全文获取类型
收费全文 | 1929篇 |
免费 | 9篇 |
国内免费 | 18篇 |
专业分类
系统科学 | 33篇 |
丛书文集 | 2篇 |
教育与普及 | 11篇 |
理论与方法论 | 49篇 |
现状及发展 | 229篇 |
研究方法 | 309篇 |
综合类 | 1214篇 |
自然研究 | 109篇 |
出版年
2021年 | 6篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 15篇 |
2017年 | 20篇 |
2016年 | 21篇 |
2015年 | 15篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 173篇 |
2011年 | 296篇 |
2010年 | 72篇 |
2009年 | 14篇 |
2008年 | 135篇 |
2007年 | 201篇 |
2006年 | 163篇 |
2005年 | 177篇 |
2004年 | 159篇 |
2003年 | 150篇 |
2002年 | 164篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 11篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1985年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1968年 | 4篇 |
1967年 | 2篇 |
1966年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有1956条查询结果,搜索用时 522 毫秒
201.
Inada N Oguri M Pindor B Hennawi JF Chiu K Zheng W Ichikawa S Gregg MD Becker RH Suto Y Strauss MA Turner EL Keeton CR Annis J Castander FJ Eisenstein DJ Frieman JA Fukugita M Gunn JE Johnston DE Kent SM Nichol RC Richards GT Rix HW Sheldon ES Bahcall NA Brinkmann J Ivezić Z Lamb DQ McKay TA Schneider DP York DG 《Nature》2003,426(6968):810-812
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model. 相似文献
202.
Reticular synthesis and the design of new materials 总被引:18,自引:0,他引:18
The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal-oxygen-carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized. 相似文献
203.
The phytochromes are a family of plant photoreceptor proteins that control several adaptive developmental strategies. For example, the phytochromes perceive far-red light (wavelengths between 700 and 800 nm) reflected or scattered from the leaves of nearby vegetation. This provides an early warning of potential shading, and triggers a series of 'shade-avoidance' responses, such as a rapid increase in elongation, by which the plant attempts to overgrow its neighbours. Other, less immediate, responses include accelerated flowering and early production of seeds. However, little is known about the molecular events that connect light perception with increased growth in shade avoidance. Here we show that the circadian clock gates this rapid shade-avoidance response. It is most apparent around dusk and is accompanied by altered expression of several genes. One of these rapidly responsive genes encodes a basic helix-loop-helix protein, PIL1, previously shown to interact with the clock protein TOC1 (ref. 4). Furthermore PIL1 and TOC1 are both required for the accelerated growth associated with the shade-avoidance response. 相似文献
204.
Daniel S. Yeung Wing W. Y. Ng Aki P. F. Chan Patrick P. K. Chan Michael Firth Eric C. C. Tsang 《系统科学与系统工程学报(英文版)》2007,16(2):166-180
Company bankruptcies cost billions of dollars in losses to banks each year. Thus credit risk prediction is a critical part of a bank's loan approval decision process. Traditional financial models for credit risk prediction are no longer adequate for describing today's complex relationship between the financial health and potential bankruptcy of a company. In this work, a multiple classifier system (embedded in a multiple intelligent agent system) is proposed to predict the financial health of a company. In our model, each individual agent (classifier) makes a prediction on the likelihood of credit risk based on only partial information of the company. Each of the agents is an expert, but has limited knowledge (represented by features) about the company. The decisions of all agents are combined together to form a final credit risk prediction. Experiments show that our model out-performs other existing methods using the benchmarking Compustat American Corporations dataset. 相似文献
205.
This is the first report of Clark’s Nutcrackers ( Nucifraga columbiana ) harvesting seeds from the cones of sugar pine ( Pinus lambertiana ), based on observations over several years in Crater Lake National Park, Oregon. The harvesting period in late September corresponded to peak cone-opening dates. Nutcrackers placed harvested seeds in their throat pouches, a behavior associated with seed caching. 相似文献
206.
