全文获取类型
收费全文 | 44271篇 |
免费 | 702篇 |
国内免费 | 410篇 |
专业分类
化学 | 20929篇 |
晶体学 | 362篇 |
力学 | 1795篇 |
综合类 | 12篇 |
数学 | 11616篇 |
物理学 | 10669篇 |
出版年
2023年 | 143篇 |
2022年 | 261篇 |
2021年 | 351篇 |
2020年 | 418篇 |
2019年 | 396篇 |
2018年 | 1425篇 |
2017年 | 1655篇 |
2016年 | 1292篇 |
2015年 | 1011篇 |
2014年 | 1009篇 |
2013年 | 1489篇 |
2012年 | 4170篇 |
2011年 | 3569篇 |
2010年 | 2436篇 |
2009年 | 2088篇 |
2008年 | 1504篇 |
2007年 | 1513篇 |
2006年 | 1496篇 |
2005年 | 5207篇 |
2004年 | 4580篇 |
2003年 | 2772篇 |
2002年 | 988篇 |
2001年 | 464篇 |
2000年 | 234篇 |
1999年 | 320篇 |
1998年 | 229篇 |
1997年 | 193篇 |
1996年 | 213篇 |
1995年 | 146篇 |
1994年 | 154篇 |
1993年 | 143篇 |
1992年 | 258篇 |
1991年 | 244篇 |
1990年 | 206篇 |
1989年 | 179篇 |
1988年 | 185篇 |
1987年 | 128篇 |
1986年 | 111篇 |
1985年 | 139篇 |
1984年 | 133篇 |
1982年 | 137篇 |
1981年 | 142篇 |
1980年 | 113篇 |
1979年 | 115篇 |
1978年 | 139篇 |
1977年 | 113篇 |
1976年 | 152篇 |
1975年 | 109篇 |
1974年 | 106篇 |
1973年 | 119篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A rectilinear drawing of a graph is one where each edge is drawn as a straight-line segment, and the rectilinear crossing number of a graph is the minimum number of crossings over all rectilinear drawings. We describe, for every integer k ≥ 4, a class of graphs of crossing number k, but unbounded rectilinear crossing number. This is best possible since the rectilinear crossing number is equal to the crossing number whenever the latter is at most 3. Further, if we consider drawings where each edge is drawn as a polygonal line segment with at most one break point, then the resulting crossing number is at most quadratic in the regular crossing number. © 1993 John Wiley & Sons, Inc. 相似文献
2.
Daniel L. Feldheim Del R. Lawson Charles R. Martin 《Journal of Polymer Science.Polymer Physics》1993,31(8):953-957
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc. 相似文献
3.
4.
Julia K Pruns Jens-Peter Vietzke Manfred Strassner Claudius Rapp Ulrich Hintze Wilfried A K?nig 《Rapid communications in mass spectrometry : RCM》2002,16(3):208-211
A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards. 相似文献
5.
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
6.
We build a metric space which is homeomorphic to a Cantor set but cannot be realized as the attractor of an iterated function system. We give also an example of a Cantor set K in R3 such that every homeomorphism f of R3 which preserves K coincides with the identity on K. 相似文献
7.
Milan Medved? 《Journal of Mathematical Analysis and Applications》2002,267(2):643-650
A criterion for the nonexplosion of solutions to semilinear evolution equations on Banach spaces is proved. The result is obtained by applying a modification of the Bihari type inequality to the case of a weakly singular nonlinear integral inequality. 相似文献
8.
Partially supported by the general research fund at the University of Kansas 相似文献
9.
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
10.
James K. Freericks Elliott H. Lieb Daniel Ueltschi 《Communications in Mathematical Physics》2002,227(2):243-279
The Falicov–Kimball model is a simple quantum lattice model that describes light and heavy electrons interacting with an
on-site repulsion; alternatively, it is a model of itinerant electrons and fixed nuclei. It can be seen as a simplification
of the Hubbard model; by neglecting the kinetic (hopping) energy of the spin up particles, one gets the Falicov–Kimball model.
We show that away from half-filling, i.e. if the sum of the densities of both kinds of particles differs from 1, the particles
segregate at zero temperature and for large enough repulsion. In the language of the Hubbard model, this means creating two
regions with a positive and a negative magnetization.
Our key mathematical results are lower and upper bounds for the sum of the lowest eigenvalues of the discrete Laplace operator
in an arbitrary domain, with Dirichlet boundary conditions. The lower bound consists of a bulk term, independent of the shape
of the domain, and of a term proportional to the boundary. Therefore, one lowers the kinetic energy of the itinerant particles
by choosing a domain with a small boundary. For the Falicov- Kimball model, this corresponds to having a single “compact”
domain that has no heavy particles.
Received: 21 June 2001 / Accepted: 4 January 2002 相似文献