全文获取类型
收费全文 | 6603篇 |
免费 | 217篇 |
国内免费 | 23篇 |
专业分类
化学 | 4728篇 |
晶体学 | 51篇 |
力学 | 117篇 |
综合类 | 1篇 |
数学 | 971篇 |
物理学 | 975篇 |
出版年
2023年 | 41篇 |
2021年 | 66篇 |
2020年 | 98篇 |
2019年 | 92篇 |
2018年 | 58篇 |
2017年 | 56篇 |
2016年 | 171篇 |
2015年 | 154篇 |
2014年 | 172篇 |
2013年 | 256篇 |
2012年 | 300篇 |
2011年 | 371篇 |
2010年 | 230篇 |
2009年 | 186篇 |
2008年 | 284篇 |
2007年 | 295篇 |
2006年 | 284篇 |
2005年 | 261篇 |
2004年 | 233篇 |
2003年 | 198篇 |
2002年 | 200篇 |
2001年 | 124篇 |
2000年 | 122篇 |
1999年 | 96篇 |
1998年 | 75篇 |
1997年 | 51篇 |
1996年 | 61篇 |
1995年 | 88篇 |
1994年 | 78篇 |
1993年 | 75篇 |
1992年 | 60篇 |
1991年 | 74篇 |
1990年 | 56篇 |
1989年 | 67篇 |
1988年 | 42篇 |
1987年 | 58篇 |
1985年 | 63篇 |
1984年 | 69篇 |
1983年 | 68篇 |
1982年 | 57篇 |
1981年 | 49篇 |
1980年 | 61篇 |
1979年 | 61篇 |
1978年 | 64篇 |
1977年 | 63篇 |
1976年 | 65篇 |
1975年 | 58篇 |
1974年 | 44篇 |
1973年 | 46篇 |
1971年 | 42篇 |
排序方式: 共有6843条查询结果,搜索用时 515 毫秒
21.
Rational approximation of vertical segments 总被引:1,自引:0,他引:1
In many applications, observations are prone to imprecise measurements. When constructing a model based on such data, an approximation rather than an interpolation approach is needed. Very often a least squares approximation is used. Here we follow a different approach. A natural way for dealing with uncertainty in the data is by means of an uncertainty interval. We assume that the uncertainty in the independent variables is negligible and that for each observation an uncertainty interval can be given which contains the (unknown) exact value. To approximate such data we look for functions which intersect all uncertainty intervals. In the past this problem has been studied for polynomials, or more generally for functions which are linear in the unknown coefficients. Here we study the problem for a particular class of functions which are nonlinear in the unknown coefficients, namely rational functions. We show how to reduce the problem to a quadratic programming problem with a strictly convex objective function, yielding a unique rational function which intersects all uncertainty intervals and satisfies some additional properties. Compared to rational least squares approximation which reduces to a nonlinear optimization problem where the objective function may have many local minima, this makes the new approach attractive. 相似文献
22.
Ohne Zusammenfassung 相似文献
23.
The category of small covariant functors from simplicial sets to simplicial sets supports the projective model structure [B. Chorny, W.G. Dwyer, Homotopy theory of small diagrams over large categories, preprint, 2005]. In this paper we construct various localizations of the projective model structure and also give a variant for functors from simplicial sets to spectra. We apply these model categories in the study of calculus of functors, namely for a classification of polynomial and homogeneous functors. In the n-homogeneous model structure, the nth derivative is a Quillen functor to the category of spectra with Σn-action. After taking into account only finitary functors—which may be done in two different ways—the above Quillen map becomes a Quillen equivalence. This improves the classification of finitary homogeneous functors by T.G. Goodwillie [T.G. Goodwillie, Calculus. III. Taylor series, Geom. Topol. 7 (2003) 645-711 (electronic)]. 相似文献
24.
Alexander G. Stepanov Sergei S. Arzumanov Horst Ernst Dieter Freude 《Chemical physics letters》2006,420(4-6):574-576
It has been demonstrated that 1H MAS NMR spectroscopy can be used as a tool for in situ monitoring the reaction kinetics of 13C-labeled carbon scrambling in alkane molecules adsorbed on zeolite catalysts at the reaction temperature of 540–570 K. The accuracy of the results and the time resolution are improved compared to 13C MAS NMR spectroscopy. 相似文献
25.
26.
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
27.
28.
Guido Momerency Karel Van Cauwenberghe Ernst A. De Bruijn Allan T. Van Oosterom Martin S. Highley Peter G. Harper 《Journal of separation science》1994,17(9):655-661
A method is described for the determination of the antitumor agent iphosphamide and seven of its metabolites in the plasma of cancer patients by multiple ion monitoring (MIM) GC-MS, mainly using the electron capture chemical ionization mode, of stable methyl and/or trifluoroacetyl derivatives. The metabolites determined were 2- and 3-dechloroethyliphosphamide, 4-ketoiphosphamide, carboxyiphosphamide, iphosphamide mustard, and two previously undetected metabolites, chloroethylamine and 1,3-oxazolidine-2-one. The isolation of the acidic and neutral metabolites was performed by solid phase extraction on to C18 adsorbent at pH 4. The weakly acidic iphosphamide mustard, isolated under these conditions with a yield of ca 50%, was measured as a stable methyltrifluoroacetyl derivative, in contrast to the corresponding phosphoramide mustard of the isomer cyclophosphamide which decomposes during derivatization. Chloroethylamine and 1,3-oxazolidine-2-one were isolated with high yield by liquid extraction with ethyl acetate at pH 10. Selective measurement of several metabolite derivatives with similar retention times was performed by multiple ion monitoring MS of specific ion masses, using a methyl phenyl siloxane capillary column previously employed in the study of cyclophosphamide metabolites. Quantitation of metabolites in patient plasma samples could be performed in the concentration range 3 ng to 20 μg per ml of original plasma. 相似文献
29.
30.