全文获取类型
收费全文 | 8203篇 |
免费 | 1454篇 |
国内免费 | 1589篇 |
专业分类
测绘学 | 856篇 |
大气科学 | 740篇 |
地球物理 | 3406篇 |
地质学 | 3250篇 |
海洋学 | 795篇 |
天文学 | 1278篇 |
综合类 | 432篇 |
自然地理 | 489篇 |
出版年
2024年 | 44篇 |
2023年 | 138篇 |
2022年 | 233篇 |
2021年 | 308篇 |
2020年 | 303篇 |
2019年 | 373篇 |
2018年 | 304篇 |
2017年 | 301篇 |
2016年 | 375篇 |
2015年 | 435篇 |
2014年 | 484篇 |
2013年 | 445篇 |
2012年 | 444篇 |
2011年 | 455篇 |
2010年 | 443篇 |
2009年 | 554篇 |
2008年 | 513篇 |
2007年 | 613篇 |
2006年 | 562篇 |
2005年 | 485篇 |
2004年 | 517篇 |
2003年 | 449篇 |
2002年 | 371篇 |
2001年 | 309篇 |
2000年 | 283篇 |
1999年 | 267篇 |
1998年 | 234篇 |
1997年 | 160篇 |
1996年 | 164篇 |
1995年 | 127篇 |
1994年 | 89篇 |
1993年 | 121篇 |
1992年 | 70篇 |
1991年 | 60篇 |
1990年 | 57篇 |
1989年 | 49篇 |
1988年 | 40篇 |
1987年 | 21篇 |
1986年 | 11篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
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
22.
模糊划分矩阵在岩土参数概率分布中的应用 总被引:1,自引:0,他引:1
讨论如何在小样本条件下用已有的过程经验与试验资料确定岩土参数概率分布,用模糊划分矩阵与BAYES方法相结合,给出由小样本试验数据确定岩土参数的概率分布。 相似文献
23.
K. Nagamine † M. Fukugita R. Cen J.P. Ostriker 《Monthly notices of the Royal Astronomical Society》2001,327(1):L10-L14
The luminosity function of galaxies is derived from a cosmological hydrodynamic simulation of a Λ cold dark matter universe with the aid of a stellar population synthesis model. At , the resulting B -band luminosity function has a flat faint-end slope of with the characteristic luminosity and the normalization in fair agreement with observations, while the dark matter halo mass function is steep with a slope of . The colour distribution of galaxies also agrees well with local observations. We also discuss the evolution of the luminosity function, and the colour distribution of galaxies from to 5. A large evolution of the characteristic mass in the stellar mass function as a result of number evolution is compensated by luminosity evolution; the characteristic luminosity increases only by 0.8 mag from to 2, and then declines towards higher redshift, while the B -band luminosity density continues to increase from to 5 (but only slowly at . 相似文献
24.
介绍了自动高精度测距系统。该系统能以5×10-7的测距精度测定多个观测点的形变,可作为跨断层动态连续观测的新仪器。 相似文献
25.
Ren Tinawi Marc Sarrazin Andr Filiatrault 《Soil Dynamics and Earthquake Engineering》1993,12(8):469-477
The 1990 edition of the National Building Code of Canada (Associate Committee of the National Building Code, National Research Council, Ottawa, 1990) makes a clear distinction between eastern and western Canada in terms of seismic acceleration and velocity zones. While it is well established that ground motions can be amplified significantly through loose clay deposits, no results are available that take into consideration the typical high frequency content of ground motions in eastern Canada. This paper develops ground amplification curves for clays having depths between 10 and 70 m excited by typical eastern Canadian ground motions scaled to two different values of peak horizontal accelerations. Simplified free-field spectral design curves, which could be used by structural designers, are proposed. The curves show that maximum spectral accelerations occur for structural periods between 0.2 and 0.5 s. In addition, soil depth does not appear to be an important parameter controlling the response of typical clay deposits in eastern Canada. 相似文献
26.
M. Sereno 《Monthly notices of the Royal Astronomical Society》2005,356(3):937-943
Constraints on an exact quintessence scalar-field model with an exponential potential are derived from gravitational lens statistics. An exponential potential can account for data from both optical quasar surveys and radio-selected sources. Based on the Cosmic Lens All-Sky Survey (CLASS) sample, lensing statistics provides, for the pressureless matter density parameter, an estimate of ΩM0 = 0.31+0.12 −0.14 . 相似文献
27.
28.
29.
30.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献