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   1400095篇
  免费   25849篇
  国内免费   7743篇
化学   708144篇
晶体学   20371篇
力学   75578篇
综合类   107篇
数学   245591篇
物理学   383896篇
  2021年   13590篇
  2020年   16011篇
  2019年   16184篇
  2018年   13036篇
  2016年   28297篇
  2015年   21011篇
  2014年   30647篇
  2013年   74792篇
  2012年   41461篇
  2011年   40244篇
  2010年   38304篇
  2009年   40113篇
  2008年   38643篇
  2007年   34841篇
  2006年   40633篇
  2005年   32723篇
  2004年   33630篇
  2003年   31218篇
  2002年   31985篇
  2001年   31926篇
  2000年   27009篇
  1999年   23577篇
  1998年   21508篇
  1997年   21388篇
  1996年   21337篇
  1995年   19350篇
  1994年   18778篇
  1993年   18364篇
  1992年   18678篇
  1991年   18905篇
  1990年   18079篇
  1989年   18072篇
  1988年   17680篇
  1987年   17533篇
  1986年   16536篇
  1985年   22942篇
  1984年   24044篇
  1983年   20159篇
  1982年   21845篇
  1981年   21026篇
  1980年   20356篇
  1979年   20653篇
  1978年   21894篇
  1977年   21511篇
  1976年   21303篇
  1975年   20055篇
  1974年   19755篇
  1973年   20221篇
  1972年   14677篇
  1967年   12799篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
911.
In order to reduce infant mortality in the municipality of Rio de Janeiro it is desired to encourage all mothers-to-be to attend the appropriate health care facilities. A 3-level hierarchical prenatal–neonatal health care system is described together with a basic model for optimising accessibility to facilities. A genetic algorithm to solve the basic model is developed and some numerical experience reported. Extensions to the basic model and planned future research are briefly indicated.  相似文献   
912.
Book rewiev     
Howard Anton and Robert C. Busby, John Wiley & Sons, Inc., 2002.  相似文献   
913.
Summary Characteristics of optimal solutions under nonlinear buckling constraints are investigated by using a bar-spring model. It is demonstrated that optimization under buckling constraints of a symmetric system often leads to a structure with hill-top branching, where a limit point and bifurcation points coincide. A general formulation is derived for imperfection sensitivity of the critical load factor corresponding to a hill-top branching point. It is shown that the critical load is not imperfection-sensitive even for the case where an asymmetric bifurcation point exists at the limit point.  相似文献   
914.
An upper bound for the best approximation of summable functions of several variables by trigonometric polynomials in the metric of L is determined in terms of Fourier coefficients. We consider functions representable by trigonometric series with certain symmetry of coefficients satisfying a multiple analog of the Sidon–Telyakovskii conditions.  相似文献   
915.
V stat'e izuchayutsya klassy periodicheskikh funktsii mnogikh peremennykh s zadannoi mazhorantoi smeshannykh modulei nepreryvnosti, prichem mazhoranta imeet spetsial'nyi vid. Rassmotreny nailuchshie priblizheniya ukazannykh klassov i vozmozhnost' kharakterizatsii 'etikh klassov cherez nailuchshie priblizheniya.  相似文献   
916.
Residuals have long been used in frequency domain parameter estimation methods to model the influence of out-of-band modes but, typically, as a fixed set of no more than two or three terms. Recently, a systematic approach to the use of residual polynomials has led to the development of a generalized residual model. The use of a generalized residual model with rational fraction polynomial frequency domain parameter estimation methods allows the contribution of out-of-band modes to be included without increasing the model order and creating additional computational poles. Of particular interest is the use of the generalized residual for single-degree-of-freedom (s.d.o.f.) techniques, which generally do not consider the residual effects and suffer accordingly. With the use of generalized residuals, it becomes possible to properly account for nearby modes and also extract accurate residues with an s.d.o.f. algorithm. The development of the generalized residual polynomial model is outlined and a new s.d.o.f. frequency domain algorithm with generalized residuals is developed.  相似文献   
917.
Alkyl isocyanides undergo a smooth reaction with alkylidene Meldrum’s acids in the presence of arylhydroxylamines to produce N 1-alkyl-2-(3,5-dioxo-2-aryltetrahydro-4-isoxazolyl)alkanamides in high yields.  相似文献   
918.
The mechanisms involved in the formation of n‐butanol during the synthesis of butyl acrylate containing latices were investigated. The experimental results showed that neither the hydrolysis of butyl acrylate nor of the ester bond in the butyl acrylate segments of the polymer played a major role in the formation of n‐butanol, which was mainly generated from the polymer backbone, by transfer reactions to polymer chain followed by cyclization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5838–5846, 2007  相似文献   
919.
Amphiphilic tris(2,2′‐bipyridine)ruthenium‐cored star‐shaped polymers consisting of one polystyrene block and two poly(N‐isopropylacrylamide) blocks were prepared by the “arm‐first” method in which RAFT polymerization and nonconvalent ligand–metal complexation were employed. The prepared amphiphilic star‐shaped metallopolymers are able to form micelles in water. The size and distribution of the micelles were studied by dynamic light scattering and transmission electron microscopy techniques. Preliminary studies indicate that the polymer concentration and the hydrophilic poly(N‐isopropylacrylamide) block length can affect the morphologies of the formed metal‐interfaced core–shell micelles in water. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4204–4210, 2007  相似文献   
920.
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13.  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