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   22699篇
  免费   2389篇
  国内免费   1079篇
化学   5893篇
晶体学   36篇
力学   1485篇
综合类   215篇
数学   12210篇
物理学   6328篇
  2024年   19篇
  2023年   197篇
  2022年   217篇
  2021年   458篇
  2020年   536篇
  2019年   571篇
  2018年   447篇
  2017年   556篇
  2016年   819篇
  2015年   631篇
  2014年   1055篇
  2013年   1765篇
  2012年   1092篇
  2011年   1111篇
  2010年   1010篇
  2009年   1329篇
  2008年   1538篇
  2007年   1543篇
  2006年   1345篇
  2005年   1067篇
  2004年   921篇
  2003年   910篇
  2002年   927篇
  2001年   745篇
  2000年   698篇
  1999年   626篇
  1998年   556篇
  1997年   432篇
  1996年   366篇
  1995年   328篇
  1994年   289篇
  1993年   247篇
  1992年   240篇
  1991年   172篇
  1990年   144篇
  1989年   139篇
  1988年   142篇
  1987年   113篇
  1986年   106篇
  1985年   147篇
  1984年   124篇
  1983年   37篇
  1982年   82篇
  1981年   63篇
  1980年   65篇
  1979年   58篇
  1978年   42篇
  1977年   47篇
  1976年   35篇
  1973年   14篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
A closed metallomolecular cage based on the tris(saloph) framework, in which its aperture can be opened by disulfide exchange, was designed. At the apertures of the molecular cage, bridging diamine ligands having a disulfide bond were introduced to close the cage structure. These bridging ligands efficiently blocked the uptake of a guest, Cs+, but the presence of a thiolate anion significantly accelerated the guest uptake. This clearly means that this cage complex became open by the addition of the thiolate anion, because the cage closure becomes more dynamic due to the disulfide exchange reaction.  相似文献   
82.
83.
We develop a fully calibrated positive mathematical programming model for Hawaii's local food systems—which captures the production and the consumer sides of the market. Then we use the model to assess two proposed policies—a general excise tax (GET) exemption on locally produced foods, and an investment in agricultural infrastructure. For the GET exemption case, our results indicate an economic gain of $118 per $100 cost. On the other hand, an investment in 1,200 acres of land injected to support local production may generate an economic gain of up to $357 per $100 annual cost of the investment. However, these estimates should be considered preliminary, and thus viewed with caution. Although the model is used to capture Hawaii's local food systems, we believe that our model is generalizable and can be adopted by other economies to assess their respective food localization policies. Recommendations for Resource Managers
  • Local food policies need to be based on quantitative terms instead of mere armchair speculation because often their potential outcomes may vary significantly.
  • The current modeling framework demonstrates the potential of using positive mathematical programming (PMP) in capturing the intricacies of local food systems. However, this exploratory exercise should be viewed as preliminary in nature and the ensuing results were taken with caution because many important factors such as labor availability may have been left out.
  • Thus, further model refinements are necessary to better capture the complexities of local food systems such as farm heterogeneity, availability of farm labor, water availability, and interisland transportation of farm products in the case of Hawaii.
  相似文献   
84.
In this paper we study a Hamiltonian function on the cotangent bundle of the space of Riemannian metrics on a 3-manifold M and prove the orbits of the constrained Hamiltonian dynamical system correspond to G2-manifolds foliated by hypersurfaces diffeomorphic to M×SO(3).  相似文献   
85.
We will focus on the existence of nontrivial solutions to the following Hamiltonian elliptic system where are numbers belonging to the interval [0, 2), V is a continuous potential bounded below on by a positive constant and the functions f and g possess exponential growth range established by Trudinger–Moser inequalities in Lorentz–Sobolev spaces. The proof involves linking theorem and a finite‐dimensional approximation.  相似文献   
86.
87.
We consider the problem of performing matrix completion with side information on row-by-row and column-by-column similarities. We build upon recent proposals for matrix estimation with smoothness constraints with respect to row and column graphs. We present a novel iterative procedure for directly minimizing an information criterion to select an appropriate amount of row and column smoothing, namely, to perform model selection. We also discuss how to exploit the special structure of the problem to scale up the estimation and model selection procedure via the Hutchinson estimator, combined with a stochastic Quasi-Newton approach. Supplementary material for this article is available online.  相似文献   
88.
In this paper, we investigate the complete moment convergence for dependent linear processes with random coefficients to form  相似文献   
89.
In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.  相似文献   
90.
We establish that solutions to the Cauchy–Dirichlet problem
?tu?div(Dξf(x,Du))=0
for functionals f:Ω×RN×n[0,) of linear growth can be obtained as limits of solutions to flows with p-growth in the limit p1. The result can be interpreted on the one hand as a stability result. On the other hand it provides an existence result for general flows with linear growth.  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