全文获取类型
收费全文 | 586790篇 |
免费 | 7878篇 |
国内免费 | 2463篇 |
专业分类
化学 | 316408篇 |
晶体学 | 8657篇 |
力学 | 25649篇 |
综合类 | 72篇 |
数学 | 70532篇 |
物理学 | 175813篇 |
出版年
2021年 | 4766篇 |
2020年 | 5338篇 |
2019年 | 5787篇 |
2018年 | 7483篇 |
2017年 | 7396篇 |
2016年 | 11205篇 |
2015年 | 7175篇 |
2014年 | 10928篇 |
2013年 | 26457篇 |
2012年 | 20885篇 |
2011年 | 25506篇 |
2010年 | 17953篇 |
2009年 | 17768篇 |
2008年 | 23614篇 |
2007年 | 23581篇 |
2006年 | 21910篇 |
2005年 | 19680篇 |
2004年 | 18045篇 |
2003年 | 15886篇 |
2002年 | 15677篇 |
2001年 | 17570篇 |
2000年 | 13334篇 |
1999年 | 10265篇 |
1998年 | 8622篇 |
1997年 | 8377篇 |
1996年 | 8170篇 |
1995年 | 7268篇 |
1994年 | 7247篇 |
1993年 | 7025篇 |
1992年 | 7769篇 |
1991年 | 7847篇 |
1990年 | 7501篇 |
1989年 | 7261篇 |
1988年 | 7176篇 |
1987年 | 7048篇 |
1986年 | 6763篇 |
1985年 | 8887篇 |
1984年 | 9029篇 |
1983年 | 7330篇 |
1982年 | 7561篇 |
1981年 | 7359篇 |
1980年 | 6809篇 |
1979年 | 7339篇 |
1978年 | 7667篇 |
1977年 | 7593篇 |
1976年 | 7525篇 |
1975年 | 6984篇 |
1974年 | 7064篇 |
1973年 | 7039篇 |
1972年 | 5162篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
The issues involved in teaching English language learners mathematics while they are learning English pose many challenges for mathematics teachers and highlight the need to focus on language-processing issues related to teaching mathematical content. Two realistic-type problems from high-stakes tests are used to illustrate the complex interactions between culture, language, and mathematical learning. The analyses focus on aspects of the problems that potentially increase cognitive demands for second-language learners. An analytical framework is presented that is designed to enable mathematics teachers to identify critical elements in problems and the learning environment that contribute to increased cognitive demands for students of English as a second language. The framework is proposed as a cycle of teacher reflection that would extend a constructivist model of teaching to include broader linguistic, cultural, and cognitive processing issues of mathematics teaching, as well as enable teachers to develop more accurate mental models of student learning. 相似文献
994.
S. Yu. Kudryashov Yu. I. Arutyunov L. A. Onuchak 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2007,81(1):102-107
New methods for calculating the mean volumetric flow rate of the carrier gas and the retention time of the unretained substance in the column under conditions of gas chromatography were proposed. The methods are based on preliminary isothermal calibrations of the flow rate and holdup time for a packed column. A theoretical substantiation of the methods was given. Procedures of plotting calibration dependences for determining the indicated quantities at a desired temperature were described. The calculation results were compared to experimental data obtained by traditional methods. It was demonstrated that the use of calculation methods substantially simplifies the determination of the specific retention volume over a wide temperature range. 相似文献
995.
We establish constructive existence conditions and construct a generalized Green operator for the construction of solutions
of a Noetherian linear boundary-value problem for a system of ordinary differential equations with switchings and pulse action
in critical and noncritical cases.
__________
Translated from Neliniini Kolyvannya, Vol. 10, No. 1, pp. 51–65, January–March, 2007. 相似文献
996.
The surgery obstruction of a normal map to a simple Poincaré pair (X, Y) lies in the relative surgery obstruction group L *(π 1(Y) → π 1(X)). A well-known result of Wall, the so-called π-π-theorem, states that in higher dimensions a normal map of a manifold with boundary to a simple Poincaré pair with π 1(X) ? π 1(Y) is normally bordant to a simple homotopy equivalence of pairs. In order to study normal maps to a manifold with a submanifold, Wall introduced the surgery obstruction groups LP * for manifold pairs and splitting obstruction groups LS *. In the present paper, we formulate and prove for manifold pairs with boundary results similar to the π-π-theorem. We give direct geometric proofs, which are based on the original statements of Wall’s results and apply obtained results to investigate surgery on filtered manifolds. 相似文献
997.
We consider queuing systems where customers are not allowed to queue, instead of that they make repeated attempts, or retrials, in order to enter service after some time. We obtain the distribution of the number of retrials produced by a tagged customer, until he finds an available server. 相似文献
998.
Litharge, the red tetragonal form of lead oxide α-PbO and massicot, the yellow orthorhombic form β-PbO, are synthesized from
lead(II) salts in aqueous media at elevated temperatures. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were
used to characterize the size, morphology, and crystallographic structural forms of the products. The role of impurities in
the experimental synthesis of the materials and microstructural variations in the final products are described. The implications
of these observations regarding the synthesis of different conducting lead oxides and other related materials are discussed.
PACS 71.20.Ps; 72.80.Jc; 74.62.Bf; 74.62.Dh; 75.50.Tt 相似文献
999.
Many recent algorithmic approaches involve the construction of a differential equation model for computational purposes, typically
by introducing an artificial time variable. The actual computational model involves a discretization of the now time-dependent differential system, usually
employing forward Euler. The resulting dynamics of such an algorithm is then a discrete dynamics, and it is expected to be
“close enough” to the dynamics of the continuous system (which is typically easier to analyze) provided that small – hence
many – time steps, or iterations, are taken. Indeed, recent papers in inverse problems and image processing routinely report
results requiring thousands of iterations to converge. This makes one wonder if and how the computational modeling process
can be improved to better reflect the actual properties sought.
In this article we elaborate on several problem instances that illustrate the above observations. Algorithms may often lend
themselves to a dual interpretation, in terms of a simply discretized differential equation with artificial time and in terms
of a simple optimization algorithm; such a dual interpretation can be advantageous. We show how a broader computational modeling
approach may possibly lead to algorithms with improved efficiency.
AMS subject classification (2000) 65L05, 65M32, 65N21, 65N22, 65D18 相似文献
1000.
In cricket, a rain-affected pitch can make batting more difficult than normal. Several other conditions such as poor light or an initially lively pitch, may also result in difficulties for the batsmen. In this note, we refer to all of them as ‘sticky wickets’. On sticky wickets, lower order batsmen are often sent into ‘hold the fort’ until conditions improve. In this paper, a stochastic dynamic programming model is used to examine the appropriateness of this policy. The model suggests that the tactic is often optimal when the sticky wicket persists until the end of the day's play, but not often when the sticky wicket is transitory. In some circumstances, it is worthwhile, on a normal wicket near the end of the day, to send in a lower order batsman to hold the fort (a night watchman): when the wicket is sticky, this tactic is even more worthwhile. 相似文献