全文获取类型
收费全文 | 727156篇 |
免费 | 6410篇 |
国内免费 | 2080篇 |
专业分类
化学 | 367817篇 |
晶体学 | 10719篇 |
力学 | 36289篇 |
综合类 | 20篇 |
数学 | 96835篇 |
物理学 | 223966篇 |
出版年
2021年 | 6553篇 |
2020年 | 7032篇 |
2019年 | 7945篇 |
2018年 | 10642篇 |
2017年 | 10890篇 |
2016年 | 15195篇 |
2015年 | 8619篇 |
2014年 | 14226篇 |
2013年 | 33319篇 |
2012年 | 25619篇 |
2011年 | 30618篇 |
2010年 | 22671篇 |
2009年 | 22614篇 |
2008年 | 28543篇 |
2007年 | 28241篇 |
2006年 | 25900篇 |
2005年 | 23070篇 |
2004年 | 21415篇 |
2003年 | 19202篇 |
2002年 | 19086篇 |
2001年 | 21046篇 |
2000年 | 16092篇 |
1999年 | 12523篇 |
1998年 | 10684篇 |
1997年 | 10472篇 |
1996年 | 9829篇 |
1995年 | 8769篇 |
1994年 | 8741篇 |
1993年 | 8448篇 |
1992年 | 9038篇 |
1991年 | 9540篇 |
1990年 | 9102篇 |
1989年 | 8988篇 |
1988年 | 8604篇 |
1987年 | 8564篇 |
1986年 | 8165篇 |
1985年 | 10425篇 |
1984年 | 10845篇 |
1983年 | 9078篇 |
1982年 | 9355篇 |
1981年 | 8791篇 |
1980年 | 8471篇 |
1979年 | 9091篇 |
1978年 | 9322篇 |
1977年 | 9160篇 |
1976年 | 9076篇 |
1975年 | 8715篇 |
1974年 | 8547篇 |
1973年 | 8844篇 |
1972年 | 6343篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
We prove a conjecture by F. Ferrari. Let X be the total space of a nonlinear deformation of a rank two holomorphic vector bundle on a smooth rational curve, such that X has trivial canonical bundle and has sections. Then the normal bundle to such sections is computed in terms of the rank of the Hessian of a suitably defined superpotential at its critical points. 相似文献
942.
We present theoretical results of the electron impact ionization rate in GaAs/AlGaAs multiquantum well structures as a function of applied electric field for various geometries, i.e., well and barrier widths. In addition, we present preliminary measurements of the current-voltage characteristics of MBE grown devices which demonstrate very low leakage current as well as sharp breakdwon behavior. It is found that the net ionization rate, determined by averaging over the constitutent GaAs and AlGaAs layers, approaches the weighted average of the constituent bulk rates at high electric field strengths; the potential discontinuity is relatively unimportant. The electron ionization rate within the well regions alone is still higher than that in bulk GaAs, but is insufficiently enhanced to compensate for the much lower rate in the AlGaAs layers. As the field is lowered to 250.0 kV/cm, the average ionization rate in the multiquantum well structure becomes larger than in the bulk. 相似文献
943.
R.S. Kraußhar 《Journal of Mathematical Analysis and Applications》2007,325(1):359-376
In this paper we study Clifford and harmonic analysis on some examples of conformal flat manifolds that have a spinor structure, or more generally, at least a pin structure. The examples treated here are manifolds that can be parametrized by U/Γ where U is a subdomain of either Sn or Rn and Γ is a Kleinian group acting discontinuously on U. The examples studied here include RPn and the Hopf manifolds S1×Sn−1. Also some hyperbolic manifolds will be treated. Special kinds of Clifford-analytic automorphic forms associated to the different choices of Γ are used to construct explicit Cauchy kernels, Cauchy integral formulas, Green's kernels and formulas together with Hardy spaces and Plemelj projection operators for Lp spaces of hypersurfaces lying in these manifolds. 相似文献
944.
We present a framework for extending Stone's representation theorem for distributive lattices to representation theorems for distributive lattices with operators. We proceed by introducing the definition of algebraic theory of operators over distributive lattices. Each such theory induces a functor on the category of distributive lattices such that its algebras are exactly the distributive lattices with operators in the original theory. We characterize the topological counterpart of these algebras in terms of suitable coalgebras on spectral spaces. We work out some of these coalgebraic representations, including a new representation theorem for distributive lattices with monotone operators. 相似文献
945.
