全文获取类型
收费全文 | 9473篇 |
免费 | 1063篇 |
国内免费 | 751篇 |
专业分类
化学 | 799篇 |
晶体学 | 17篇 |
力学 | 769篇 |
综合类 | 278篇 |
数学 | 5418篇 |
物理学 | 4006篇 |
出版年
2024年 | 9篇 |
2023年 | 78篇 |
2022年 | 143篇 |
2021年 | 245篇 |
2020年 | 199篇 |
2019年 | 208篇 |
2018年 | 240篇 |
2017年 | 324篇 |
2016年 | 335篇 |
2015年 | 306篇 |
2014年 | 546篇 |
2013年 | 668篇 |
2012年 | 458篇 |
2011年 | 557篇 |
2010年 | 482篇 |
2009年 | 623篇 |
2008年 | 644篇 |
2007年 | 630篇 |
2006年 | 582篇 |
2005年 | 489篇 |
2004年 | 407篇 |
2003年 | 451篇 |
2002年 | 388篇 |
2001年 | 304篇 |
2000年 | 274篇 |
1999年 | 263篇 |
1998年 | 256篇 |
1997年 | 243篇 |
1996年 | 178篇 |
1995年 | 133篇 |
1994年 | 104篇 |
1993年 | 86篇 |
1992年 | 73篇 |
1991年 | 57篇 |
1990年 | 37篇 |
1989年 | 32篇 |
1988年 | 31篇 |
1987年 | 30篇 |
1986年 | 19篇 |
1985年 | 30篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1974年 | 3篇 |
1959年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
We propose a new approach towards proving that the fixed point property for ordered sets is preserved by products. This approach uses a characterization of fixed points in products via isotone relations. First explorations of classes of isotone relations are presented. These first explorations give us hope that this approach could lead to advances on the Product Problem. 相似文献
72.
本文结合代数教学中的实际情况.针对代数教学过程中出现的问题。提出了在代数教学过程中改进教学方法的几条措施. 相似文献
73.
《Surface and interface analysis : SIA》2004,36(9):1304-1313
For rough heterogeneous samples, the contrast observed in XPS images may result from both changes in elemental or chemical composition and sample topography. Background image acquisition and subtraction are frequently utilized to minimize topographical effects so that images represent concentration variations in the sample. This procedure may significantly increase the data acquisition time. Multivariate statistical methods can assist in resolving topographical and chemical information from multispectral XPS images. Principal component analysis (PCA) is one method for identification of the highest correlation/variation between the images. Topography, which is common to all of the images, will be resolved in the first most significant component. The score of this component contains spatial information about the topography of the surface, whereas the loading is a quantitative representation of the topography contribution to each elemental/chemical image. The simple‐to‐use self‐modelling mixture analysis (Simplisma) method is a pure variable method that searches for the source of most differences in the data and therefore has the potential to distinguish between chemical and topographical phases in images. The mathematical background correction scheme is developed and validated by comparing results to the experimental background correction for samples with differing degrees of topography. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
74.
A new type of optoelectronic cellular neural network has been developed by providing the capability of coefficients adjusment of cellular neural network (CNN) using Widrow based perceptron learning algorithm. The new supervised cellular neural network is called Widrow-CNN. Despite the unsupervised CNN, the proposed learning algorithm allows to use the Widrow-CNN for various image processing applications easily. Also, the capability of CNN for image processing and feature extraction has been improved using basic joint transform correlation architecture. This hardware application presents high speed processing capability compared to digital applications. The optoelectronic Widrow-CNN has been tested for classic CNN feature extraction problems. It yields the best results even in case of hard feature extraction problems such as diagonal line detection and vertical line determination. 相似文献
75.
激光水下扫描成像系统的图像修正机理 总被引:2,自引:0,他引:2
同步扫描系统可以克服后向散射,但同时也产生了两种非线性,一是扫描线速度的非线性,一是由于距离对光能衰减造成的非线性,两种非线性都需要修正,本文从原理上讨论水下激光成像时能量衰减的修正原理与修正方法,并给出仿真结果。 相似文献
76.
77.
Nikolaos S. PAPAGEORGIOU Nikolaos YANNAKAKIS 《数学学报(英文版)》2005,21(5):977-996
This is the first part of a work on second order nonlinear, nonmonotone evolution inclusions defined in the framework of an evolution triple of spaces and with a multivalued nonlinearity depending on both x(t) and x(t). In this first part we prove existence and relaxation theorems. We consider the case of an usc, convex valued nonlinearity and we show that for this problem the solution set is nonempty and compact in C^1 (T, H). Also we examine the Isc, nonconvex case and again we prove the existence of solutions. In addition we establish the existence of extremal solutions and by strengthening our hypotheses, we show that the extremal solutions are dense in C^1 (T, H) to the solutions of the original convex problem (strong relaxation). An example of a nonlinear hyperbolic optimal control problem is also discussed. 相似文献
78.
Athanossios Tzouvaras 《Mathematical Logic Quarterly》2003,49(3):284-292
The powerset operator, ??, is compared with other operators of similar type and logical complexity. Namely we examine positive operators whose defining formula has a canonical form containing at most a string of universal quantifiers. We call them ?‐operators. The question we address in this paper is: How is the class of ?‐operators generated ? It is shown that every positive ?‐operator Γ such that Γ(??) ≠ ??, is finitely generated from ??, the identity operator Id, constant operators and certain trivial ones by composition, ∪ and ∩. This extends results of [3] concerning bounded positive operators. 相似文献
80.
A. Truffert 《Annals of Operations Research》1991,30(1):115-156
The conditional expectation of integrands and random sets is the main tool of stochastic optimization. This work wishes to make up for the lack of real synthesis about this subject. We improve the existing hypothesis and simplify the corresponding proofs. In the convex case we especially study the problem of the exchange of conditional expectation and subdifferential operators. 相似文献