全文获取类型
收费全文 | 17398篇 |
免费 | 748篇 |
国内免费 | 1194篇 |
专业分类
化学 | 10069篇 |
晶体学 | 194篇 |
力学 | 143篇 |
综合类 | 70篇 |
数学 | 6798篇 |
物理学 | 2066篇 |
出版年
2023年 | 100篇 |
2022年 | 237篇 |
2021年 | 245篇 |
2020年 | 202篇 |
2019年 | 707篇 |
2018年 | 415篇 |
2017年 | 592篇 |
2016年 | 351篇 |
2015年 | 419篇 |
2014年 | 558篇 |
2013年 | 1014篇 |
2012年 | 829篇 |
2011年 | 1104篇 |
2010年 | 815篇 |
2009年 | 1124篇 |
2008年 | 1200篇 |
2007年 | 1104篇 |
2006年 | 980篇 |
2005年 | 757篇 |
2004年 | 757篇 |
2003年 | 646篇 |
2002年 | 561篇 |
2001年 | 440篇 |
2000年 | 386篇 |
1999年 | 397篇 |
1998年 | 327篇 |
1997年 | 280篇 |
1996年 | 258篇 |
1995年 | 290篇 |
1994年 | 252篇 |
1993年 | 230篇 |
1992年 | 205篇 |
1991年 | 131篇 |
1990年 | 196篇 |
1989年 | 195篇 |
1988年 | 133篇 |
1987年 | 128篇 |
1986年 | 140篇 |
1985年 | 130篇 |
1984年 | 107篇 |
1983年 | 42篇 |
1982年 | 45篇 |
1981年 | 52篇 |
1980年 | 51篇 |
1979年 | 46篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1976年 | 28篇 |
1974年 | 16篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
51.
Y.P. Venkata Subbaiah 《Applied Surface Science》2006,253(5):2409-2415
ZnS films have been deposited on glass substrates by close-spaced evaporation (CSE) technique. The films were grown at different temperatures in the range, 200-350 °C. The layers have been characterized with X-ray diffractometer (XRD), atomic force microscope (AFM), energy dispersive analysis of X-rays (EDAX) and optical spectrophotometer to evaluate the quality of the layers for photovoltaic applications. The studies showed that the optimum substrate temperature for the growth of ZnS layers was 300 °C. The films grown at these temperatures exhibited cubic structure with nearly stoichiometric composition. The AFM data revealed that the films had nano-sized grains with a grain size of ∼40 nm. The optical studies exhibited direct allowed transition with an energy band gap of 3.61 eV. The other structural and optical parameters such as lattice stress, dislocation density, refractive index and extinction coefficient were also evaluated. The temperature-dependent conductivity measured in the range, 303-523 K showed a change in the conduction mechanism at 120 °C. The activation energy values evaluated using the temperature dependence of electrical conductivity are 7 and 29 meV at low and high temperature regions, respectively. 相似文献
52.
Lévy processes in matrix Lie groups are studied. Subordination (random time change) is used to show that quasi-invariance of the Brownian motion in a Lie group induces absolute continuity of the laws of the corresponding pure jump processes. These results are applied to several examples which are discussed in detail. 相似文献
53.
Roberto E. Madariaga-Garcia 《Topology and its Applications》2007,154(7):1470-1480
Tkachenko showed in 1990 the existence of a countably compact group topology on the free Abelian group of size c using CH. Koszmider, Tomita and Watson showed in 2000 the existence of a countably compact group topology on the free Abelian group of size c2 using a forcing model in which CH holds.Wallace's question from 1955, asks whether every both-sided cancellative countably compact semigroup is a topological group. A counterexample to Wallace's question has been called a Wallace semigroup. In 1996, Robbie and Svetlichny constructed a Wallace semigroup under CH. In the same year, Tomita constructed a Wallace semigroup from MAcountable.In this note, we show that the examples of Tkachenko, Robbie and Svetlichny, and Koszmider, Tomita and Watson can be obtained using a family of selective ultrafilters. As a corollary, the constructions presented here are compatible with the total failure of Martin's Axiom. 相似文献
54.
