全文获取类型
收费全文 | 19590篇 |
免费 | 666篇 |
国内免费 | 122篇 |
专业分类
化学 | 12572篇 |
晶体学 | 114篇 |
力学 | 404篇 |
综合类 | 2篇 |
数学 | 3930篇 |
物理学 | 3356篇 |
出版年
2023年 | 124篇 |
2022年 | 227篇 |
2021年 | 413篇 |
2020年 | 376篇 |
2019年 | 351篇 |
2018年 | 747篇 |
2017年 | 541篇 |
2016年 | 657篇 |
2015年 | 524篇 |
2014年 | 591篇 |
2013年 | 1166篇 |
2012年 | 1194篇 |
2011年 | 1260篇 |
2010年 | 821篇 |
2009年 | 777篇 |
2008年 | 1120篇 |
2007年 | 1069篇 |
2006年 | 1012篇 |
2005年 | 927篇 |
2004年 | 731篇 |
2003年 | 651篇 |
2002年 | 587篇 |
2001年 | 338篇 |
2000年 | 400篇 |
1999年 | 254篇 |
1998年 | 188篇 |
1997年 | 203篇 |
1996年 | 232篇 |
1995年 | 165篇 |
1994年 | 172篇 |
1993年 | 157篇 |
1992年 | 144篇 |
1991年 | 123篇 |
1990年 | 134篇 |
1989年 | 114篇 |
1988年 | 90篇 |
1987年 | 91篇 |
1986年 | 96篇 |
1985年 | 112篇 |
1984年 | 129篇 |
1983年 | 96篇 |
1982年 | 104篇 |
1981年 | 102篇 |
1980年 | 82篇 |
1979年 | 62篇 |
1978年 | 83篇 |
1977年 | 67篇 |
1976年 | 69篇 |
1975年 | 63篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
81.
We study percolation in the following random environment: let Z be a Poisson process of constant intensity on ℝ2, and form the Voronoi tessellation of ℝ2 with respect to Z. Colour each Voronoi cell black with probability p, independently of the other cells. We show that the critical probability is 1/2. More precisely, if p>1/2 then the union of the black cells contains an infinite component with probability 1, while if p<1/2 then the distribution of the size of the component of black cells containing a given point decays exponentially. These
results are analogous to Kesten's results for bond percolation in ℤ2.
The result corresponding to Harris' Theorem for bond percolation in ℤ2 is known: Zvavitch noted that one of the many proofs of this result can easily be adapted to the random Voronoi setting.
For Kesten's results, none of the existing proofs seems to adapt. The methods used here also give a new and very simple proof
of Kesten's Theorem for ℤ2; we hope they will be applicable in other contexts as well.
Research supported in part by NSF grant ITR 0225610 and DARPA grant F33615-01-C-1900
Research partially undertaken during a visit to the Forschungsinstitut für Mathematik, ETH Zürich, Switzerland 相似文献
82.
A. Cerdán Vidal A. R. Maurí Aucejo M. Llobat Estellés C. Pascual Martí J. Simeón Martí 《Fresenius' Journal of Analytical Chemistry》1994,350(12):706-711
The possibility of identification of substances which have similar spectral behaviour by means of Apparent Content Curves has been studied. This study is carried out with absorption, excitation and emission spectra of several amphetamines of widespread pharmaceutical use. Results obtained show that amphetamine, phenylpropanolamine, pseudoephedrine, phenylephrine, epinephrine, dopamine and methoxyamphetamine can be identified with a probability of >95%. 相似文献
83.
R. López-Martens J. Mauritsson P. Johnsson K. Varjú A. L’Huillier W. Kornelis J. Biegert U. Keller M. Gaarde K. Schafer 《Applied physics. B, Lasers and optics》2004,78(7-8):835-840
We characterize the temporal structure of high-order harmonic radiation on both the femtosecond and attosecond time scales. The harmonic emission is characterized by mixed-color two-photon ionization with an infrared femtosecond laser using a Mach–Zehnder interferometer where both pump and probe arms travel completely separate paths. In a first experiment, we measure the duration and chirp of individual harmonics. In a second experiment, we resolve, for the first time with this type of setup, the attosecond beating of several harmonics generated under conditions similar to the first experiment. We suggest that the results of both measurements can be combined to determine the full attosecond time structure of the harmonic emission. PACS 32.80.Rm; 42.65.Ky 相似文献
84.
