全文获取类型
收费全文 | 52368篇 |
免费 | 726篇 |
国内免费 | 408篇 |
专业分类
化学 | 26623篇 |
晶体学 | 382篇 |
力学 | 1937篇 |
综合类 | 12篇 |
数学 | 13164篇 |
物理学 | 11384篇 |
出版年
2021年 | 273篇 |
2020年 | 318篇 |
2019年 | 329篇 |
2018年 | 1411篇 |
2017年 | 1630篇 |
2016年 | 1167篇 |
2015年 | 980篇 |
2014年 | 932篇 |
2013年 | 1915篇 |
2012年 | 4190篇 |
2011年 | 3675篇 |
2010年 | 2481篇 |
2009年 | 2207篇 |
2008年 | 1770篇 |
2007年 | 1838篇 |
2006年 | 1807篇 |
2005年 | 5571篇 |
2004年 | 4889篇 |
2003年 | 3170篇 |
2002年 | 1274篇 |
2001年 | 625篇 |
2000年 | 363篇 |
1999年 | 415篇 |
1998年 | 381篇 |
1997年 | 372篇 |
1996年 | 399篇 |
1995年 | 278篇 |
1994年 | 320篇 |
1993年 | 282篇 |
1992年 | 405篇 |
1991年 | 392篇 |
1990年 | 324篇 |
1989年 | 337篇 |
1988年 | 335篇 |
1987年 | 257篇 |
1986年 | 236篇 |
1985年 | 327篇 |
1984年 | 364篇 |
1983年 | 244篇 |
1982年 | 385篇 |
1981年 | 352篇 |
1980年 | 329篇 |
1979年 | 346篇 |
1978年 | 346篇 |
1977年 | 310篇 |
1976年 | 358篇 |
1975年 | 287篇 |
1974年 | 287篇 |
1973年 | 282篇 |
1972年 | 168篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Parameters of Gaussian multivariate models are often estimated using the maximum likelihood approach. In spite of its merits, this methodology is not practical when the sample size is very large, as, for example, in the case of massive georeferenced data sets. In this paper, we study the asymptotic properties of the estimators that minimize three alternatives to the likelihood function, designed to increase the computational efficiency. This is achieved by applying the information sandwich technique to expansions of the pseudo-likelihood functions as quadratic forms of independent normal random variables. Theoretical calculations are given for a first-order autoregressive time series and then extended to a two-dimensional autoregressive process on a lattice. We compare the efficiency of the three estimators to that of the maximum likelihood estimator as well as among themselves, using numerical calculations of the theoretical results and simulations. 相似文献
92.
In this paper, by using elementary analysis, we establish some new Lyapunov-type inequalities for nonlinear systems of differential equations, special cases of which contain the well-known equations such as Emden-Fowler-type and half-linear equations. The inequalities obtained here can be used as handy tools in the study of qualitative behaviour of solutions of the associated equations. 相似文献
93.
94.
95.
Robert G. Payton 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2007,44(6):876-888
A cylindrically orthotropic elastic solid is excited by a point impulsive body force. The solid contains a semi-infinite stress
free crack. The resulting anti-plane wave motion problem has been solved in the form of a finite series representing the incident
and reflected pulses plus an integral representing the diffraction pulse. The series part of the solution has been previously
treated. In the present investigation the diffraction integral is integrated when λ (which measures the anisotropy of the
solid) is an odd integer number. The diffraction integral is also integrated when λ is half an odd integer, for the special
case in which the source lies in the plane of the crack and parallel to the crack edge. The displacement jump across the circular
diffraction wave front is given for unrestricted (positive) values of λ. 相似文献
96.
We give a number of characterizations of bodies of constant width in arbitrary dimension. As an application, we describe a way to construct a body of constant width in dimension n, one of its (n – 1)‐dimensional projection being given. We give a number of examples, like a four‐dimensional body of constant width whose 3D‐projection is the classical Meissner's body. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
97.
MDF materials are chemically bonded ceramic materials free of the macrodefects typical of hydraulic cement-based materials. MDF materials arising through reactions of sulfo-aluminate-ferrite belitic (SAFB) clinkers and/or Portland cements (PC) with two types of water-soluble polymer (hydroxy-propylmethyl cellulose {HPMC}, polyphosphate glass {poly-P}) are discussed. Mixes of low energy SAFB clinkers with Portland cement, HPMC and, especially poly-P comprise promising cross-linked compositions additional to the better known MDF materials formed from high alumina cement with polyvinylalcohol/acetate. The principles of co-ordination of P and C atoms (of the polymer) with Al and Fe atoms (originating from the cement) are highlighted from spectroscopic information on next-nearest-neighbour interactions, along with the effects of second co-ordination spheres. Polymers modify the interface through functional bonding/grafting of polymer chains onto the surfaces of cement grains. Both the cross-linked atomic structure and the interface coincide well with the model of functional polymers and represent a new type of atomic-level structure in polymer-modified cements. Interpretation is based on previous magnetic resonance and thermal analysis studies. The compactness of Al(Fe)-O-P cross-links reduces transport through the interfaces, increasing the interfacial interactions and resisting the unfavourable uptake of moisture and carbonation. 相似文献
98.
E. Beinrohr M. Németh P. Tsch?pel G. T?lg 《Fresenius' Journal of Analytical Chemistry》1992,344(3):93-99
Summary Flow-through electrochemical cells with porous working electrodes made of crushed reticulated vitreous carbon and plated with mercury were used for absolute analysis of trace amounts of lead by anodic stripping coulometry with collection (ASCWC) in a flow system. The role of mercury coating, flow rate and pH were investigated. The coulombic content of the collection peak corresponded to the theoretical values calculated by Faraday's law in a concentration range from about 10–9 to 10–6 mol/l. The relative error and the relative standard deviation was +0.15% and 0.8%, respectively for 2×10–6 mol/l analyte concentration. The absolute detection limit (3 s) was 0.1 ng of Pb, the linear response range 7×104.
One leave from: Department of Analytical Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia 相似文献
99.
Celik Tarimci Anda? Karamustafa Kutalmi? Güven Mehmet Kabak Akgül Ye?ilada 《Analytical sciences》2003,19(9):1347-1348
The crystal structure of 1-[3-(3,4-dimethoxyphenyl)-2-propenoyl]pyrrolidine (C15H19NO3) (I) has been determined by X-ray analysis. It crystallizes orthorhombic space group Pbca with a = 24.295(3), b = 15.086(3), c = 7.552(3)A, V = 2768(1)A3, Z = 8, Dcalc = 1.254 g/cm3, mu = (Mo K(alpha)) = 0.87 cm(-1). The title compound has analgesic activity of cycloaliphatic amine part. The molecule is deviated from planar configuration. 相似文献
100.
We introduce and investigate the properties of Hochschild cohomology of algebras in an abelian monoidal category M. We show that the second Hochschild cohomology group of an algebra in M classifies extensions of A up to an equivalence. We characterize algebras of Hochschild dimension 0 (separable algebras), and of Hochschild dimension ≤1 (formally smooth algebras). Several particular cases and applications are included in the last section of the paper. 相似文献