全文获取类型
收费全文 | 28328篇 |
免费 | 693篇 |
国内免费 | 164篇 |
专业分类
化学 | 19037篇 |
晶体学 | 227篇 |
力学 | 758篇 |
综合类 | 1篇 |
数学 | 4741篇 |
物理学 | 4421篇 |
出版年
2021年 | 227篇 |
2020年 | 270篇 |
2019年 | 337篇 |
2018年 | 271篇 |
2017年 | 235篇 |
2016年 | 510篇 |
2015年 | 403篇 |
2014年 | 513篇 |
2013年 | 1421篇 |
2012年 | 1334篇 |
2011年 | 1672篇 |
2010年 | 859篇 |
2009年 | 692篇 |
2008年 | 1510篇 |
2007年 | 1551篇 |
2006年 | 1553篇 |
2005年 | 1531篇 |
2004年 | 1306篇 |
2003年 | 1113篇 |
2002年 | 1062篇 |
2001年 | 407篇 |
2000年 | 320篇 |
1999年 | 261篇 |
1998年 | 316篇 |
1997年 | 332篇 |
1996年 | 416篇 |
1995年 | 314篇 |
1994年 | 299篇 |
1993年 | 307篇 |
1992年 | 293篇 |
1991年 | 256篇 |
1990年 | 238篇 |
1989年 | 255篇 |
1988年 | 280篇 |
1987年 | 269篇 |
1986年 | 270篇 |
1985年 | 401篇 |
1984年 | 458篇 |
1983年 | 337篇 |
1982年 | 447篇 |
1981年 | 429篇 |
1980年 | 369篇 |
1979年 | 339篇 |
1978年 | 367篇 |
1977年 | 355篇 |
1976年 | 295篇 |
1975年 | 285篇 |
1974年 | 271篇 |
1973年 | 248篇 |
1972年 | 154篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
The multilevel generalized assignment problem is a problem of assigning agents to tasks where the agents can perform tasks at more than one efficiency level. A profit is associated with each assignment and the objective of the problem is profit maximization. Two heuristic solution methods are presented for the problem. The heuristics are developed from solution methods for the generalized assignment problem. One method uses a regret minimization approach whilst the other method uses a repair approach on a relaxation of the problem. The heuristics are able to solve moderately large instances of the problem rapidly and effectively. Procedures for deriving an upper bound on the solution of the problem are also described. On larger and harder instances of the problem one heuristic is particularly effective. 相似文献
12.
A multiple image cryptosystem based on different apertures in an optical set-up under a holographic arrangement is proposed. The system is a security architecture that uses different pupil aperture mask in the encoding lens to encrypt different images. Based on this approach multiple encryption is achieved by changing the pupil aperture arrangement of the optical system among exposures. In addition to the classical speckle phase mask, the geometrical parameters characterizing the apertures are introduced to increase the system security. Even when an illegal user steals the speckle phase mask, the system cannot be broken into without the correct pupil geometrical parameters. The experimental set-up is based on a volume photorefractive BSO crystal as storing device. Information retrieval is done via a phase conjugation operation. We also have to stress that the multiple storage under this scheme, is only possible with the help of the aperture mask. Simulation and experimental results are further introduced to verify the proposed method. 相似文献
13.
For a ? R\alpha \in \mathbf{R}, the class of a-\alpha -order spherical harmonic functions in an open set W í\Omega \subseteq Sn-1\mathbf{S}^{n-1}, Ha(W)H^{\alpha }(\Omega ) is defined as the C2-C^{2}-solutions of Dau=0\Delta _{\alpha }u=0; where Da=Ds+a(n+a-2)\Delta _{\alpha }=\Delta _{s}+\alpha (n+\alpha -2) is the spherical Laplace--Beltrami operator of order a\alpha and Ds\Delta _{s} is the radially independent part of the Laplace operator. We obtain a Green's integral formula for the functions in Ha(W)H^{\alpha }(\Omega ) with kernel expressed as a Gegenbauer function. As generalizations, higher order spherical iterated Dirac operators are defined in a polynomial form. Integral representations of the null solutions to these operators and an intertwining formula relating these operators on the sphere and their analogues in Euclidean space are presented. 相似文献
14.
