全文获取类型
收费全文 | 1471篇 |
免费 | 23篇 |
国内免费 | 6篇 |
专业分类
化学 | 1142篇 |
晶体学 | 17篇 |
力学 | 10篇 |
数学 | 167篇 |
物理学 | 164篇 |
出版年
2023年 | 18篇 |
2022年 | 62篇 |
2021年 | 64篇 |
2020年 | 36篇 |
2019年 | 26篇 |
2018年 | 30篇 |
2017年 | 24篇 |
2016年 | 27篇 |
2015年 | 38篇 |
2014年 | 34篇 |
2013年 | 78篇 |
2012年 | 82篇 |
2011年 | 90篇 |
2010年 | 59篇 |
2009年 | 44篇 |
2008年 | 85篇 |
2007年 | 81篇 |
2006年 | 82篇 |
2005年 | 69篇 |
2004年 | 60篇 |
2003年 | 70篇 |
2002年 | 57篇 |
2001年 | 11篇 |
2000年 | 13篇 |
1999年 | 13篇 |
1998年 | 18篇 |
1997年 | 16篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1940年 | 2篇 |
排序方式: 共有1500条查询结果,搜索用时 15 毫秒
1.
In this paper a tripartite qualitative design combining abservation, stimulated recall and interview is presented and discussed. This three-step-design makes it possible to get insight into the interaction of internal and external processes when solving mathematical tasks. The data analysis depends on the research question and the methodological approach. In the light of two research projects in mathematics education two different methods of data analysis are presented and methodologically reflected. 相似文献
2.
A consistent realization of an operatorial deformed Heisenberg-Weyl (HW) algebra is given. The explicit construction of the
deformation matrix is discussed. The corresponding generalized Fock-like space is analyzed. 相似文献
3.
Maria Rita Casali 《Acta Appl Math》1998,54(1):75-97
The present paper adopts a computational approach to the study of nonorientable 3-manifolds: in fact, we describe how to create an automaticcatalogue of all nonorientable 3-manifolds admitting coloured triangulationswith a fixed number of tetrahedra. In particular, the catalogue has been effectively produced and analysed for up to 26 tetrahedra, to reach the complete classification of all involved 3-manifolds. As a consequence, the following summarising result can be stated:THEOREM I. Exactly seven closed connected prime nonorientable3-manifolds exist, which admit a coloured triangulation consisting of atmost 26 tetrahedra.More precisely, they are the four Euclidean nonorientable 3-manifolds, the nontrivial S2 bundle overS1, the topological product between thereal projective plane RP2 andS1, and the torus bundle overS1, with monodromy induced by matrix(10 -11). 相似文献
4.
5.
Mixed acetals 7 of benzeneseleninylacetaldehyde, prepared by a simple 2-step procedure from mono- and bicyclic allylic alcohols 5 , undergo benzeneselenenic acid elimination to transient ketene acetals 8 which afford γ, δ-unsaturated esters 9 via the ester Claisen rearrangement (Scheme 2). Under the same conditions selenoxide 7h derived from benzyl alcohol 5h is converted back to benzyl alcohol with the concomitant formation of ethylphenylselenoacetate 12 . 相似文献
6.
7.
Some laws in physics describe the change of a flux and are represented by parabolic equations of the form (*) \documentclass{article}\pagestyle{empty}\begin{document}$$\frac{{\partial u}}{{\partial t}}=\frac{\partial}{{\partial x_j }}(\eta \frac{{\partial u}}{{ax_j}}-vju),$$\end{document} j≤m, where η and vj are functions of both space and time. We show under quite general assumptions that the solutions of equation (*) with homogeneous Dirichlet boundary conditions and initial condition u(x, 0) = uo(x) satisfy The decay rate d > 0 only depends on bounds for η, v and G § Rm the spatial domain, while the constant c depends additionally on which norm is considered. For the solutions of equation (*) with homogeneous Neumann boundary conditions and initial condition u0(x) ≥ 0 we derive bounds d1u1 ≤ u(x, t) ≤ d2u2, Where di, i = 1, 2, depend on bounds for η, v and G, and the ui are bounds on the initial condition u0. 相似文献
8.
Die erste Röntgenstrukturanalyse eines 3,7-Dehydrotropon ( 1 )-Derivates, nämlich von 2-Diisopropylammo-3,7-dehydrotropon ( 4 ) zeigt das Ringsystem in 4 als planaren Bicyclus mit nahezu C2V-Symmetrie. Dementsprechend enthalten Dehydrotropone formell zwei trans-konfigurierte Doppelbindungen in einem 7-gliedrigen Ring; auch das O-Atom und das N-Atom mit seinen Ligandatomen liegen in derselben Ebene wie die Ringatome. Die Bindung zwischen den beiden C-Atomen, welche die Heteroatome tragen, (C(1) and C(2)), ist wesentlich langer (1,56 Å) als die anderen Bindungen im Ringsystem (1,37-1,46 Å). Dies impliziert eine ‘push-pull’-π-Elektronen-Delokalisierung, in der auch das O-und das N-Atom involviert sind, und macht eine ‘aromatische’ Ring-Delokalisierung weniger wahr-scheinlich. Im Gegensatz zu 2-(t-Butyl)-3,7-dehydrotropon ( 3 ) existiert das 2-Diiso-propylamino-3,7-dehydrotropon ( 4 ) im Kristall als Monomeres, was dem stabili-sierenden Einfluss der ‘push-pull’-Delokausierung zugeschrieben werden kann. 相似文献
9.
Wolfgang Stadlbauer Rita Laschober Thomas Kappe 《Monatshefte für Chemie / Chemical Monthly》1991,122(10):853-861
Summary 4-Chloro-3-aryl-coumarins and quinolones2 a–e undergo thermolytic ring closure by reaction with sodium azide in refluxing dimethyl formamide to yield indolo[3,2-c]coumarins and indolo[3,2-c]quinolin-6(5H)-ones6 a–e. In the case of the coumarin2 a the azido coumarin5 can be isolated. The mono- and diazacoumestrol-dimethylethers6 a–c are converted into the coumestrol analogues7 a–c and their diacetyl derivatives8 a–c.
Potentielle nichtsteroidale Östrogene und Antiöstrogene, 4. Mitt.: Organische Azide in der Heterocyclensynthese, Teil 13: Synthese von Aza- und Diazacumöstrolen über Azidzwischenstufen
Zusammenfassung 4-Chlor-3-arylcumarine und-chinolone2 a–e reagieren thermolytisch mit Natriumazid in siedendem Dimethylfomamid unter Ringschluß zu Indolo[3,2-c]cumarinen und Indolo[3,2-c]chinolin-6(5H)-onen6 a–e. Nur aus dem Cumarinderivat2 a kann das zwischenzeitlich gebildete Azidocumarin5 isoliert werden. Die so erhaltenen Mono- und Diazacumöstroldimethylether6 a–c werden in die entsprechenden Cumöstrole7 a–c und ihre Diacetylderivate8 a–c umgewandelt.相似文献
10.
Rita Khanna 《Pramana》1983,20(4):267-277
Using the continuum theory of linear elasticity, Huang diffuse scattering (HDS) has been calculated from self-interstitials in a general orthorhombic and tetragonal lattice. Various defect configurations are represented according to the point group symmetry of the defect site. The contribution toHDS from all possible equivalent orientations (assumed to be equally populated) of the defect configuration is averaged. The limitations ofHDS in discriminating between defect configurations having the same long-range symmetry are discussed, considering some special cases. 相似文献