全文获取类型
收费全文 | 21059篇 |
免费 | 902篇 |
国内免费 | 148篇 |
专业分类
化学 | 14415篇 |
晶体学 | 144篇 |
力学 | 548篇 |
数学 | 3259篇 |
物理学 | 3743篇 |
出版年
2023年 | 180篇 |
2022年 | 156篇 |
2021年 | 515篇 |
2020年 | 539篇 |
2019年 | 535篇 |
2018年 | 388篇 |
2017年 | 311篇 |
2016年 | 817篇 |
2015年 | 655篇 |
2014年 | 791篇 |
2013年 | 1201篇 |
2012年 | 1484篇 |
2011年 | 1727篇 |
2010年 | 953篇 |
2009年 | 855篇 |
2008年 | 1346篇 |
2007年 | 1243篇 |
2006年 | 1195篇 |
2005年 | 1053篇 |
2004年 | 912篇 |
2003年 | 645篇 |
2002年 | 638篇 |
2001年 | 270篇 |
2000年 | 231篇 |
1999年 | 207篇 |
1998年 | 168篇 |
1997年 | 175篇 |
1996年 | 209篇 |
1995年 | 144篇 |
1994年 | 152篇 |
1993年 | 161篇 |
1992年 | 128篇 |
1991年 | 116篇 |
1990年 | 97篇 |
1989年 | 86篇 |
1988年 | 104篇 |
1987年 | 83篇 |
1986年 | 76篇 |
1985年 | 114篇 |
1984年 | 122篇 |
1983年 | 79篇 |
1982年 | 124篇 |
1981年 | 144篇 |
1980年 | 107篇 |
1979年 | 96篇 |
1978年 | 129篇 |
1977年 | 101篇 |
1976年 | 82篇 |
1975年 | 80篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
22.
23.
Aidan Mooney John G. Keating Daniel M. Heffernan 《Chaos, solitons, and fractals》2006,30(5):1088-1097
A digital watermark is a visible, or preferably invisible, identification code that is permanently embedded in digital media, to prove owner authentication and provide protection for documents. Given the interest in watermark generation using chaotic functions a detailed study of one chaotic function for this purpose is performed. In this paper, we present an approach for the generation of watermarks using the logistic map. Using this function, in conjunction with seed management, it is possible to generate chaotic sequences that may be used to create highpass or lowpass digital watermarks. In this paper we provide a detailed study on the generation of optically detectable watermarks and we provide some guidelines on successful chaotic watermark generation using the logistic map, and show using a recently published scheme, how care must be taken in the selection of the function seed. 相似文献
24.
Daniel B. Szyld 《Numerical Algorithms》2006,42(3-4):309-323
Given an oblique projector P on a Hilbert space, i.e., an operator satisfying P
2=P, which is neither null nor the identity, it holds that ||P|| = ||I –P||. This useful equality, while not widely-known, has been proven repeatedly in the literature. Many published proofs are reviewed, and simpler ones are presented. 相似文献
25.
26.
A Garcia-Quiroz David L AzevedoE.C da Silva 《Journal of Physics and Chemistry of Solids》2002,63(10):1863-1866
Properties of oxygenated carbon nitride films have attracted the attention of physics researchers due to their magnetic and physical properties, as well as for their usefulness in the industry. The free radicals were investigated using electron paramagnetic resonance applied in the study of spin concentration due to the different mechanism of preparation of carbon nitride films by RF-discharge with different kinds of plasma. Unpaired spin concentrations, in the order of 1020 per cm3, were measured and their time recombination dependency was important in those films. The films were grown by plasma enhanced chemical vapor deposition using mixtures of hydrocarbons, N2 and O2 in different proportions. 相似文献
27.
A simple simulation scheme that simultaneously describes the growth kinetics of SiO2 films at the nanometer scale and the SiOx/Si interface dynamics (its extent, and spatial/temporal evolution) is presented. The simulation successfully applies to experimental data in the region above and below 10 nm, reproduces the Deal and Grove linear-parabolic law and the oxide growth rate enhancement in the very thin film regime (the so-called anomalous region). According to the simulation, the oxidation is governed mainly by two processes: (a) the formation of a transition suboxide layer and (b) its subsequent drift towards the silicon bulk. We found that it is the superposition of these two processes that produces the crossover from the anomalous oxidation region behavior to the linear-parabolic law. 相似文献
28.
Partially supported by the general research fund at the University of Kansas 相似文献
29.
Daniel Michelson 《Journal d'Analyse Mathématique》1989,53(1):1-138
The general problem studied has as a prototype the full non-linear Navier-Stokes equations for a slightly viscous compressible
fluid including the heat transfer. The boundaries are of inflow-outflow type, i.e. non-characteristic, and the boundary conditions
are the most general ones with any order of derivatives. It is assumed that the uniform Lopatinsky condition is satisfied.
The goal is to prove uniform existence and boundedness of solution as the viscosity tends to zero and to justify the boundary
layer asymptotics. The paper consists of two parts. In Part I the linear problem is studied. Here, uniform lower and higher
order tangential estimates are derived and the existence of a solution is proved. The higher order estimates depend on the
smoothness of coefficients; however this smoothness does not exceed the smoothness of the solution. In Part II the quasilinear
problem is studied. It is assumed that for zero viscosity the overall initial-boundary value problem has a smooth solutionu
0 in a time interval 0≦t≦T
0. As a result the boundary laye, is weak and is uniformlyC
1 bounded. This makes the linear theory applicable. an iteration scheme is set and proved to converge to the viscous solution.
The convergence takes place for small viscosity and over the original time interval 0≦t≦T
0. 相似文献
30.
Software failures have become the major factor that brings the system down or causes a degradation in the quality of service. For many applications, estimating the software failure rate from a user's perspective helps the development team evaluate the reliability of the software and determine the release time properly. Traditionally, software reliability growth models are applied to system test data with the hope of estimating the software failure rate in the field. Given the aggressive nature by which the software is exercised during system test, as well as unavoidable differences between the test environment and the field environment, the resulting estimate of the failure rate will not typically reflect the user‐perceived failure rate in the field. The goal of this work is to quantify the mismatch between the system test environment and the field environment. A calibration factor is proposed to map the failure rate estimated from the system test data to the failure rate that will be observed in the field. Non‐homogeneous Poisson process models are utilized to estimate the software failure rate in both the system test phase and the field. For projects that have only system test data, use of the calibration factor provides an estimate of the field failure rate that would otherwise be unavailable. For projects that have both system test data and previous field data, the calibration factor can be explicitly evaluated and used to estimate the field failure rate of future releases as their system test data becomes available. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献