全文获取类型
收费全文 | 75000篇 |
免费 | 932篇 |
国内免费 | 617篇 |
专业分类
测绘学 | 2171篇 |
大气科学 | 5603篇 |
地球物理 | 14098篇 |
地质学 | 29199篇 |
海洋学 | 5744篇 |
天文学 | 15888篇 |
综合类 | 313篇 |
自然地理 | 3533篇 |
出版年
2021年 | 417篇 |
2020年 | 455篇 |
2019年 | 512篇 |
2018年 | 3884篇 |
2017年 | 3607篇 |
2016年 | 2923篇 |
2015年 | 1084篇 |
2014年 | 1517篇 |
2013年 | 2760篇 |
2012年 | 2626篇 |
2011年 | 4562篇 |
2010年 | 4105篇 |
2009年 | 4729篇 |
2008年 | 3873篇 |
2007年 | 4478篇 |
2006年 | 2072篇 |
2005年 | 2137篇 |
2004年 | 1932篇 |
2003年 | 1954篇 |
2002年 | 1731篇 |
2001年 | 1400篇 |
2000年 | 1314篇 |
1999年 | 1170篇 |
1998年 | 1131篇 |
1997年 | 1119篇 |
1996年 | 968篇 |
1995年 | 910篇 |
1994年 | 848篇 |
1993年 | 733篇 |
1992年 | 667篇 |
1991年 | 668篇 |
1990年 | 692篇 |
1989年 | 642篇 |
1988年 | 574篇 |
1987年 | 706篇 |
1986年 | 588篇 |
1985年 | 757篇 |
1984年 | 826篇 |
1983年 | 801篇 |
1982年 | 749篇 |
1981年 | 665篇 |
1980年 | 665篇 |
1979年 | 570篇 |
1978年 | 587篇 |
1977年 | 538篇 |
1976年 | 504篇 |
1975年 | 508篇 |
1974年 | 491篇 |
1973年 | 518篇 |
1972年 | 339篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
103.
104.
Chiang E. I. Lovering J. R. Millis R. L. Buie M. W. Wasserman L. H. Meech K. J. 《Earth, Moon, and Planets》2003,92(1-4):49-62
We review ongoing efforts to identify occupants of mean-motion resonances(MMRs) and collisional families in the Edgeworth–Kuiper belt. Directintegrations of trajectories of Kuiper belt objects (KBOs) reveal the 1:1(Trojan), 5:4, 4:3, 3:2 (Plutino), 5:3, 7:4, 9:5, 2:1 (Twotino), and 5:2 MMRsto be inhabited. Apart from the Trojan, resonant KBOs typically have largeorbital eccentricities and inclinations. The observed pattern of resonanceoccupation is consistent with resonant capture and adiabatic excitation bya migratory Neptune; however, the dynamically cold initial conditions priorto resonance sweeping that are typically assumed by migration simulationsare probably inadequate. Given the dynamically hot residents of the 5:2 MMRand the substantial inclinations observed in all exterior MMRs, a fraction ofthe primordial belt was likely dynamically pre-heated prior to resonancesweeping. A pre-heated population may have arisen as Neptune gravitationallyscattered objects into trans-Neptunian space. The spatial distribution of Twotinosoffers a unique diagnostic of Neptune's migration history. The Neptunian Trojanpopulation may rival the Jovian Trojan population, and the former's existence isargued to rule out violent orbital histories for Neptune. Finally, lowest-order seculartheory is applied to several hundred non-resonant KBOs with well-measured orbitsto update proposals of collisional families. No convincing family is detected. 相似文献
105.
106.
A non-static exact solution of the Einstein equations corresponding to a conformally invariant scalar field with trace-free energy momentum tensor is obtained for the Robertson-Walker type metric. Some physical properties of the solution are discussed. 相似文献
107.
108.
109.
K. R. Subramanian R. Ramesh M. S. Sundara Rajan Ch. V. Sastry 《Astrophysics and Space Science》1996,243(1):259-262
Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented. 相似文献
110.
The emergence of artificial neural network (ANN) technology has provided many promising results in the field of hydrology and water resources simulation. However, one of the major criticisms of ANN hydrologic models is that they do not consider/explain the underlying physical processes in a watershed, resulting in them being labelled as black‐box models. This paper discusses a research study conducted in order to examine whether or not the physical processes in a watershed are inherent in a trained ANN rainfall‐runoff model. The investigation is based on analysing definite statistical measures of strength of relationship between the disintegrated hidden neuron responses of an ANN model and its input variables, as well as various deterministic components of a conceptual rainfall‐runoff model. The approach is illustrated by presenting a case study for the Kentucky River watershed. The results suggest that the distributed structure of the ANN is able to capture certain physical behaviour of the rainfall‐runoff process. The results demonstrate that the hidden neurons in the ANN rainfall‐runoff model approximate various components of the hydrologic system, such as infiltration, base flow, and delayed and quick surface flow, etc., and represent the rising limb and different portions of the falling limb of a flow hydrograph. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献