全文获取类型
收费全文 | 2967篇 |
免费 | 242篇 |
国内免费 | 376篇 |
专业分类
化学 | 124篇 |
晶体学 | 2篇 |
力学 | 59篇 |
综合类 | 48篇 |
数学 | 2782篇 |
物理学 | 570篇 |
出版年
2024年 | 4篇 |
2023年 | 42篇 |
2022年 | 56篇 |
2021年 | 64篇 |
2020年 | 78篇 |
2019年 | 84篇 |
2018年 | 73篇 |
2017年 | 82篇 |
2016年 | 93篇 |
2015年 | 92篇 |
2014年 | 149篇 |
2013年 | 182篇 |
2012年 | 140篇 |
2011年 | 152篇 |
2010年 | 130篇 |
2009年 | 186篇 |
2008年 | 248篇 |
2007年 | 199篇 |
2006年 | 181篇 |
2005年 | 164篇 |
2004年 | 123篇 |
2003年 | 145篇 |
2002年 | 140篇 |
2001年 | 92篇 |
2000年 | 115篇 |
1999年 | 99篇 |
1998年 | 89篇 |
1997年 | 69篇 |
1996年 | 67篇 |
1995年 | 31篇 |
1994年 | 38篇 |
1993年 | 26篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 16篇 |
1984年 | 10篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1936年 | 4篇 |
排序方式: 共有3585条查询结果,搜索用时 15 毫秒
101.
Abstract A comparison and evaluation is made of recent proposals for multivariate matched sampling in observational studies, where the following three questions are answered: (1) Algorithms: In current statistical practice, matched samples are formed using “nearest available” matching, a greedy algorithm. Greedy matching does not minimize the total distance within matched pairs, though good algorithms exist for optimal matching that do minimize the total distance. How much better is optimal matching than greedy matching? We find that optimal matching is sometimes noticeably better than greedy matching in the sense of producing closely matched pairs, sometimes only marginally better, but it is no better than greedy matching in the sense of producing balanced matched samples. (2) Structures: In common practice, treated units are matched to one control, called pair matching or 1–1 matching, or treated units are matched to two controls, called 1–2 matching, and so on. It is known, however, that the optimal structure is a full matching in which a treated unit may have one or more controls or a control may have one or more treated units. Optimal 1 — k matching is compared to optimal full matching, finding that optimal full matching is often much better. (3) Distances: Matching involves defining a distance between covariate vectors, and several such distances exist. Three recent proposals are compared. Practical advice is summarized in a final section. 相似文献
102.
Based on QCD sum rules we explore the consequences of a scenario for the ρ meson, where the chiral symmetry breaking condensates are set to zero whereas the chirally symmetric condensates remain at their vacuum values. This clean-cut scenario causes a lowering of the ρ spectral moment by about 120 MeV. The complementarity of mass shift and broadening is discussed. A simple parametrization of the ρ spectral function leads to a width of about 280 MeV if no shift of the peak position is assumed. 相似文献
103.
《Optimization》2012,61(4):799-814
The purpose of this paper is to present two new iterative algorithms to find the minimum norm fixed point of nonexpansive nonself-mappings in the framework of Hilbert spaces. The results presented in this paper improve and extend the corresponding ones announced by Yao and Xu [Yao Y, Xu HK. Iterative methods for finding minimum norm fixed point of mappings with applications. Optimization. 2011;60:645–658] and many others. Some applications to convex minimization and split feast problems(SFP) are also included. 相似文献
104.
AbstractIn this article, we first discuss the subduality and orthogonality of the cones and the dual cones when the norm is monotone in Banach spaces. Then, under different assumptions, the necessary and sufficient conditions for the ordering increasing property of the metric projection onto cones and order intervals are studied. Moreover, representations of the metric projection onto cones and order intervals are obtained. As applications, the solvability and approximation results of solutions to nonlinear discontinuous variational inequality and complementarity problems are proved by partial ordering methods. 相似文献
105.
《Acta Crystallographica. Section C, Structural Chemistry》2017,73(11):896-904
Since the discovery of electrochemically active LiFePO4, materials with tunnel and layered structures built up of transition metals and polyanions have become the subject of much research. A new quaternary arsenate, sodium calcium trinickel aluminium triarsenate, NaCa1–x Ni3–2x Al2x (AsO4)3 (x = 0.23), was synthesized using the flux method in air at 1023 K and its crystal structure was determined from single‐crystal X‐ray diffraction (XRD) data. This material was also characterized by qualitative energy‐dispersive X‐ray spectroscopy (EDS) analysis and IR spectroscopy. The crystal structure belongs to the α‐CrPO4 type with the space group Imma . The structure is described as a three‐dimensional framework built up of corner‐edge‐sharing NiO6, (Ni,Al)O6 and AsO4 polyhedra, with channels running along the [100] and [010] directions, in which the sodium and calcium cations are located. The proposed structural model has been validated by bond‐valence‐sum (BVS) and charge‐distribution (CHARDI) tools. The sodium ionic conduction pathways in the anionic framework were investigated by means of the bond‐valence site energy (BVSE) model, which predicted that the studied material will probably be a very poor Na+ ion conductor (bond‐valence activation energy ∼7 eV). 相似文献
106.
ZHANG YongBing 《中国科学A辑(英文版)》2009,(8)
We use the contact Yamabe flow to find solutions of the contact Yamabe problem on K-contact manifolds. 相似文献
107.
The aim of the paper is to estimate the density functions or distribution functions measured by Wasserstein metric, a typical kind of statistical distances, which is usually required in the statistical learningBased on the classical Bernstein approximation, a scheme is presented.To get the error estimates of the scheme, the problem turns to estimating the L1 norm of the Bernstein approximation for monotone C-1functions, which was rarely discussed in the classical approximation theoryFinally, we get a probability estimate by the statistical distance. 相似文献
108.
In this paper, we study the weighted Korn inequality on some irregular domains, e.g., s-John domains and domains satisfying quasihyperbolic boundary conditions. Examples regarding sharpness of the Korn inequality on these domains are presented. Moreover, we show that Korn inequalities imply certain Poincaré inequality. 相似文献
109.
110.
Kazuaki Z. Takahashi 《Journal of computational chemistry》2014,35(11):865-875
In our previous study (Takahashi et al., J. Chem. Theory Comput. 2012, 8, 4503), we developed the linear‐combination‐based isotropic periodic sum (LIPS) method. The LIPS method is based on the extended isotropic periodic sum theory that produces a ubiquitous interaction potential function to estimate homogeneous and heterogeneous systems. The LIPS theory also provides the procedure to design a periodic reaction field. To demonstrate this, in the present work, a novel reaction field of the LIPS method was developed. The novel reaction field was labeled LIPS‐SW, because it provides an interaction potential function with a shape that resembles that of the switch function method. To evaluate the ability of the LIPS‐SW method to describe in homogeneous and heterogeneous systems, we carried out molecular dynamics (MD) simulations of bulk water and water–vapor interfacial systems using the LIPS‐SW method. The results of these simulations show that the LIPS‐SW method gives higher accuracy than the conventional interaction potential function of the LIPS method. The accuracy of simulating water–vapor interfacial systems was greatly improved, while that of bulk water systems was maintained using the LIPS‐SW method. We conclude that the LIPS‐SW method shows great potential for high‐accuracy, high‐performance computing to allow large scale MD simulations. © 2014 Wiley Periodicals, Inc. 相似文献