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   469034篇
  免费   3569篇
  国内免费   1213篇
化学   223048篇
晶体学   7467篇
力学   25204篇
综合类   11篇
数学   66211篇
物理学   151875篇
  2020年   3452篇
  2019年   4116篇
  2018年   12064篇
  2017年   12557篇
  2016年   10851篇
  2015年   4909篇
  2014年   7530篇
  2013年   17252篇
  2012年   16363篇
  2011年   25249篇
  2010年   17192篇
  2009年   17408篇
  2008年   22731篇
  2007年   25423篇
  2006年   13650篇
  2005年   16725篇
  2004年   13768篇
  2003年   12876篇
  2002年   11284篇
  2001年   11639篇
  2000年   8985篇
  1999年   6777篇
  1998年   5717篇
  1997年   5718篇
  1996年   5501篇
  1995年   4913篇
  1994年   4932篇
  1993年   4883篇
  1992年   5290篇
  1991年   5495篇
  1990年   5208篇
  1989年   5226篇
  1988年   5193篇
  1987年   5072篇
  1986年   4849篇
  1985年   6370篇
  1984年   6655篇
  1983年   5389篇
  1982年   5606篇
  1981年   5486篇
  1980年   5241篇
  1979年   5652篇
  1978年   5699篇
  1977年   5697篇
  1976年   5814篇
  1975年   5337篇
  1974年   5304篇
  1973年   5473篇
  1972年   3974篇
  1971年   3448篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Some features of software implementation of the Pulay scaling procedure are considered. The advantages of the single value decomposition method for maintaining well-conditionality of the scale factor determination problem are demonstrated. The necessity of using a rational number of scale factors is shown. The possibility of obtaining transferable scale factors with the Pulay method and thus predict the vibrational spectra of related compounds is emphasized.  相似文献   
972.
A velocity and absorption coefficient of sound for magnetic fluid (MF) based on water are studied in the frequency range of 12–132 MHz as a function of the concentration of dispersed phase, the uniform magnetic field, the storage time, and the temperature. The MF dispersed phase consist of magnetite Fe3O4 particles stabilized with sodium oleate. The parameters of sound propagation are measured within temperature interval 0–80°C. Densities of MF and sodium oleate are also measured as a function of temperature. Volume concentrations of magnetic fluid components (water, magnetite, and sodium oleate adsorbed on the surface of magnetic particles) are determined. Densities, heat capacities, coefficients of heat conductivity and thermal expansion of aggregates are estimated. Aggregate size distribution in the studied magnetic fluid is described by the log-normal function. Parameters characterizing the aggregate size distribution are determined and their interpretation is given.  相似文献   
973.
The dynamic and equilibrium surface tensions of aqueous dodecylamidoethyldimethylbenzylammonium chloride solutions of various concentrations at 16, 20, 25, 30, and 35°C are studied for the first time. The effects of the concentration and temperature on the surface tension relaxation are discussed. The possibility of two-dimensional phase transition and its effect on the dynamic behavior of surface tension are considered.  相似文献   
974.
Conditions are determined for the determination of salicylhydroxamic and acetylhydroxamic acids by thermal-lens spectrometry using iron(III) in an aqueous medium. The detection limits are 6 × 10–8 and 3 × 10–8 M, respectively. The sensitivity can be enhanced 3–6 times with the use of aqueous–organic media.  相似文献   
975.
IntroductionPolyoxometalatesarewidelystudiedfortheirin terestingapplicationsincatalysis ,electronicconduc tivity ,magnetism ,nonlinearopticsandmedicine[1— 4 ] .Whilethemechanismofthesynthesisofpolyoxometalatesremainselusiveandisoftende scribedasself assembly ,thereseemstobeanincreas inguseofthecombinationofthehydrothermalmethodandthestructure directingtemplate .There searchwiththismethodhasdemonstratedthatanum beroftransitionmetaloxides ,typicallyreducedmolybdenumoxides ,mixedvalancevanadiumo…  相似文献   
976.
A two-dimensional mathematical model for the transport of reactants in a fuel cell with a solid polymer electrolyte is developed. The model is used for analyzing spatial distributions of the concentration of reactants and current density over the cell. The effect of the catalytic-layer activity, reactant speed, bipolar-plate geometry, thickness and porosity of current collector and/or gas-diffusion sublayer, and the reaction mixture composition on the fuel cell efficiency is estimated theoretically and experimentally.  相似文献   
977.
Cyclic Hypomonotonicity,Cyclic Submonotonicity,and Integration   总被引:9,自引:1,他引:8  
Rockafellar has shown that the subdifferentials of convex functions are always cyclically monotone operators. Moreover, maximal cyclically monotone operators are necessarily operators of this type, since one can construct explicitly a convex function, which turns out to be unique up to a constant, whose subdifferential gives back the operator. This result is a cornerstone in convex analysis and relates tightly convexity and monotonicity. In this paper, we establish analogous robust results that relate weak convexity notions to corresponding notions of weak monotonicity, provided one deals with locally Lipschitz functions and locally bounded operators. In particular, the subdifferentials of locally Lipschitz functions that are directionally hypomonotone [respectively, directionally submonotone] enjoy also an additional cyclic strengthening of this notion and in fact are maximal under this new property. Moreover, every maximal cyclically hypomonotone [respectively, maximal cyclically submonotone] operator is always the Clarke subdifferential of some directionally weakly convex [respectively, directionally approximately convex] locally Lipschitz function, unique up to a constant, which in finite dimentions is a lower C2 function [respectively, a lower C1 function].  相似文献   
978.
In our previous work, a generic well-posedness result (with respect to variations of the integrand of the integral functional) without the convexity condition was established for a class of optimal control problems satisfying the Cesari growth condition. In this paper, we extend this generic well-posedness result to classes of constrained variational problems in which the values at the endpoints and constraint maps are also subject to variations.  相似文献   
979.
The spectral projected gradient method SPG is an algorithm for large-scale bound-constrained optimization introduced recently by Birgin, Martínez, and Raydan. It is based on the Raydan unconstrained generalization of the Barzilai-Borwein method for quadratics. The SPG algorithm turned out to be surprisingly effective for solving many large-scale minimization problems with box constraints. Therefore, it is natural to test its perfomance for solving the sub-problems that appear in nonlinear programming methods based on augmented Lagrangians. In this work, augmented Lagrangian methods which use SPG as the underlying convex-constraint solver are introduced (ALSPG) and the methods are tested in two sets of problems. First, a meaningful subset of large-scale nonlinearly constrained problems of the CUTE collection is solved and compared with the perfomance of LANCELOT. Second, a family of location problems in the minimax formulation is solved against the package FFSQP.  相似文献   
980.
In this paper, a model of a linear multilevel programming problem with dominated objective functions (LMPPD(l)) is proposed, where multiple reactions of the lower levels do not lead to any uncertainty in the upper-level decision making. Under the assumption that the constrained set is nonempty and bounded, a necessary optimality condition is obtained. Two types of geometric properties of the solution sets are studied. It is demonstrated that the feasible set of LMPPD(l) is neither necessarily composed of faces of the constrained set nor necessarily connected. These properties are different from the existing theoretical results for linear multilevel programming problems.  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