首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   9247篇
  免费   1072篇
  国内免费   1226篇
测绘学   774篇
大气科学   1418篇
地球物理   2235篇
地质学   1207篇
海洋学   927篇
天文学   4137篇
综合类   507篇
自然地理   340篇
  2024年   39篇
  2023年   91篇
  2022年   150篇
  2021年   196篇
  2020年   226篇
  2019年   272篇
  2018年   189篇
  2017年   230篇
  2016年   229篇
  2015年   309篇
  2014年   346篇
  2013年   388篇
  2012年   378篇
  2011年   412篇
  2010年   383篇
  2009年   706篇
  2008年   610篇
  2007年   744篇
  2006年   684篇
  2005年   653篇
  2004年   656篇
  2003年   594篇
  2002年   463篇
  2001年   445篇
  2000年   387篇
  1999年   392篇
  1998年   455篇
  1997年   154篇
  1996年   132篇
  1995年   113篇
  1994年   131篇
  1993年   92篇
  1992年   85篇
  1991年   62篇
  1990年   32篇
  1989年   36篇
  1988年   15篇
  1987年   16篇
  1986年   10篇
  1985年   4篇
  1984年   3篇
  1983年   7篇
  1982年   4篇
  1981年   2篇
  1980年   3篇
  1978年   3篇
  1977年   2篇
  1973年   5篇
  1972年   3篇
  1971年   2篇
排序方式: 共有10000条查询结果,搜索用时 575 毫秒
1.
In order to model non‐Fickian transport behaviour in groundwater aquifers, various forms of the time–space fractional advection–dispersion equation have been developed and used by several researchers in the last decade. The solute transport in groundwater aquifers in fractional time–space takes place by means of an underlying groundwater flow field. However, the governing equations for such groundwater flow in fractional time–space are yet to be developed in a comprehensive framework. In this study, a finite difference numerical scheme based on Caputo fractional derivative is proposed to investigate the properties of a newly developed time–space fractional governing equations of transient groundwater flow in confined aquifers in terms of the time–space fractional mass conservation equation and the time–space fractional water flux equation. Here, we apply these time–space fractional governing equations numerically to transient groundwater flow in a confined aquifer for different boundary conditions to explore their behaviour in modelling groundwater flow in fractional time–space. The numerical results demonstrate that the proposed time–space fractional governing equation for groundwater flow in confined aquifers may provide a new perspective on modelling groundwater flow and on interpreting the dynamics of groundwater level fluctuations. Additionally, the numerical results may imply that the newly derived fractional groundwater governing equation may help explain the observed heavy‐tailed solute transport behaviour in groundwater flow by incorporating nonlocal or long‐range dependence of the underlying groundwater flow field.  相似文献   
2.
3.
4.
5.
6.
7.
A wide-field time-series CCD photometric survey of variable stars in the field of the open cluster NGC 2168 was carried out using the BATC Schmidt telescope. In total 13 new variable stars are discovered with three W UMa systems, one EA type and two EB type eclipsing binaries (one of them could be a W UMa system), and seven pulsating stars including three candidates of δScuti stars.  相似文献   
8.
9.
10.
Intermediate orbit for general planetary theory is constructed in the form of multivariate Fourier series with numerical coefficients. The structure and efficiency of the derived series are illustrated by giving various statistical properties of the coefficients.The ability of the recently proposed elliptic function approach to compress the Fourier series representing the intermediate orbit is investigated. Our results confirm that when mutual perturbations of a pair of planets are considered the elliptic function approach is quite efficient and allows one to compress the series substantially. However, when perturbations of three or more planets are under study the elliptic function approach does not give any advantages.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号