Summary We consider a class of infinite delay equations of neutral type which includes Volterrra type integral und integrodifferential equations. Using abstract approximation results (cf. Trotter-Kato-type) for strongly continuous semigroups we develop an approximation scheme which is based on approximation of the system state by Laguerre (and Legendre) polynomials. Numerical examples demonstrate the feasibility of the scheme and show infinite order convergence for smooth data.Supported in part by the Air Force Office of Scientific Research under Contracts AFORS-84-0398 and AFORS-85-0303 and the National Aeronautics and Space Administration under NASA Grant NAG-1-517 and by NSF under Grant UINT-8521208Supported in part by the Air Force Office of Scientific Research under Contract AFORS-84-0398 and in part by the Fonds zur Förderung der wissenschaftlichen Forschung, Austria, under project No. S3206 相似文献
55.
We give a new and comparably short proof of Gittins’ index theorem for dynamic allocation problems of the multi-armed bandit type in continuous time under minimal assumptions. This proof gives a complete characterization of optimal allocation strategies as those policies which follow the current leader among the Gittins indices while ensuring that a Gittins index is at an all-time low whenever the associated project is not worked on exclusively. The main tool is a representation property of Gittins index processes which allows us to show that these processes can be chosen to be pathwise lower semi-continuous from the right and quasi-lower semi-continuous from the left. Both regularity properties turn out to be crucial for our characterization and the construction of optimal allocation policies. 相似文献
56.
57.
Christos Kravvaritis Marilena Mitrouli Jennifer Seberry 《Linear and Multilinear Algebra》2007,55(5):471-490
In the present article we concentrate our study on the growth problem for the weighing matrix W(12,11) and show that the unique W(12,11) has three pivot structures. An improved algorithm for extending a k × k (0,+,-) matrix to a W(n,n-1), if possible, has been developed to simplify the proof. For the implementation of the algorithm special emphasis is given to the notions of data structures and parallel processing. 相似文献
58.
Piotr Borodulin-Nadzieja 《Topology and its Applications》2007,154(18):3107-3124
We investigate properties of minimally generated Boolean algebras. It is shown that all measures defined on such algebras are separable but not necessarily weakly uniformly regular. On the other hand, there exist Boolean algebras small in terms of measures which are not minimally generated. We prove that under CH a measure on a retractive Boolean algebra can be nonseparable. Some relevant examples are indicated. Also, we give two examples of spaces satisfying some kind of Efimov property. 相似文献
59.
Despite the large cost of bodily injury (BI) claims in motor insurance, relatively little research has been done in this area. Many companies estimate (and therefore reserve) bodily injury compensation directly from initial medical reports. This practice may underestimate the final cost, because the severity is often assessed during the recovery period. Since the evaluation of this severity is often only qualitative, in this paper we apply an ordered multiple choice model at different moments in the life of a claim reported to an insurance company. We assume that the information available to the insurer does not flow continuously, because it is obtained at different stages. Using a real data set, we show that the application of sequential ordered logit models leads to a significant improvement in the prediction of the BI severity level, compared to the subjective classification that is used in practice. We also show that these results could improve the insurer’s reserves notably. 相似文献
60.
T. Nakamura 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):33-37
We present the recent experimental results on the 6He structure studied by the 6Li(t, 3He)6He reaction at 336 MeV. Above the conspicuous peaks for ground and first excited states for 6He, we have observed a broad structures at E
x∼ 5 MeV, and E
x∼ 15 MeV. The angular distribution of this structure exhibits the dominance of a ΔL = 1 transition, indicating the existence of intruder dipole states at low excitation energies in 6He. A slight admixture of positive-parity states in this structure has been indicated as well.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献