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《Endeavour》1987,11(2):67-72
In the Mediterranean region there is a marked discontinuity between the fossil remains found in Miocene deposits and those in the more recent Pliocene deposits which overlay them. There exist nothing in the way of missing links to bridge the gap. Deep drilling has revealed that the explanation is that the earlier fauna derived from an Indo-Pacific ocean became extinct when the Mediterranean dried up; the later ones came with an Atlantic inundation. 相似文献
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基于第一性原理总能量平面波全势方法,采用广义梯度近似（ＧＧＡ）和局域密度近似（ＬＤＡ）等不同的交换关联势,计算了体心结构ＰｂＳ的总能,并给出了体系平衡时的晶格常数．讨论了不同交换关联势对能带结构、态密度、分态密度及能隙的影响．与面心结构ＰｂＳ结果对比,能量的计算结果表明,面心结构ＰｂＳ较体心结构更稳定．另外,体心结构ＰｂＳ的能带计算结果表明存在能隙,这与同族铅盐化合物中存在能隙的实验结论一致．上述ＰｂＳ的计算结果有助于实验研究． 相似文献
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应用相对论多通道理论,计算锂原子1s2ns2S,1s2np2P,1s2nd 2D和1s2nf2F系列束缚态和相邻连续态的本征量子数亏损,并拟合得到量子亏损函数,进而计算1s2np2P系列n=7 ～60的电离能和量子数亏损,计算结果与最新的实验值相符合.采用相对论多组态相互作用方法计算锂原子1s22s→1s2np1/2偶极跃迁光学振子强度,结果表明,通过包含单、双电子激发组态考虑电子关联作用,计算结果有很大的改善. 相似文献
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中国东西部地区发展差距拉大的原因分析 总被引：2，自引：0，他引：2
根据梯度转移理论,不同国家或地区间存在着高梯度产业地区和低梯度产业地区,高梯度地区的产业会自发地向低梯度地区转移.而对我国东西部整体经济和各项经济要素进行分析后,发现东西部地区经济差距不仅没有缩小,而且东部经济极化效应愈加明显,东部的低梯度产业向西部转移的力度很小,相反西部地区资本、自然资源和廉价的劳动力等要素加速向东部地区集中,进一步减缓了东部传统产业的成本压力,使其竞争优势得以继续保持,延长了东部低梯度产业的生命周期,同时延缓了西部经济发展的步伐,造成东西部差距进一步扩大. 相似文献
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合法性匹配效应与创新生态系统构建 《科学学研究》2019,37(11):2064-2072
创新活动的复杂性和不确定性导致大多数不连续创新无法孤立完成,企业需从外部获取互补性资源以保障新产品或新技术的开发。在搜寻与获取互补性资源的过程中,各创新主体逐渐联结,并开启创新生态系统构建的过程。除联结各类型互补资源提供者外,创新生态系统的构建过程亦是与外部环境不断互动以获取合法性的过程。那么,在不连续创新背景下,企业应当如何构建创新生态系统。通过对以盛大盒子为核心产品的创新生态系统进行案例研究,发现：1）过高的创新不连续性对创新生态系统的构建产生负向影响,2）在创新生态系统内部,创新不连续性主要通过匹配效应作用于系统构建过程,即企业需搜寻、联结与创新活动匹配度较高的互补企业;3）在创新生态系统外部,创新不连续性主要通过合法性作用于系统的构建过程,即创新生态系统构建过程中需不断与外部环境互动,并获得相应的合法性。 相似文献
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Ralph Bown 《Journal of The Franklin Institute》1917,183(1):41-60
Not many years ago it was quite generally believed that iron was unable to follow rapid magnetic changes. Experiments which showed an apparent decrease in the permeability of the iron with an increase in the frequency of the magnetic cycle furnished a basis for a theory that iron was magnetically sluggish. Further and more accurate experiments proved, however, that the effects which had previously been ascribed to a peculiarity of the material were in reality caused by eddy currents in the sample. Theoretical calculations were made which demostrated that eddy currents in an iron test piece increased as the square of the frequency and that for even the lower frequencies it was necessary to use quite thin laminations in magnetic circuits in order to eliminate deleterious effects. Furthermore, it was found that due to eddy currents and the magnetic properties of iron, the magnetization in high frequency fields was confined to a thin surface layer of the piece. This “Magnetic Skin Effect” reduced the cross section of the iron which was magnetically active even though the laminations were extremely thin. Careful experimental measurements compared with theoretical calculations proved that the real permeability of iron remained unchanged at frequencies up to about 106 and that previous results had been is serious error due to neglect of the factors mentioned. This fact having been established, efforts were made to see what practical use could be made of iron in high frequency work and to that end some extensive experimental investigations of the saturation curves and core losses were made upon specimens laminated as thinly as was commercially practicable. The resulting data have furnished a basis for design.It is a demostrated fact that the permeability of all metals is unity for the magnetic cycles imposed upon them by heat and light waves. In the region between frequencies of about 106, where the true permeability of iron is practically the same as at zero frequency, and frequencies of about 1010 where the true permeability of iron approaches unity, the experimental values of μ decrease smoothly with the frequency. What happens to μ in the range of frequencies between the lingest heat waves and the shortest Hertzian waves which have yet been made is a question which has many interesting features but which has not yet yielded to the experimenter. 相似文献