Relying on reliability growth testing to improve system designis neither usually effective nor efficient. Instead it is importantto design in reliability. This requires models to estimate reliabilitygrowth in the design that can be used to assess whether goalreliability will be achieved within the target timescale forthe design process. Many models have been developed for analysisof reliability growth on test, but there has been much lessattention given to reliability growth in design. This paperdescribes and compares two models: one motivated by the practicalengineering process; the other by extending the reasoning ofstatistical reliability growth modelling. Both models are referencedin the recently revised edition of international standard IEC61164. However, there has been no reported evaluation of theirproperties. Therefore, this paper explores the commonalitiesand differences between these models through an assessment oftheir logic and their application to an industrial example.Recommendations are given for the use of reliability growthmodels to aid management of the design process and to informproduct development. 相似文献
15.
In response to the bio-activity found in fluorine-containing 4-alkyl-3, 5-pyridinedicarboxylates, a series of novel 4-substituted derivatives, not directly available by Hantzsch sequences, were prepared. Starting 4-alkylpyridines, 1 , were converted via enamine 2 to materials 3–8 . Derivatives 9–16 in turn were derived from aldehyde 3 , while acid derivatives 28–36 were prepared from 14 . Addition of oxygen, sulfur, and carbenoids effected conversion of 4-allylpyridine 16 to epoxy and cyclopropyl derivative 16–22 . A number of neighboring group effects were noted, including those forming the fused-ring systems 23-27 . 相似文献
16.
E. Rodriguez E. Jimenez G.J. Jacob A.A.R. Neves C.L. Cesar L.C. Barbosa 《Physica E: Low-dimensional Systems and Nanostructures》2005,26(1-4):361
Multilayer PbTe quantum dots (QDs) and SiO2 were grown by pulsed laser deposition (PLD) and Plasma enhanced chemical vapor deposition (PECVD) techniques. The crystalline structure, QD size and size dispersion were observed by high-resolution transmission electron microscopy (HRTEM) measurements. This technique allows one to grow PbTe QDs as small as 1.8 nm diameter and 0.6 nm size dispersion. The whole structure can be used in a Fabry–Perot cavity for an optical device operating at the mid-infrared region. 相似文献
17.
18.
For ϕ a δ-subharmonic function, sharp results are obtained that connectA(r, ϕ), B(r, ϕ) andT(r, ϕ), whereA(r, ϕ)=inf|z|=r
ϕ(z),B(r, ϕ)=sup|z|=r
ϕ(z), andT(r, ϕ) is the Nevanlinna characteristics. 相似文献
19.
Ricaurte Rodríguez Manuel Nogueras Justo Cobo John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(7):o376-o378
Molecules of the title compound, C12H13ClN4, are linked by two independent N—H...N hydrogen bonds into a chain of edge‐fused R22(8) rings. The significance of this study lies in its attempt to rationalize the patterns of supramolecular aggregation in the title compound and in a range of analogous 4,6‐disubstituted 2‐aminopyrimidines. 相似文献
20.
Jacob Schach Møller 《Annales Henri Poincare》2005,6(6):1091-1135
In this paper we analyze the bottom of the energy-momentum spectrum of the translation invariant Nelson model, describing
one electron linearly coupled to a second quantized massive scalar field. Our results are valid for all values of the coupling
constant and include an HVZ theorem, non-degeneracy of ground states, existence of isolated groundstates in dimensions 1 and
2, non-existence of ground states embedded in the bottom of the essential spectrum in dimensions 3 and 4, (i.e., at total
momenta where no isolated groundstate eigenvalue exists), and we study regularity and monotonicity properties of the bottom
of the essential spectrum, as a function of total momentum.
Communicated by Joel Feldman
submitted 04/11/04, accepted 17/02/05 相似文献