全文获取类型
收费全文 | 94069篇 |
免费 | 350篇 |
国内免费 | 879篇 |
专业分类
95298篇 |
出版年
2023年 | 12篇 |
2022年 | 26篇 |
2021年 | 29篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 11856篇 |
2017年 | 10683篇 |
2016年 | 7471篇 |
2015年 | 627篇 |
2014年 | 326篇 |
2013年 | 363篇 |
2012年 | 4264篇 |
2011年 | 12858篇 |
2010年 | 12036篇 |
2009年 | 8251篇 |
2008年 | 9838篇 |
2007年 | 11402篇 |
2006年 | 292篇 |
2005年 | 540篇 |
2004年 | 1013篇 |
2003年 | 1059篇 |
2002年 | 810篇 |
2001年 | 288篇 |
2000年 | 186篇 |
1999年 | 45篇 |
1998年 | 15篇 |
1997年 | 30篇 |
1996年 | 20篇 |
1994年 | 10篇 |
1993年 | 36篇 |
1992年 | 38篇 |
1991年 | 43篇 |
1990年 | 10篇 |
1989年 | 13篇 |
1988年 | 24篇 |
1987年 | 19篇 |
1984年 | 7篇 |
1983年 | 21篇 |
1979年 | 7篇 |
1978年 | 13篇 |
1975年 | 9篇 |
1972年 | 252篇 |
1971年 | 276篇 |
1970年 | 16篇 |
1969年 | 10篇 |
1968年 | 11篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Extreme learning machine (ELM) is a novel and fast learning method to train single layer feed-forward networks. However due to the demand for larger number of hidden neurons, the prediction speed of ELM is not fast enough. An evolutionary based ELM with differential evolution (DE) has been proposed to reduce the prediction time of original ELM. But it may still get stuck at local optima. In this paper, a novel algorithm hybridizing DE and metaheuristic coral reef optimization (CRO), which is called differential evolution coral reef optimization (DECRO), is proposed to balance the explorative power and exploitive power to reach better performance. The thought and the implement of DECRO algorithm are discussed in this article with detail. DE, CRO and DECRO are applied to ELM training respectively. Experimental results show that DECRO-ELM can reduce the prediction time of original ELM, and obtain better performance for training ELM than both DE and CRO. 相似文献
992.
Gabriela Castaño-Meneses 《Arthropod-Plant Interactions》2016,10(5):403-417
The canopy of forests has been considered “the last biotic frontier,” and study of its elements is very important in explaining the global functionality in ecosystems. Epiphytic plants and arthropods are essential elements in canopy habitats, and their relationships have been studied in order to understand the high diversity in tropical forests. Nevertheless, there are few studies on this development in temperate forests. The arthropod community was studied during the rainy and dry seasons at two altitudes, and a total of 240 T. violacea plants of three sizes were collected from Abies religiosa and Quercus spp. host trees. A total of 163,043 arthropods were collected and about 200 morphospecies identified. The highest abundance was obtained during the dry season, while high diversity was found during the rainy season. There was a significant effect of plant size, host trees and collecting season on abundance and diversity, and there were seasonal variations in community composition. The community hosted on A. religiosa epiphytes showed higher abundance and density than that of Quercus. 相似文献
993.
994.
Ariel Cohen 《Morphology》2016,26(1):91-103
-Er nominals usually obey the External Argument Generalization: the argument of the nominal receives the thematic role that the verb assigns to its external argument. I argue that this syntactic generalization, as well as the exceptions to it, can be explained by semantics.Specifically, -er nominals usually express properties that are inherent in the subject; I argue that they therefore belong to the class of expressions that express such properties—dynamic modals. Crucially, dynamic modals are subject oriented; hence, the proposal that -er nominals are dynamic modals naturally provides an account of the External Argument Generalization. 相似文献
995.
Stephanie J. Peacock Andrew W. Bateman Martin Krkošek Mark A. Lewis 《Theoretical Ecology》2016,9(3):365-380
The dynamics of coupled populations have mostly been studied in the context of metapopulation viability with application to, for example, species at risk. However, when considering pests and pathogens, eradication, not persistence, is often the end goal. Humans may intervene to control nuisance populations, resulting in reciprocal interactions between the human and natural systems that can lead to unexpected dynamics. The incidence of these human-natural couplings has been increasing, hastening the need to better understand the emergent properties of such systems in order to predict and manage outbreaks of pests and pathogens. For example, the success of the growing aquaculture industry depends on our ability to manage pathogens and maintain a healthy environment for farmed and wild fish. We developed a model for the dynamics of connected populations subject to control, motivated by sea louse parasites that can disperse among salmon farms. The model includes exponential population growth with a forced decline when populations reach a threshold, representing control interventions. Coupling two populations with equal growth rates resulted in phase locking or synchrony in their dynamics. Populations with different growth rates had different periods of oscillation, leading to quasiperiodic dynamics when coupled. Adding small amounts of stochasticity destabilized quasiperiodic cycles to chaos, while stochasticity was damped for periodic or stable dynamics. Our analysis suggests that strict treatment thresholds, although well intended, can complicate parasite dynamics and hinder control efforts. Synchronizing populations via coordinated management among farms leads to more effective control that is required less frequently. Our model is simple and generally applicable to other systems where dispersal affects the management of pests and pathogens. 相似文献
996.
