全文获取类型
收费全文 | 25414篇 |
免费 | 3698篇 |
国内免费 | 5246篇 |
专业分类
测绘学 | 3299篇 |
大气科学 | 3991篇 |
地球物理 | 6351篇 |
地质学 | 10744篇 |
海洋学 | 3236篇 |
天文学 | 1093篇 |
综合类 | 1751篇 |
自然地理 | 3893篇 |
出版年
2024年 | 129篇 |
2023年 | 407篇 |
2022年 | 758篇 |
2021年 | 972篇 |
2020年 | 1121篇 |
2019年 | 1330篇 |
2018年 | 925篇 |
2017年 | 1231篇 |
2016年 | 1237篇 |
2015年 | 1269篇 |
2014年 | 1611篇 |
2013年 | 1901篇 |
2012年 | 1597篇 |
2011年 | 1682篇 |
2010年 | 1320篇 |
2009年 | 1714篇 |
2008年 | 1684篇 |
2007年 | 1735篇 |
2006年 | 1679篇 |
2005年 | 1313篇 |
2004年 | 1229篇 |
2003年 | 1039篇 |
2002年 | 811篇 |
2001年 | 715篇 |
2000年 | 640篇 |
1999年 | 599篇 |
1998年 | 602篇 |
1997年 | 517篇 |
1996年 | 427篇 |
1995年 | 381篇 |
1994年 | 316篇 |
1993年 | 313篇 |
1992年 | 218篇 |
1991年 | 173篇 |
1990年 | 135篇 |
1989年 | 128篇 |
1988年 | 105篇 |
1987年 | 59篇 |
1986年 | 54篇 |
1985年 | 51篇 |
1984年 | 29篇 |
1983年 | 13篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1978年 | 14篇 |
1977年 | 19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 0 毫秒
11.
O. Tamuz T. Mazeh S. Zucker † 《Monthly notices of the Royal Astronomical Society》2005,356(4):1466-1470
We suggest a new algorithm to remove systematic effects in a large set of light curves obtained by a photometric survey. The algorithm can remove systematic effects, such as those associated with atmospheric extinction, detector efficiency, or point spread function changes over the detector. The algorithm works without any prior knowledge of the effects, as long as they linearly appear in many stars of the sample. The approach, which was originally developed to remove atmospheric extinction effects, is based on a lower rank approximation of matrices, an approach which has already been suggested and used in chemometrics, for example. The proposed algorithm is especially useful in cases where the uncertainties of the measurements are unequal. For equal uncertainties, the algorithm reduces to the Principal Component Analysis (PCA) algorithm. We present a simulation to demonstrate the effectiveness of the proposed algorithm and we point out its potential, in the search for transit candidates in particular. 相似文献
12.
1INTRODUCTIONGrain production has always been top-priority issue con-cerned by government and people (MA etal., 2002). Af-ter China put household contract responsibility system in force, its domestic issue of grain was basically settled in the mid 1980s (MA and XU, 1999). However, in the days when idea of sustainable development is broadly accepted, efficiency and sustainability of grain produc-tion are now in question, which have prompted interest in reviewing grain production patterns… 相似文献
13.
张敏先 《成都信息工程学院学报》2001,16(3):209-212
文献[1][2]认为M-PN空间是局部凸的。通过定理说明这个结论对t-模T(a,b)≥min(a,b)时成立。又通过反例表明,当t-模型T(a,b)=max(a b-1,0)时无论邻域Nθ(ε,λ)概率有界还是概率半有界结论均不成立。还讨论了Takahashi凸性并对S-凸和K-凸作了比较。 相似文献
14.
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
15.
16.
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
17.
18.
19.
案例推理的地学应用背景和方法 总被引:3,自引:1,他引:2
案例推理 (CBR)研究 ,是人类认知的一种合理性模型 ,接近于人类认识、解决问题最原始的思维方式 ,具有在无法获取机理模型、确定规则或统计模型时 ,采用简单的历史相似性推理实现问题的定量求解和预测 ,成为人工智能领域近年来的研究热点。 相似文献
20.
Dan Gill 《Mathematical Geology》1993,25(4):471-482
The ability of association analysis to discriminate sedimentary facies was tested on Purdy's modal analyses of modern sediments of the Great Bahama Bank. Purdy's data set has served in the past as a standard reference for evaluating various multivariate classification algorithms. In order to adapt Purdy's data to association analysis, the percent abundance of the 12 constituents was converted to binary form by dichotomizing each variable on its mean value. The results obtained by association analysis are virtually identical to those obtained by Purdy and other authors. The same four main sedimentary facies were discriminated; 86% of the samples were identically classified (97% when misclassified borderline cases are counted as matches); the total partition variance of the classification is only negligibly greater (4%); and the grouping of the variables yielded the same four groups. The rank order of the three division-attributes responsible for the sample classification is fines, oolites, and corals. Association analysis has been employed by other authors to differentiate meaningful facies groups in studies of ancient reef carbonates, modern reef sediments, and heavy minerals in stream sediments. In all these studies, the results were found to be compatible with those obtained by using the continuous quantitative measurements, indicating that qualitative binary data may often be sufficient for the purpose of facies discrimination in many branches of geology and that association analysis is an effective method for this purpose. 相似文献