全文获取类型
收费全文 | 3379篇 |
免费 | 21篇 |
国内免费 | 7篇 |
专业分类
系统科学 | 56篇 |
教育与普及 | 1篇 |
理论与方法论 | 60篇 |
现状及发展 | 1924篇 |
研究方法 | 297篇 |
综合类 | 1000篇 |
自然研究 | 69篇 |
出版年
2020年 | 15篇 |
2018年 | 61篇 |
2017年 | 46篇 |
2016年 | 59篇 |
2015年 | 42篇 |
2014年 | 48篇 |
2013年 | 52篇 |
2012年 | 149篇 |
2011年 | 212篇 |
2010年 | 82篇 |
2009年 | 31篇 |
2008年 | 130篇 |
2007年 | 106篇 |
2006年 | 134篇 |
2005年 | 123篇 |
2004年 | 123篇 |
2003年 | 100篇 |
2002年 | 98篇 |
2001年 | 50篇 |
2000年 | 36篇 |
1999年 | 36篇 |
1992年 | 24篇 |
1991年 | 22篇 |
1990年 | 28篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 31篇 |
1986年 | 18篇 |
1985年 | 27篇 |
1984年 | 27篇 |
1982年 | 32篇 |
1981年 | 24篇 |
1980年 | 37篇 |
1979年 | 86篇 |
1978年 | 71篇 |
1977年 | 99篇 |
1976年 | 26篇 |
1975年 | 42篇 |
1974年 | 97篇 |
1973年 | 90篇 |
1972年 | 108篇 |
1971年 | 78篇 |
1970年 | 92篇 |
1969年 | 96篇 |
1968年 | 70篇 |
1967年 | 92篇 |
1966年 | 79篇 |
1965年 | 59篇 |
1964年 | 18篇 |
1960年 | 12篇 |
排序方式: 共有3407条查询结果,搜索用时 62 毫秒
1.
A physically based model for ground‐level ozone forecasting is evaluated for Santiago, Chile. The model predicts the daily peak ozone concentration, with the daily rise of air temperature as input variable; weekends and rainy days appear as interventions. This model was used to analyse historical data, using the Linear Transfer Function/Finite Impulse Response (LTF/FIR) formalism; the Simultaneous Transfer Function (STF) method was used to analyse several monitoring stations together. Model evaluation showed a good forecasting performance across stations—for low and high ozone impacts—with power of detection (POD) values between 70 and 100%, Heidke's Skill Scores between 40% and 70% and low false alarm rates (FAR). The model consistently outperforms a pure persistence forecast. Model performance was not sensitive to different implementation options. The model performance degrades for two‐ and three‐days ahead forecast, but is still acceptable for the purpose of developing an environmental warning system at Santiago. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
2.
Abnormal pattern detected in fragile-X patients by pulsed-field gel electrophoresis. 总被引:26,自引:0,他引:26
The fragile-X syndrome is the most frequent inherited form of mental retardation, with an incidence of 1 in 1,500 males. It is characterized by the presence of a fragile site at Xq27.3 induced in vitro by folate deprivation or by inhibitors of deoxynucleotide synthesis. Its mode of inheritance is unusual for an X-linked trait, with incomplete penetrance in both males and females. Some phenotypically normal males transmit the mutation to all their daughters who rarely express any symptoms, but penetrance is high in sons and daughters of these carrier women. Genetic and physical mapping of the Xq27-q28 region has confirmed that the disease locus is located at or very near the fragile site. Hypotheses proposed to account for the abnormalities in the inheritance of the disease include sequence rearrangements by meiotic recombination or a mutation that affects reactivation of an inactive X chromosome during differentiation of female germ cells. To detect such rearrangements, or methylation changes that may reflect a locally inactive X chromosome, we used pulsed-field gel analysis of DNA from fragile-X patients with probes close to the fragile-X locus. The probe Do33 (DXS465) detected abnormal patterns in fragile-X patients, but not in normal controls or in non-expressing male transmitters. 相似文献
3.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada. 相似文献
4.
Maria Luisa Dalla Chiara 《Foundations of Science》2002,7(1-2):1-9
In recent times, a particular attention has been devoted to thesignificance of Quantum Theory for other disciplines. The articlescollected in this issue discuss some interesting cases,characterized by an interaction between Quantum Theory andother fields. Some basic notrons of the mathematical formalismof the theory are here summarized. 相似文献
5.
Ramsés Fuenmayor 《Systemic Practice and Action Research》1991,4(5):473-490
Both an ontoepistemology for reductionist modern science (counter-ontoepistemology) and an ontology for interpretive Systemology have been outlined in the two preceding papers in this special issue ofSystems Practice. In the present article, the notion of “truth” is interpreted in terms of both the ontoepistemology of “reductionism” and the ontology of interpretive systemology. Both interpretations are discussed. Such a discussion represents the objective of this paper, that is, to outline the epistemological “face” of the ontoepistemology of interpretive systemology. In order to design that “epistemological face,” the relation between ontology and epistemology must be clarified. Such a relation is seen from the standpoint already provided by the ontology. After the discussion on the notion of truth, the general shape of a systemic-interpretive inquiring process is outlined. 相似文献
6.
7.
This paper stresses the restrictive nature of the standard unit root/cointegration assumptions and examines a more general type of time heterogeneity, which might characterize a number of economic variables, and which results in parameter time dependence and misleading statistical inference. We show that in such cases ‘operational’ models cannot be obtained, and the estimation of time‐varying parameter models becomes necessary. For instance, economic processes subject to endemic change can only be adequately modelled in a state space form. This is a very important point, because unstable models will break down when used for forecasting purposes. We also discuss a new test for the null of cointegration developed by Quintos and Phillips (1993), which is based on parameter constancy in cointegrating regressions. Finally, we point out that, if it is possible to condition on a subset of superexogenous variables, parameter instability can be handled by estimating a restricted system. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
8.
J. P. Barthélemy 《Journal of Classification》1988,5(1):85-87
The theorem of the paper Aggregation of Equivalence Relations, by Fishburn and Rubinstein, states a result already known. This theorem improves a result from Mirkin (1975) and appears as a corollary occurring in Leclerc (1984).
Resume L'unique théorème de l'article Aggregation of Equivalence Relations de Fishburn et Rubinstein est déja connu. Il améliore, en fait, un résultat de Mirkin (1975) et apparait en tant que corollaire dans Leclerc (1984).相似文献
9.
D. Porquet M. Appel T. Fournier O. Bertaux D. Biou J. Féger 《Cellular and molecular life sciences : CMLS》1992,48(3):257-261
Both in vivo and in vitro models have certain disadvantages for the study of the chronic hepatotoxicity of drugs. The aim of this work was to evaluate a new approach based on an in vivo/in vitro model. After chronic in vivo treatment of rats with Vincamine and Vindeburnol (an eburnamenine derivative which exhibits hepatotoxic properties in man) liver cells were isolated, and functional and metabolic disorders (metabolic utilization of fructose and protein biosynthesis) were studied to determine injury. The results showed no modification of blood parameters, but a direct relationship between the dose of Vindeburnol administered in vivo and the metabolic disorders observed in vitro, evidencing the high sensitivity and reliability of this model. 相似文献
10.
The median procedure for n-trees 总被引:2,自引:2,他引:0
Let (X,d) be a metric space The functionM:X
k 2
x
defined by
is the minimum } is called themedian procedure and has been found useful in various applications involving the notion of consensus Here we present axioms that characterizeM whenX is a certain class of trees (hierarchical classifications), andd is the symmetric difference metricWe would like to thank the referees and Editor for helpful comments 相似文献