全文获取类型
收费全文 | 10931篇 |
免费 | 102篇 |
国内免费 | 60篇 |
专业分类
系统科学 | 97篇 |
丛书文集 | 90篇 |
教育与普及 | 13篇 |
理论与方法论 | 85篇 |
现状及发展 | 5327篇 |
研究方法 | 633篇 |
综合类 | 4685篇 |
自然研究 | 163篇 |
出版年
2018年 | 98篇 |
2017年 | 88篇 |
2016年 | 99篇 |
2014年 | 88篇 |
2013年 | 89篇 |
2012年 | 220篇 |
2011年 | 377篇 |
2010年 | 128篇 |
2009年 | 79篇 |
2008年 | 257篇 |
2007年 | 262篇 |
2006年 | 245篇 |
2005年 | 253篇 |
2004年 | 334篇 |
2003年 | 219篇 |
2002年 | 216篇 |
2001年 | 346篇 |
2000年 | 332篇 |
1999年 | 249篇 |
1992年 | 141篇 |
1991年 | 149篇 |
1990年 | 138篇 |
1989年 | 121篇 |
1988年 | 121篇 |
1987年 | 153篇 |
1986年 | 135篇 |
1985年 | 189篇 |
1984年 | 150篇 |
1983年 | 109篇 |
1982年 | 115篇 |
1981年 | 145篇 |
1980年 | 149篇 |
1979年 | 349篇 |
1978年 | 279篇 |
1977年 | 273篇 |
1976年 | 177篇 |
1975年 | 199篇 |
1974年 | 301篇 |
1973年 | 265篇 |
1972年 | 295篇 |
1971年 | 297篇 |
1970年 | 367篇 |
1969年 | 311篇 |
1968年 | 270篇 |
1967年 | 295篇 |
1966年 | 258篇 |
1965年 | 211篇 |
1958年 | 114篇 |
1957年 | 84篇 |
1954年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
3.
4.
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
5.
6.
7.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献
8.
9.
The timing of sleep and sleep EEG parameters in 10 healthy male subjects were investigated in four seasons under controlled conditions. The phase of nocturnal sleep was delayed about one and a half hours in winter as compared to that in summer. The duration of stage 4 sleep decreased and REM sleep increased significantly in winter compared with summer. The seasonality in the timing of sleep can be explained by photoperiodic time cues, but the changes in sleep EEG parameters are difficult to explain in terms of photoperiod. 相似文献
10.
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