全文获取类型
收费全文 | 732篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 16篇 |
理论与方法论 | 26篇 |
现状及发展 | 447篇 |
研究方法 | 38篇 |
综合类 | 195篇 |
自然研究 | 26篇 |
出版年
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 14篇 |
2015年 | 11篇 |
2014年 | 18篇 |
2013年 | 13篇 |
2012年 | 22篇 |
2011年 | 47篇 |
2010年 | 21篇 |
2009年 | 7篇 |
2008年 | 27篇 |
2007年 | 15篇 |
2006年 | 20篇 |
2005年 | 30篇 |
2004年 | 16篇 |
2003年 | 23篇 |
2002年 | 17篇 |
2001年 | 18篇 |
2000年 | 11篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1993年 | 5篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 21篇 |
1978年 | 20篇 |
1977年 | 9篇 |
1976年 | 20篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 14篇 |
1972年 | 8篇 |
1971年 | 16篇 |
1970年 | 10篇 |
1969年 | 12篇 |
1968年 | 20篇 |
1967年 | 11篇 |
1966年 | 9篇 |
1965年 | 18篇 |
1964年 | 6篇 |
1954年 | 5篇 |
1948年 | 6篇 |
1947年 | 6篇 |
排序方式: 共有748条查询结果,搜索用时 31 毫秒
1.
In agglomerative hierarchical clustering, pair-group methods suffer from a problem of non-uniqueness when two or more distances
between different clusters coincide during the amalgamation process. The traditional approach for solving this drawback has
been to take any arbitrary criterion in order to break ties between distances, which results in different hierarchical classifications
depending on the criterion followed. In this article we propose a variable-group algorithm that consists in grouping more
than two clusters at the same time when ties occur. We give a tree representation for the results of the algorithm, which
we call a multidendrogram, as well as a generalization of the Lance andWilliams’ formula which enables the implementation of the algorithm in a recursive
way.
The authors thank A. Arenas for discussion and helpful comments. This work was partially supported by DGES of the Spanish
Government Project No. FIS2006–13321–C02–02 and by a grant of Universitat Rovira i Virgili. 相似文献
2.
Hormogón M 《Annals of science》1991,48(2):187-190
3.
4.
Summary Cyclic voltammetry of molecular oxygen in aprotic media (dimethylformamide) and in the presence of bilirubin and other bile pigments shows that superoxide anion (
) undergoes proton-induced dismutation. Lactam hydrogens of bile pigments are sufficiently acid to induce
disproportionation to O2 and H2O2. Because of its characteristic lipophilic behavior, a biological role for natural bilirubin similar to that of other non-enzymatic lipophilic scavengers of
is suggested. 相似文献
5.
B. Stankiewicz-Choroszucha J. Górski 《Cellular and molecular life sciences : CMLS》1978,34(3):357-358
Summary It was shown in the rat that mobilization of intramuscular triglyceride during exercise is controlled by the adrenergic system.Acknowledgments. The work was supported by the Polish Academy of Sciences, grant No. 10.4.2.01.3.2. 相似文献
6.
Summary Natural cell-mediated cytotoxicity of normal murine Peyer's patch cells against Ehrlich ascites carcinoma was found in a short-term51Cr release assay. Peyer's patch and lymph node cells showed natural cytotoxicity at approximately the same level.Acknowledgments. This work was supported by grant No. 10.5 from the Polish Academy of Sciences. 相似文献
7.
Summary The active calcium transport along the small intestine of adult rat was shown to exhibit daily rhythmic variations of considerable amplitude. Both the light-dark cycle and the time of food presentation acted as synchronizers of the rhythm observed.This work has been supported by Polish Academy of Sciences within the project MR II.1.1.6. 相似文献
8.
V. R. Osório E Castro M. G. P. Vale A. P. Carvalho 《Cellular and molecular life sciences : CMLS》1976,32(4):424-426
Summary The effect of the antibiotic X-537A on the phosphorylated ATPase (EP) was investigated. The results show that X-537A depresses the level of EP which is dependent on the Ca2+ gradient, while the Ca2+-independent EP is not affected.
Acknowledgments. This work was supported by grants from the Instituto de Alta Cultura of the Portuguese Ministry of Education (No. CB/2) and the Calouste Gulbenkian Foundation. 相似文献
9.
10.
Patricia A. Burrowes María Celeste Martes Mónica Torres-Ríos Ana V. Longo 《Journal of Natural History》2017,51(11-12):643-656
Pathogen-mediated changes in host behaviour can result from hosts altering their habitat preferences. Although infection risk with pathogenic fungus Batrachochytrium dendrobatidis in amphibians is associated with environments favouring its growth, the relationship with microhabitat use has not been examined. Here, we aim to determine if microhabitats used by frogs during their nocturnal activity predict B. dendrobatidis prevalence and infection intensity. Our focal host, Eleutherodactylus coqui, is a habitat generalist that uses multiple habitats from the forest floor to the canopy. We analysed data on B. dendrobatidis occurrence in 157 adults and 122 juveniles at El Yunque National forest in Puerto Rico. We categorized each individual’s nocturnal microhabitat as forest floor, curled palm fronds in the floor, arboreal bromeliads and foliage or tree trunks 50 cm to 2.5 m above ground. We found that frogs on the forest floor had the greatest B. dendrobatidis prevalence (73%), compared with those active in vegetation above ground (55%). Overall, the probability of B. dendrobatidis infection in frogs using microhabitats on the forest floor was twice as great as for those on arboreal substrates. Differences in B. dendrobatidis prevalence and intensity in E. coqui may be explained by specific abiotic conditions of microenvironments (temperature and humidity) affecting both pathogen and host, and by the age-specific ecological requirements of hosts. Adults were found to be most active in microhabitats where individuals had lower infection burdens, suggesting pathogen-modulated habitat choice. This work has important implications for the evolutionary dynamics of enzootic diseases and provides data that may inform potential mitigation strategies against a generalist amphibian pathogen. 相似文献