Coexistence and emergent neutrality generate synchrony among competitors in fluctuating environments
Many competitive communities exhibit a puzzling amount of species diversity. In this study, we model a community of symmetric competitors in a fluctuating environment. We use biologically realistic temperature-dependent growth curves with a widely hypothesized trade-off between maximum growth and nice breadth to control the shapes of the curves of different species. We perform three analyses of the community dynamics to investigate the role of environmental fluctuations in community composition and species diversity. We initiate communities with equal abundances of all species and randomize the temperature fluctuations so that there is no correlation between species responses, only noise. We initiate single populations and allow other species to randomly invade the community. We also knock out extant species one by one from an established community and allow them to reinvade after the remaining species have adjusted. We find that competitors with sufficiently different temperature niches coexist via temporal niche differentiation. We also find long-term persistence of species that are very similar to a dominant competitor. This creates communities with species clumped along a temperature niche axis, with stable coexistence between groups and near neutrality within groups. The near neutrality results in interspecific synchrony within the groups, providing an explanation for the maintenance of high diversity in competitive communities where synchrony is commonly observed. 相似文献
997.
Jordan L. Woehl Daisuke Takahashi Alvaro I. Herrera Brian V. Geisbrecht Om Prakash 《Biomolecular NMR assignments》2016,10(2):301-305
The pathogenic bacterium Staphylococcus aureus has evolved to actively evade many aspects of the human innate immune system by expressing a series of secreted inhibitory proteins. Among these, the extracellular adherence protein (Eap) has been shown to inhibit the classical and lectin pathways of the complement system. By binding to complement component C4b, Eap is able to inhibit formation of the CP/LP C3 pro-convertase. Secreted full-length, mature Eap consists of four ~98 residue domains, all of which adopt a similar beta-grasp fold, and are connected through a short linker region. Through multiple biochemical approaches, it has been determined that the third and fourth domains of Eap are responsible for C4b binding. Here we report the backbone and side-chain resonance assignments of the 11.3 kDa fourth domain of Eap. The assignment data has been deposited in the BMRB database under the accession number 26726. 相似文献
998.
999.
The attractiveness of herbivore-induced plant volatiles (HIPVs) from a specific plant species to natural enemies has been well established. However, under natural conditions and polycultural agriculture systems, the interactions among trophic levels are thought to be more complex. For instance, complex mixtures of volatiles emitted from diverse host plant species infested by polyphagous herbivores might affect responses of natural enemies. In this study, we investigated whether a mixture of HIPVs emitted from herbivore-damaged multiple host plant species affect responses of a predatory bug. Therefore, we report (1) olfactory responses of the predatory bug (Orius strigicollis) to volatiles emitted from cotton bollworm (Helicoverpa armigera) first instar larvae-damaged multiple plant species (tomato, French bean and sweet corn), (2) chemical analyses of volatiles emitted from the three plant species exposed to different treatments and (3) olfactory responses of the predators to a reconstituted HIPV blend from multiple plant species based on chemical analyses. O. strigicollis significantly preferred volatiles emanating from H. armigera-damaged multiple plant species to volatiles emanating from a single plant species. In all the three plant species, H. armigera-damaged seedlings emitted significantly a greater amount of volatiles as well as a larger number of volatile compounds than an undamaged or a mechanically injured seedling. The predators preferred the reconstituted HIPVs from multiple plant species to the reconstituted HIPVs from a single plant species. Thus, the mixture of HIPVs from multiple plant species enhanced the attractiveness to the predators. 相似文献
1000.
Amy Rose Deal 《Morphology》2016,26(3-4):313-339
This paper analyzes two patterns of number marking in the DP in Nez Perce (Sahaptian) within the framework of Distributed Morphology. The first involves under-realization of plural on nouns. Number has classically been understood as a feature inherent to nouns, rather than to adjectives that modify them. In Nez Perce, however, only a small set of nouns show number morphology, whereas number morphology is highly productive on adjectival modifiers. Adjectives in fact may realize the plural more than once per word—an instance of multiple exponence. I show that the puzzle of under-realization for nouns can be solved through conditioned allomorphy, providing new evidence for the presence of gender features on n (Lowenstamm 2007; Kramer 2014). The puzzle of multiple exponence (over-realization) for adjectives can be solved through Local Dislocation (Embick and Noyer 2001) combined with very late insertion of reduplicative content (Haugen 2008, 2011), demonstrating one way the DM architecture may produce multiple exponence without recourse to dedicated mechanisms. 相似文献