What is the maximum number of unit distances between the vertices of a convex n-gon in the plane? We review known partial results for this and other open questions on multiple occurrences of the same interpoint distance in finite planar subsets. Some new results are proved for small n. Challenging conjectures, both old and new, are highlighted. 相似文献
85.
Nicolás Andruskiewitsch 《Selecta Mathematica, New Series》2005,11(2):203-246
Quivers over a fixed base set form a monoidal category with tensor product given by pullback. The quantum Yang–Baxter equation,
or more properly the braid equation, is investigated in this setting. A solution of the braid equation in this category is
called a “solution” for short. Results of Etingof–Schedler–Soloviev, Lu–Yan–Zhu and Takeuchi on the set-theoretical quantum
Yang–Baxter equation are generalized to the context of quivers, with groupoids playing the role of groups. The notion of “braided
groupoid” is introduced. Braided groupoids are solutions and are characterized in terms of bijective 1-cocycles. The structure
groupoid of a non-degenerate solution is defined; it is shown that it is a braided groupoid. The reduced structure groupoid
of a non-degenerate solution is also defined. Non-degenerate solutions are classified in terms of representations of matched
pairs of groupoids. By linearization we construct star-triangular face models and realize them as modules over quasitriangular
quantum groupoids introduced in papers by M. Aguiar, S. Natale and the author. 相似文献
86.
Agnès Tourin 《Numerische Mathematik》1992,62(1):75-85
Summary The reconstruction from a shaded image of a Lambertian and not self-shadowing surface illuminated by a single distant pointwise light source may be written as a first-order Hamilton-Jacobi equation.In this paper, we continue the investigation begun in E. Rouy and A. Tourin into the uniqueness of the solution of this equation; the approach is based on the viscosity solutions theory and the dynamic programming principle.More precisely, we concentrate here on the uniqueness of the viscosity solution of this equation in case the measured luminous intensity reflected by the surface is discontinuous along a smooth curve. We prove a general comparison result for a piecewise Lipschitz continuous Hamiltonian and illustrate it by numerical experiments. 相似文献
87.
L Soltés 《Biomedical chromatography : BMC》1992,6(1):43-49
The paper presents results and experience obtained in the isolation of a number of basic polar/non-polar solutes (drugs, their metabolites and structural derivatives) from biological samples by using the separation minicolumns Separcol SI and Separcol SI C18 L/T. The aim was to provide insight into some simple procedures yielding optimal selectivity/specificity of the isolation of the studied solutes either by proper selection of the sorbent or by applying sophisticated elution schemes which use to advantage different properties of the eluents/solvents. 相似文献
88.
In this work we study semifield planes of orderq
n(q=p
r
,p prime) with a collineation whose order is ap-primitive divisor ofq
n–1.Research supported in part by NSF Grant No. DMS-9107372Research supported in part by NSF grants RII-9014056, component IV of the EPSCoR of Puerto Rico grant and ARO grant for Cornell MSI. 相似文献
89.
Dávid Szegvári Romána Zelkó Péter Horváth András Gergely 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):169-172
Enantioselective solubility of rac-norgestrel was found in the presence of γ-cyclodextrin or hydroxypropyl-γ-cyclodextrin.
In both cases the efficacious enantiomer was dissolved in greater extent. Calculating the molar absorptivity and molar ellipticity
spectra of the γ- and hydroxypropyl-γ-cyclodextrin aqueous complexes, a simple and rapid direct circular dichroism (CD) spectrometric
method was obtained for the determination of the enantiomer ratio in aqueous solutions. 相似文献
90.
Enrique Bendito ngeles Carmona Andr s M. Encinas 《Applied Numerical Mathematics》2004,50(3-4):343-370
Our aim is to set the foundations of a discrete vectorial calculus on uniform n-dimensional grids, that can be easily reformulated on general irregular grids. As a key tool we first introduce the notion of tangent space to any grid node. Then we define the concepts of vector field, field of matrices and inner products on the space of grid functions and on the space of vector fields, mimicking the continuous setting. This allows us to obtain the discrete analogous of the basic first order differential operators, gradient and divergence, whose composition define the fundamental second order difference operator. As an application, we show that all difference schemes, with constant coefficients, for first and second order differential operators with constant coefficients can be seen as difference operators of the form
for suitable choices of q,
and
. In addition, we characterize special properties of the difference scheme, such as consistency, symmetry and positivity in terms of q,
and
. 相似文献