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Abolhassan Razminia Mehdi Asadizadehshiraz Hamid Reza Shaker 《Journal of The Franklin Institute》2019,356(3):1590-1603
In this paper, a generalized trajectory tracking problem for a closed-loop control system is formulated in the optimal control context. A linear time varying plant is considered to track a closed-loop desired trajectory generated by a given mechanism. The theoretical results are obtained based on the Hamilton-Jacobi-Bellman theory in which some generalized semiquadratic value functions are employed as the Lagrangian. In addition, we employ a non-integer order integral of Riemann-Liouville type as the cost functional, so that the trajectory tracking process can be evaluated in an extended optimum manner wherein the fractionality plays the main role. By selecting a suitable fractional order of the integral, a satisfactory optimal control system can be deduced in which least concentration on selecting the weighting matrices is needed. To show the effectiveness of the results, some numerical examples illustrate the potentials. 相似文献
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政府研发投入对中国大学技术转移合同的影响——基于三螺旋理论的视角 总被引：1，自引：0，他引：1
三螺旋理论强调政府、企业和大学的协同演化和互动,受到了世界各国政府、企业和大学的高度重视,政府在企业—大学互动中的作用更是三螺旋理论讨论的热点问题。从三螺旋理论的视角,以我国大学2003—2010年的省级面板数据为依据,通过实证分析,研究了政府研发投入对大学技术转移合同的影响。研究发现,各省市大学获得的政府科技经费投入非常显著地促进了该省市大学技术转移合同数量和收入两方面的增长;并且,国家科技支撑计划、863计划、国家自然科学基金项目和973计划的数量也是大学技术转移合同数量/收入的促进因素。 相似文献
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解学梅 《科学技术与辩证法》2011,28(1):95-99
在分析协同学基本原理基础上剖析了都市圈协同创新的内在机理;分析了都市圈协同创新的序参量、控制参量和自组织演化过程;指出都市圈协同创新系统是不同创新子系统间协同及系统内各要素的相互作用,形成企业、政府、中介机构、研究组织为序参量的有序集合,其中非线性机制是都市圈协同创新系统要素耦合的根本机制;放大涨落是都市圈协同创新系统的基本效应。由此,为提升都市圈区域创新能力,需遵循协同学机理,构建都市圈跨区域的协同创新网络,形成有序的自组织创新机制。 相似文献
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高技术产业与区域经济之间共同演化水平的高低直接关系到经济的可持续发展。本文基于共同演化理论,构建我国高技术产业系统与所在区域经济系统的演化模型;并运用协同学的分析方法,选取相应的序变量,形成两系统共同演化水平评价指标体系,采用全国30个省市(西藏除外)的数据,对我国各地区两系统的演化水平进行实证分析。研究发现:不同地区两系统的共同演化水平呈现显著的四梯度分布,演化过程表现为持续提升和往复波动并存,总体演化水平较低且东西部差距明显。 相似文献
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目前,京津冀区域医药产业已经形成一定规模的专利合作网络,该网络结构及其对医药企业专利产出的影响有待进一步探索。基于2011至2015年间的专利数据,以京津冀区域医药上市企业为搜索起点,依据组织间共同申请专利行为获取区域专利合作网络;在此基础上采用社会网络以及渗流理论分析方法解析该网络结构;最后结合上市企业年报数据实证该网络结构对上市企业专利产出的影响。结果显示京津冀区域医药产业专利合作网络整体以及北京地区子网和天津地区子网都是连通的,并且该网络整体的连通性主要由北京和天津地区所属结点来维持;该区域上市医药企业专利产出受到企业自身拥有的子网内以及子网间连接数量的正向影响,受其所属子网内部连通程度的负向影响。 相似文献
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We investigate in the context of crowdsourcing how seekers can increase open innovation performance, measured as received solver attention, by making two strategic decisions: selecting innovation tasks that are well suited for crowdsourcing and choosing between the potentials of status signalling through identity disclosure versus enjoying the benefits of anonymity. Drawing on uncertainty reduction theory, we suggest that a well-articulated problem statement reduces uncertainties of potential solvers and increases their willingness to participate. We argue that the ability of seekers to draft high-quality problem statements depends on the distance between the problem domain and their current knowledge stock. An analysis of 637 crowdsourcing projects finds that problem-seeker knowledge distance and received solver attention are curvilinear related such that moderate levels of knowledge distance maximize solver participation. However, high-status seekers who engage in identity-based status signalling are able to benefit from crowdsourcing across all levels of problem-seeker knowledge distance. 相似文献
