全文获取类型
收费全文 | 9472篇 |
免费 | 914篇 |
国内免费 | 1114篇 |
专业分类
化学 | 2443篇 |
晶体学 | 52篇 |
力学 | 1167篇 |
综合类 | 154篇 |
数学 | 5967篇 |
物理学 | 1717篇 |
出版年
2024年 | 16篇 |
2023年 | 101篇 |
2022年 | 179篇 |
2021年 | 202篇 |
2020年 | 282篇 |
2019年 | 264篇 |
2018年 | 254篇 |
2017年 | 330篇 |
2016年 | 301篇 |
2015年 | 256篇 |
2014年 | 424篇 |
2013年 | 766篇 |
2012年 | 479篇 |
2011年 | 567篇 |
2010年 | 471篇 |
2009年 | 669篇 |
2008年 | 618篇 |
2007年 | 613篇 |
2006年 | 540篇 |
2005年 | 518篇 |
2004年 | 436篇 |
2003年 | 409篇 |
2002年 | 366篇 |
2001年 | 276篇 |
2000年 | 298篇 |
1999年 | 290篇 |
1998年 | 286篇 |
1997年 | 244篇 |
1996年 | 155篇 |
1995年 | 119篇 |
1994年 | 116篇 |
1993年 | 79篇 |
1992年 | 83篇 |
1991年 | 68篇 |
1990年 | 55篇 |
1989年 | 35篇 |
1988年 | 42篇 |
1987年 | 42篇 |
1986年 | 40篇 |
1985年 | 27篇 |
1984年 | 31篇 |
1983年 | 8篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 19篇 |
1979年 | 22篇 |
1978年 | 20篇 |
1977年 | 13篇 |
1976年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
71.
72.
现场抽样调查中,由于测量误差的存在,使得所测变量实测值的方差增大,通过增加每个体的测量次数可以控制测量误差,但这样每个体调查费用增大。本文对测量信度R,每个体测量次数m与相应所需的样本含量nm、调查费用Tn的关系进行了探讨,并介绍了如何根据R,及每个体测量费用占其总费用构成比C,确定最佳测量次数m值,以达到最佳控制调查费用的目的,这对我们在大型现场调查中进行经济效益分析具有重大的理论指导意义。 相似文献
73.
Youssef Saab 《Journal of Heuristics》1997,3(3):207-224
A heuristic optimization methodology, Dynamic Contraction (DC), is introduced as an approach for solving a wide variety of hard combinatorial problems. Contraction is an operation that maps an instance of a problem to a smaller instance of the same problem. DC is an iterative improvement strategy that relies on contraction as a mechanism for escaping local minima. As a byproduct of contraction, efficiency is improved due to a reduction of problem size. Effectiveness of DC is shown through simple applications to two classical combinatorial problems: The graph bisection problem and the traveling salesman problem. 相似文献
74.
Algorithms for systematical search of inverse in arithmetic operations with fuzzy numbers are discussed. The fuzzy number derived is considered either in a discrete representation of its support or described by the parameters of the corresponding membership function formula. Both algorithms are easy to be programmed for desk calculators. 相似文献
75.
The blends composed of polyamide 6 (PA6) and polyamide 66 (PA66) were obtained using two different preparation methods, one of which was the melt‐mixing through a twin‐screw extruder and the subsequent injection molding; and the other, the in situ blending through anionic polymerization of ε‐caprolactam in the presence of PA66. For the former, there existed a remarkable improvement in toughness but a drastic drop in strength and modulus; however, for the latter, a reverse but less significant trend of mechanical properties change appeared. Various characterizations were conducted, including the analyses of crystalline morphology, crystallographic form, and crystallization and melting behaviors using polarized optical microscopy (POM), wide‐angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively; observation of morphology of fractured surface with scanning electron microscope (SEM); measurement of glass transition through dynamic mechanical analysis (DMA); and the intermolecular interaction as well as the interchange reaction between the two components by Fourier transform infrared spectrometry (FT‐IR) and 13C solution NMR. The presence and absence of interchange reaction was verified for the in situ and melt‐mixed blends, respectively. It is believed that the transreaction resulted in a drop in glass transition temperature (Tg) for the in situ blends, contrary to an increase of Tg with increasing PA66 content for the melt‐mixed ones. And the two kinds of fabrication methods led to significant differences in the crystallographic form, spherulite size and crystalline content and perfection as well. Accordingly, it is attempted to explain the reasons for the opposite trends of changes in the mechanical properties for these two blends. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1176–1186, 2007 相似文献
76.
S.V. Astashkin F.A. Sukochev 《Journal of Mathematical Analysis and Applications》2007,336(2):1231-1258
We catalogue all Marcinkiewicz function and sequence spaces with the Banach-Saks property and present necessary and sufficient conditions for a wide subclass of spaces to possess the p-Banach-Saks property, 1<p<∞. We apply our results to several open problems. 相似文献
77.
平衡规划问题的熵函数方法及其在混合交通流中的应用 总被引:1,自引:0,他引:1
将参变极值问题的极大熵函数方法应用到求解平衡规划问题中,通过先验分布信息和Kullback熵概念,给出了平衡规划问题基于Kullback熵表示的熵函数求解方法,并将平衡规划的极大熵函数方法应用于求解混合交通平衡分配问题. 相似文献
78.
Bai-suo JIN~ 《中国科学A辑(英文版)》2007,50(9):1303-1315
In the factor analysis model with large cross-section and time-series dimensions,we pro- pose a new method to estimate the number of factors.Specially if the idiosyncratic terms satisfy a linear time series model,the estimators of the parameters can be obtained in the time series model. The theoretical properties of the estimators are also explored.A simulation study and an empirical analysis are conducted. 相似文献
79.
80.
In this paper we study higher Chow groups of smooth, projective surfaces over a field k of characteristic zero, using some new Hodge theoretic methods which we develop for this purpose. In particular we investigate the subgroup of CH
r+1 (X,r) with r = 1,2 consisting of cycles that are supported over a normal crossing divisor Z on X. In this case, the Hodge theory of the complement forms an interesting variation of mixed Hodge structures in any geometric deformation of the situation. Our main result is a structure theorem in the case where X is a very general hypersurface of degree d in projective 3-space for d sufficiently large and Z is a union of very general hypersurface sections of X. In this case we show that the subgroup of CH
r+1 (X,r) we consider is generated by obvious cycles only arising from rational functions on X with poles along Z. This can be seen as a generalization of the Noether–Lefschetz theorem for r = 0. In the case r = 1 there is a similar generalization by Müller-Stach, but our result is more precise than it, since it is geometric and not only cohomological. The case r = 2 is entirely new and original in this paper. For small d, we construct some explicit examples for r = 1 and 2 where the corresponding higher Chow groups are indecomposable, i.e. not the image of certain products of lower order groups. In an appendix Alberto Collino constructs even more indecomposable examples in CH
3 (X,2) which move in a one-dimensional family on the surface X.Contribution to appendix. 相似文献