=-0.327,p<0.01)在单双车道与多车道空间中呈现相反的作用。 相似文献
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The moments of bivariate normal distribution, which is truncated with respect to both the random variables, are obtained by using the orthogonal expansion of the distribution and the properties of HERMITE polynomials. In particular the correlation coefficient of the truncated distribution is derived in terms of the actual correlation coefficient. In order to study the effect of truncation tables have been prepared of this correlation coefficient for certain given values of the actual correlation coefficient and for a few selected values of the points of truncation. A listing of the computer program for this purpose is also given. 相似文献
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Comparison of skewness coefficient,coefficient of variation,and Gini coefficient as inequality measures within populations 总被引:6,自引:0,他引:6
Summary The moment skewness coefficient, coefficient of variation and Gini coefficient are contrasted as statistical measures of inequality among members of plant populations. Constructed examples, real data examples, and distributional considerations are used to illustrate pertinent properties of these statistics to assess inequality. All three statistics possess some undesirable properties but these properties are shown to be often unimportant with real data. If the underlying distribution of the variable follows the often assumed two-parameter lognormal model, it is shown that all three statistics are likely to be highly and positively correlated. In contrast, for distributions which are not two-parameter lognormally distributed, and when the distribution is not concentrated near zero, the coefficient of variation and Gini coefficient, which are sensitive to small shifts in the mean, are often of little practical use in ordering the equality of populations. The coefficent of variation is more sensitive to individuals in the right-hand tail of a distribution than is the Gini coefficient. Therefore, the coefficient of variation may often be recommended over the Gini coefficient if a measure of relative precision is selected to assess inequality. The skewness coeficient is suggested when the distribution is either three-parameter lognormally distributed (or close to such), or when a measure of relative precision is not indicated.Scientific Paper no 7830. College of Agriculture and Home Economics Research Center, Washington State University 相似文献