全文获取类型
收费全文 | 2889篇 |
免费 | 42篇 |
国内免费 | 87篇 |
专业分类
化学 | 897篇 |
晶体学 | 5篇 |
力学 | 78篇 |
综合类 | 14篇 |
数学 | 1381篇 |
物理学 | 643篇 |
出版年
2024年 | 5篇 |
2023年 | 60篇 |
2022年 | 43篇 |
2021年 | 45篇 |
2020年 | 44篇 |
2019年 | 43篇 |
2018年 | 46篇 |
2017年 | 30篇 |
2016年 | 47篇 |
2015年 | 35篇 |
2014年 | 88篇 |
2013年 | 302篇 |
2012年 | 251篇 |
2011年 | 143篇 |
2010年 | 97篇 |
2009年 | 173篇 |
2008年 | 146篇 |
2007年 | 147篇 |
2006年 | 137篇 |
2005年 | 133篇 |
2004年 | 109篇 |
2003年 | 70篇 |
2002年 | 72篇 |
2001年 | 56篇 |
2000年 | 65篇 |
1999年 | 62篇 |
1998年 | 61篇 |
1997年 | 63篇 |
1996年 | 65篇 |
1995年 | 67篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 27篇 |
1991年 | 27篇 |
1990年 | 17篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 19篇 |
1986年 | 15篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 20篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1969年 | 1篇 |
排序方式: 共有3018条查询结果,搜索用时 15 毫秒
91.
In this paper, we investigate a constrained optimization problem with a quadratic cost functional and two quadratic equality constraints. It is assumed that the cost functional is positive definite and that the constraints are both feasible and regular (but otherwise they are unrestricted quadratic functions). Thus, the existence of a global constrained minimum is assured. We develop a necessary and sufficient condition that completely characterizes the global minimum cost. Such a condition is of essential importance in iterative numerical methods for solving the constrained minimization problem, because it readily distinguishes between local minima and global minima and thus provides a stopping criterion for the computation. The result is similar to one obtained previously by the authors. In the previous result, we gave a characterization of the global minimum of a constrained quadratic minimization problem in which the cost functional was an arbitrary quadratic functional (as opposed to positive-definite here) and the constraints were at least positive-semidefinite quadratic functions (as opposed to essentially unrestricted here). 相似文献
92.
Elmor L. Peterson 《Mathematical Programming》1977,12(1):392-405
F.E. Clark has shown that if at least one of the feasible solution sets for a pair of dual linear programming problems is nonempty then at least one of them is both nonempty and unbounded. Subsequently, M. Avriel and A.C. Williams have obtained the same result in the more general context of (prototype posynomial) geometric programming. In this paper we show that the same result is actually false in the even more general context of convex programming — unless a certain regularity condition is satisfied.We also show that the regularity condition is so weak that it is automatically satisfied in linear programming (prototype posynomial) geometric programming, quadratic programming (with either linear or quadratic constraints),l
p
-regression analysis, optimal location, roadway network analysis, and chemical equilibrium analysis. Moreover, we develop an equivalent regularity condition for each of the usual formulations of duality.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR-73-2516. 相似文献
93.
本文应用周期分析方法对江陵县棉红蜘蛛—朱砂叶螨的预测预报技术作了新的尝试.经对江陵县36年历史资料拟合,符合率高达94.4%.运用这一技术对该县1987—1996年棉红蜘蛛发生程度进行了10年长期预测预报. 相似文献
94.
Taguchi method is the usual strategy in robust design and involves conducting experiments using orthogonal arrays and estimating the combination of factor levels that optimizes a given performance measure, typically a signal-to-noise ratio. The problem is more complex in the case of multiple responses since the combinations of factor levels that optimize the different responses usually differ. In this paper, an Artificial Neural Network, trained with the experiments results, is used to estimate the responses for all factor level combinations. After that, Data Envelopment Analysis (DEA) is used first to select the efficient (i.e. non-dominated) factor level combinations and then for choosing among them the one which leads to a most robust quality loss penalization. Mean Square Deviations of the quality characteristics are used as DEA inputs. Among the advantages of the proposed approach over traditional Taguchi method are the non-parametric, non-linear way of estimating quality loss measures for unobserved factor combinations and the non-parametric character of the performance evaluation of all the factor combinations. The proposed approach is applied to a number of case studies from the literature and compared with existing approaches. 相似文献
95.
Ali Emrouznejad Abdel Latef Anouze Emmanuel Thanassoulis 《European Journal of Operational Research》2010
Data Envelopment Analysis (DEA) is a nonparametric method for measuring the efficiency of a set of decision making units such as firms or public sector agencies, first introduced into the operational research and management science literature by Charnes, Cooper, and Rhodes (CCR) [Charnes, A., Cooper, W.W., Rhodes, E., 1978. Measuring the efficiency of decision making units. European Journal of Operational Research 2, 429–444]. The original DEA models were applicable only to technologies characterized by positive inputs/outputs. In subsequent literature there have been various approaches to enable DEA to deal with negative data. 相似文献
96.
We present new results on the correspondence between symmetries, conservation laws and variational principles for field equations in general non-abelian gauge theories. Our main result states that second order field equations possessing translational and gauge symmetries and the corresponding conservation laws are always derivable from a variational principle. We also show by the way of examples that the above result fails in general for third order field equations. 相似文献
97.
98.
99.
Functional connectivity patterns derived from neuroimaging data may be represented as graphs or networks, with individual image voxels or anatomically-defined structures representing the nodes, and a measure of correlation between the responses in each pair of nodes determining the edges. This explicit network representation allows network-analysis approaches to be applied to the characterization of functional connections within the brain. Much recent research in complex networks has focused on methods to identify community structure, i.e. cohesive clusters of strongly interconnected nodes. One class of such algorithms determines a partition of a network into 'sub-networks' based on the optimization of a modularity parameter, thus also providing a measure of the degree of segregation versus integration in the full network. Here, we demonstrate that a community structure algorithm based on the maximization of modularity, applied to a functional connectivity network calculated from the responses to acute fluoxetine challenge in the rat, can identify communities whose distributions correspond to anatomically meaningful structures and include compelling functional subdivisions in the brain. We also discuss the biological interpretation of the modularity parameter in terms of segregation and integration of brain function. 相似文献
100.