K-means clustering is arguably the most popular technique for partitioning data. Unfortunately, K-means suffers from the well-known
problem of locally optimal solutions. Furthermore, the final partition is dependent upon the initial configuration, making
the choice of starting partitions all the more important. This paper evaluates 12 procedures proposed in the literature and
provides recommendations for best practices. 相似文献
207.
Michael Hofreiter Torsten Schöneberg 《Cellular and molecular life sciences : CMLS》2010,67(15):2591-2603
Variation in pigmentation is one of the most conspicuous phenotypic traits in vertebrates. Although mammals show less variation
in body pigmentation than other vertebrate groups, the genetics of colour determination and variation is best understood for
them. More than 150 genes have been identified that influence pigmentation, and in many cases, the cause for variation in
pigmentation has been identified down to the underlying nucleotide changes. These studies show that while some genes are often
responsible for deviating pigmentation, similar or almost identical phenotypes even in the same species may be due to mutations
in different genes. In this review we will first discuss the current knowledge about the genes and their functions underlying
the biochemical pathways that determine pigmentation and then give examples where the mutations responsible for colour variation
have been determined. Finally, we will discuss potential evolutionary causes for and consequences of differences in pigmentation
between individuals. 相似文献
208.
Maria C. Shina Annette Müller-Taubenberger Can Ünal Michael Schleicher Michael Steinert Ludwig Eichinger Rolf Müller Rosemarie Blau-Wasser Gernot Glöckner Angelika A. Noegel 《Cellular and molecular life sciences : CMLS》2011,68(2):303-313
Dictyostelium discoideum harbors a short (CRN12) and a long coronin (CRN7) composed of one and two beta-propellers, respectively. They are primarily
present in the cell cortex and cells lacking CRN12 (corA
−) or CRN7 (corB
−) have defects in actin driven processes. We compared the characteristics of a mutant cell line (corA
−
/corB
−) lacking CRN12 and CRN7 with the single mutants focusing on cytokinesis, phagocytosis, chemotaxis and development. Cytokinesis,
uptake of small particles, and developmental defects were not enhanced in the corA
−
/corB
− strain as compared to the single mutants, whereas motility and phagocytosis of yeast particles were more severely impaired.
It appears that although both proteins affect the same processes they do not act in a redundant manner. Rather, they often
act antagonistically, which is in accordance with their proposed roles in the actin cytoskeleton where CRN12 acts in actin
disassembly whereas CRN7 stabilizes actin filaments and protects them from disassembly. 相似文献
209.
Accurate modelling of volatility (or risk) is important in finance, particularly as it relates to the modelling and forecasting of value‐at‐risk (VaR) thresholds. As financial applications typically deal with a portfolio of assets and risk, there are several multivariate GARCH models which specify the risk of one asset as depending on its own past as well as the past behaviour of other assets. Multivariate effects, whereby the risk of a given asset depends on the previous risk of any other asset, are termed spillover effects. In this paper we analyse the importance of considering spillover effects when forecasting financial volatility. The forecasting performance of the VARMA‐GARCH model of Ling and McAleer (2003), which includes spillover effects from all assets, the CCC model of Bollerslev (1990), which includes no spillovers, and a new Portfolio Spillover GARCH (PS‐GARCH) model, which accommodates aggregate spillovers parsimoniously and hence avoids the so‐called curse of dimensionality, are compared using a VaR example for a portfolio containing four international stock market indices. The empirical results suggest that spillover effects are statistically significant. However, the VaR threshold forecasts are generally found to be insensitive to the inclusion of spillover effects in any of the multivariate models considered. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
210.
Michael C. Jackson 《Systemic Practice and Action Research》1990,3(2):177-182
This brief article looks at the influence of russ Ackoff's work, using as a focus of attention a maxim of the poet and philosopher William Blake. In particular, it considers Ackoff s contribution to the development of community operational research. 相似文献