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基于创新过程技术间断性,把技术创新失败分为工艺创新问题、结构创新问题和核心技术创新问题,利用来自国内362个企业的样本分析工艺创新间断、结构创新间断和核心技术创新间断对创新失败的影响。结果表明,工艺创新间断、结构创新间断和核心技术创新间断对创新失败有显著影响,它们是造成技术创新失败的主要因素;在影响程度上,三类技术间断对创新失败的影响存在差异。 相似文献
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Classical test theory offers theoretically derived reliability measures such as Cronbach’s alpha, which can be applied to measure the reliability of a set of Information Retrieval test results. The theory also supports item analysis, which identifies queries that are hampering the test’s reliability, and which may be candidates for refinement or removal. A generalization of Classical Test Theory, called Generalizability Theory, provides an even richer set of tools. It allows us to estimate the reliability of a test as a function of the number of queries, assessors (relevance judges), and other aspects of the test’s design. One novel aspect of Generalizability Theory is that it allows this estimation of reliability even before the test collection exists, based purely on the numbers of queries and assessors that it will contain. These calculations can help test designers in advance, by allowing them to compare the reliability of test designs with various numbers of queries and relevance assessors, and to spend their limited budgets on a design that maximizes reliability. Empirical analysis shows that in cases for which our data is representative, having more queries is more helpful for reliability than having more assessors. It also suggests that reliability may be improved with a per-document performance measure, as opposed to a document-set based performance measure, where appropriate. The theory also clarifies the implicit debate in IR literature regarding the nature of error in relevance judgments. 相似文献
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在一刀切的专利法律制度背景下,专利货币型奖励政策的可操作性强,靶向性可控,对优化专利制度作用突出,但政策制定尚缺乏完整的理论依据与有效量化工具。从专利弹性视角对此进行探讨,通过研究专利弹性与净损失之间的关系,建立了净损失率评价模型,这是弹性研究在专利领域的尝试,从而使深入挖掘专利奖励潜能,提升专利奖励相对效率,实现降低净损失率的目标有据可依,并以中国专利申请数据做了实证,检验中国专利制度执行以来绩效,证明本方法切实可行、有效。 相似文献
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An examination has been made of the rate of death of euglena cells when exposed to the alpha particle radiation from radium emanation dissolved in the containing liquid. It was found that the rate of death of the cells was pro. portional to the strength of the dissolved emanation and to the number of live euglena cells present, a conclusion in harmony with the view that death of a cell is caused by a single impact of some sensitive region with an alpha particle. The theory of the case where two impacts of the sensitive region with an alpha particle are necessary to cause death, has been worked out with a generality which allows for a recovery of the cells between the two impacts.It is found that the results of the experiment are inconsistent with such a hypothesis.From the experimental data, it is possible to calculate the size of the sensitive entity which when struck by the alpha particle causes death to the cell. The diameter of this entity comes out about equal to that of the nucleus of the euglena cell. 相似文献
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本文基于代理理论研究投资区域在阶段投资对风险投资机构投资绩效影响中的调节作用。以2000年~2008年间首次接受风险投资的991家创业企业为样本,使用logit回归模型,本文研究发现:当风险投资机构投资于本地(本省或本市)的创业企业时,总投资轮次增加对投资绩效有显著的负向影响;而当风险投资机构跨区域投资时,总投资轮次增加对投资绩效有显著的正向影响。上述结论表明,阶段投资的相对收益和相对成本会随着投资区域的不同而发生变化,风险投资机构在决定是否通过阶段投资来降低代理成本时需考虑投资区域这一情境因素。 相似文献
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快速城镇化的战略背景下,农户内部逐渐产生了分化现象而不再同质,这势必将深刻影响他们的土地流转行为,而土地的多重功能意味着农户的土地依赖很可能在其中发挥了重要作用。因此,为探究农户社会阶层对其土地流转行为的影响机制,本文以韦伯的社会分层理论为研究视角,构建起农户社会阶层影响土地流转行为的分析框架,并基于中国家庭追踪调查（CFPS）三期面板数据,应用中介效应检验模型进行了实证检验。研究发现：①农户社会阶层显著促进了土地流转行为的发生,平均促进效应为4.05%。②农户的土地依赖在社会阶层与土地流转行为之间的因果关系中发挥着中介作用,其中,就业依赖与收入依赖的中介效应分别约为17.26%与16.00%。③农户社会阶层与土地流转行为之间的因果关系存在地区异质性,影响效应呈现出“西部>全国>中部”的态势,东部则无影响。本文结论不仅深化了有关农户社会阶层、土地依赖与土地流转行为之间关系的认知,丰富了既有理论研究,而且为推动农村土地流转市场的发展提供了政策启示。 相似文献