全文获取类型
收费全文 | 28978篇 |
免费 | 4348篇 |
国内免费 | 6560篇 |
专业分类
测绘学 | 3434篇 |
大气科学 | 4172篇 |
地球物理 | 6975篇 |
地质学 | 13854篇 |
海洋学 | 3820篇 |
天文学 | 1272篇 |
综合类 | 2020篇 |
自然地理 | 4339篇 |
出版年
2024年 | 110篇 |
2023年 | 327篇 |
2022年 | 813篇 |
2021年 | 1020篇 |
2020年 | 1182篇 |
2019年 | 1493篇 |
2018年 | 1081篇 |
2017年 | 1347篇 |
2016年 | 1356篇 |
2015年 | 1438篇 |
2014年 | 1842篇 |
2013年 | 2179篇 |
2012年 | 1842篇 |
2011年 | 2006篇 |
2010年 | 1604篇 |
2009年 | 2015篇 |
2008年 | 1973篇 |
2007年 | 2070篇 |
2006年 | 2031篇 |
2005年 | 1640篇 |
2004年 | 1500篇 |
2003年 | 1285篇 |
2002年 | 1005篇 |
2001年 | 869篇 |
2000年 | 790篇 |
1999年 | 731篇 |
1998年 | 699篇 |
1997年 | 601篇 |
1996年 | 501篇 |
1995年 | 454篇 |
1994年 | 383篇 |
1993年 | 367篇 |
1992年 | 258篇 |
1991年 | 218篇 |
1990年 | 157篇 |
1989年 | 143篇 |
1988年 | 117篇 |
1987年 | 70篇 |
1986年 | 58篇 |
1985年 | 56篇 |
1984年 | 31篇 |
1983年 | 19篇 |
1982年 | 27篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1978年 | 14篇 |
1977年 | 21篇 |
1976年 | 25篇 |
1973年 | 17篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
41.
Olac Fuentes 《Experimental Astronomy》2001,12(1):21-31
In this article we show how machine learning methods can beeffectively applied to the problem of automatically predictingstellar atmospheric parameters from spectral information, a veryimportant problem in stellar astronomy. We apply feedforwardneural networks, Kohonen's self-organizing maps andlocally-weighted regression to predict the stellar atmosphericparameters effective temperature, surface gravity and metallicityfrom spectral indices. Our experimental results show that thethree methods are capable of predicting the parameters with verygood accuracy. Locally weighted regression gives slightly betterresults than the other methods using the original dataset asinput, while self-organizing maps outperform the other methods when significant amounts of noise are added. We also implemented a heterogeneous ensemble of predictors, combining the results given by the three algorithms. This ensemble yields better results than any of the three algorithms alone, using both the original and the noisy data. 相似文献
42.
应用地壳波浪与镶嵌构造学说对富氏谱分析法提取地壳垂直形变信息的科学性做了地质学意义上的阐释 ,并提出了根据多期形变资料提取特定波段上构造策应力的数学模型 相似文献
43.
Abdul Ahmad 《Celestial Mechanics and Dynamical Astronomy》1995,61(2):181-196
The periodic solutions of the restricted three-body problem representing analytic continuations of Keplerian rectilinear periodic motions are well known (Kurcheeva, 1973). Here the stability of these solutions are examined by applying Poncaré's characteristic equation for periodic solutions. It is found that the isoperiodic solutions are stable and all other solutions are unstable. 相似文献
44.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
45.
46.
PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER
下载免费PDF全文
![点击此处可从《Acta Meteorologica Sinica》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSOyear,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters andtheir influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta-tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that inthe winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone aremainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982—1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonalchanges of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinalprogression or regression of about 40 day period.There are marked differences between propagat-ing passages of low frequency modes responsible for changes of subtropical jet stream in the normalyear(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude showobvious phases.In general,the one in late winter is stronger than that in early winter,strongestone occurs in February. 相似文献
47.
We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data. 相似文献
48.
基于可靠度理论的黄土高边坡优化设计 总被引:11,自引:2,他引:9
对黄土高边坡进行可靠性分析,应用数学原理和优化原理,建立了黄土高边坡的优化模型,对铜黄一级公路黄土高边坡进行分析验证。结果表明,该方法计算的结果与实际较接近.应用中易操作。 相似文献
49.
50.
Measurements of salinity perturbations in a partially mixed estuary have been used to evaluate the usefulness of an inductive salinometer and to determine some of the characteristics of the salinity perturbations. The salinometer performed satisfactorily under most conditions. Although internal wave like effects were present, the turbulence fluctuations were dominant. The salinity fluctuations and the turbulent fluxes and were found to behave in a manner similar to the density fluctuations in a thermally stratified atmospheric boundary layer and a laboratory open channel flow. A quadrant analysis suggested that the contribution of each quadrant to the turbulent flux changed with Ri. The turbulence parameters ν and cγ were found to decrease and increase respectively as Ri increases. 相似文献