N C Proudlove S Black A Fletcher 《The Journal of the Operational Research Society》2007,58(2):145-158
This paper considers efforts to improve in-patient flows, a particularly urgent issue in the National Health Service (NHS). The context is described and related to reasons why OR has been making relatively little contribution. The paper argues that large complex models may often be unnecessary and even get in the way of providing clear insight and guidance for problem owners. The importance of understanding the generic working of systems to lead to improvement, and the limitations of simply describing them, is stressed. It is demonstrated that some very simple models can be of significant practical value in understanding and managing complex systems, changing mindsets and driving collection and use of operationally valuable data. Recommendations for more effective engagement with the NHS are offered. 相似文献
946.
R. Jayakumar Y.‐S. Lee S. Nanjundan 《Journal of polymer science. Part A, Polymer chemistry》2003,41(18):2865-2878
The calcium salt of mono(hydroxyethoxyethyl)phthalate [Ca(HEEP)2] was synthesized by the reaction of diethylene glycol, phthalic anhydride, and calcium acetate. Calcium‐containing poly(urethane ether)s (PUEs) were synthesized by the reaction of hexamethylene diisocyanate (HMDI) or tolylene 2,4‐diisocyanate (TDI) with a mixture of Ca(HEEP)2 and poly(ethylene glycol) (PEG300 or PEG400) with di‐n‐butyltin dilaurate as a catalyst. A series of calcium‐containing PUEs of different compositions were synthesized with Ca(HEEP)2/PEG300 (or PEG400)/diisocyanate (HMDI or TDI) molar ratios of 2:2:4, 3:1:4, and 1:3:4 so that the coating properties of the PUEs could be studied. Blank PUEs without calcium‐containing ionic diols were also prepared by the reaction of PEG300 or PEG400 with HMDI or TDI. The PUEs were well characterized by Fourier transform infrared, 1H and 13C NMR, solid‐state cross‐polarity/magic‐angle‐spinning 13C NMR, viscosity, solubility, and X‐ray diffraction studies. The thermal properties of the polymers were also studied with thermogravimetric analysis and differential scanning calorimetry. The PUEs were applied as top coats on acrylic‐coated leather, and their physicomechanical properties were also studied. The coating properties of PUEs, such as the tensile strength, elongation at break, tear strength, water vapor permeability, flexing endurance, cold crack resistance, abrasion resistance, color fastness, and adhesive strength, were better than the standard values. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2865–2878, 2003 相似文献
947.
We give counterexamples to the following conjecture of Auslander: given a finitely generated module M over an Artin algebra Λ, there exists a positive integer nM such that for all finitely generated Λ-modules N, if ExtΛi(M,N)=0 for all i?0, then ExtΛi(M,N)=0 for all i?nM. Some of our examples moreover yield homologically defined classes of commutative local rings strictly between the class of local complete intersections and the class of local Gorenstein rings. 相似文献
948.
A. Gawalowski 《Fresenius' Journal of Analytical Chemistry》1909,48(7-8):486-487
Ohne Zusammenfassung 相似文献
949.
K. K. Kadyrzhanov S. Khazhekber S. N. Lukashenko V. P. Solodukhin I. V. Kazachevskiy V. L. Poznyak B. B. Knyazev Ch. Rofer 《Czechoslovak Journal of Physics》2003,53(1):A39-A44
Data on the spatial distribution of radionuclides (241Am, 239Pu, 137Cs and 152Eu) formed during nuclear explosions of different types near P2 SNTS test site are presented. Radionuclide contamination induced by the explosions varies in the concentrations of individual radionuclides, their proportions and species. Examination of the variations is a crucial task to plan remediation activities as well as those aimed at decrease of radiation risk for population and prevention of repeated contamination. Concentrations of 241Am and 239+240Pu that are the most toxic radionuclides in the area lie in hundred thousands of Bqkg?1. The most contaminated areas are classified by the radionuclide concentration, ratio and form present in soil. 相似文献
950.
O. A. Zmeev 《Russian Physics Journal》2003,46(3):318-324
In the present work, the diffusion approximation of the capital is constructed for the mathematical model of a social insurance fund with random expenditures for social programs. The discontinuous hysteresis control of the fund capital is considered. 相似文献