全文获取类型
收费全文 | 19154篇 |
免费 | 449篇 |
国内免费 | 131篇 |
专业分类
化学 | 13386篇 |
晶体学 | 119篇 |
力学 | 441篇 |
数学 | 3124篇 |
物理学 | 2664篇 |
出版年
2022年 | 110篇 |
2021年 | 173篇 |
2020年 | 194篇 |
2019年 | 195篇 |
2018年 | 169篇 |
2017年 | 157篇 |
2016年 | 331篇 |
2015年 | 367篇 |
2014年 | 398篇 |
2013年 | 817篇 |
2012年 | 895篇 |
2011年 | 1138篇 |
2010年 | 594篇 |
2009年 | 541篇 |
2008年 | 999篇 |
2007年 | 1011篇 |
2006年 | 1032篇 |
2005年 | 1075篇 |
2004年 | 939篇 |
2003年 | 796篇 |
2002年 | 677篇 |
2001年 | 253篇 |
2000年 | 248篇 |
1999年 | 201篇 |
1998年 | 193篇 |
1997年 | 238篇 |
1996年 | 300篇 |
1995年 | 211篇 |
1994年 | 192篇 |
1993年 | 191篇 |
1992年 | 195篇 |
1991年 | 178篇 |
1990年 | 165篇 |
1989年 | 157篇 |
1988年 | 168篇 |
1987年 | 166篇 |
1986年 | 129篇 |
1985年 | 281篇 |
1984年 | 249篇 |
1983年 | 197篇 |
1982年 | 262篇 |
1981年 | 219篇 |
1980年 | 270篇 |
1979年 | 236篇 |
1978年 | 244篇 |
1977年 | 245篇 |
1976年 | 214篇 |
1975年 | 177篇 |
1974年 | 171篇 |
1973年 | 164篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Jürg Fröhlich Tai-Peng Tsai Horng-Tzer Yau 《Communications in Mathematical Physics》2002,225(2):223-274
We consider the nonlinear Hartree equation describing the dynamics of weakly interacting non-relativistic Bosons. We show
that a nonlinear M?ller wave operator describing the scattering of a soliton and a wave can be defined. We also consider the dynamics of a
soliton in a slowly varying background potential W(ɛx). We prove that the soliton decomposes into a soliton plus a scattering wave (radiation) up to times of order ɛ−1. To leading order, the center of the soliton follows the trajectory of a classical particle in the potential W(ɛx).
Received: 30 June 2000 / Accepted: 25 June 2001 相似文献
42.
Tung-Yang Ho Ting-Yi Sung Lih-Hsing Hsu Chang-Hsiung Tsai Jeng-Yan Hwang 《Journal of Algorithms in Cognition, Informatics and Logic》1998,28(2):216-257
Given a series-parallel network (network, for short)N, its dual networkN′ is given by interchanging the series connection and the parallel connection of networkN. We usually use a series-parallel graph to represent a network. LetG[N] andG[N′] be graph representations ofNandN′, respectively. A sequence of edgese1, e2,…,ekis said to form a common trail on (G[N], G[N′]) if it is a trail on bothG[N] andG[N′]. If a common trail covers all of the edges inG[N] andG[N′], it is called adouble Euler trail.However, there are many different graph representations for a network. We say that a networkNhas a double Euler trail (DET) if there is a common Euler trail for someG[N] and someG[N′]. Finding a DET in a network is essential for optimizing the layout area of a complementary CMOS functional cell. Maziasz and Hayes (IEEE Trans. Computer-Aided Design9(1990), 708–719) gave a linear time algorithm for solving the layout problem in fixedG[N] andG[N′] and an exponential algorithm for finding the optimal cover in a network without fixing graph representations. In this paper, we study properties of subnetworks of a DET network. According to these properties, we propose an algorithm that automatically generates the rules for composition of trail cover classes. On the basis of these rules, a linear time algorithm for recognizing DET networks is presented. Furthermore, we also give a necessary and sufficient condition for the existence of a double Euler circuit in a network. 相似文献
43.
44.
Summary There has been a rapid growth of interest in techniques for site-directed drug design, fuelled by the increasing availability of structural models of proteins of therapeutic importance, and by studies reported in the literature showing that potent chemical leads can be obtained by these techniques. Structure generation programs offer the prospect of discovering highly original lead structures from novel chemical families. Due to the fact that this technique is more-or-less still in its infancy, there are no case studies available that demonstrate the use of structure generation programs for site-directed drug design. Such programs were first proposed in 1986, and became commercially available in early 1992. They have shown their ability to reproduce, or suggest reasonable alternatives for, ligands in well-defined binding sites. This brief review will discuss the recent advances that have been made in the field of site-directed structure generation. 相似文献
45.
46.
A relatively rare phenomenon, the induction of thermotropic mesophases from combinations of homologues of non-mesogenic compounds, is examined in some detail. Thus, monotropic nematic phases have been obtained from binary mixtures of 4-alkyl-N-(4-cyanophenyl)piperidines (I). Except for the hexyl homologue, the compounds I with butyl to nonyl as alkyl groups are non-mesomorphic as neat materials. From phase diagrams, the nematic clearing temperatures of the binary mixtures are found to vary linearly with composition and to increase as the molecular lengths of the components are increased. By introducing a methyl group at the 1-position of the alkyl chain, the melting points of the neat components are depressed, and the nematic clearing temperatures of the binary mixtures are lowered. Factors associated with the induction of this mesomorphism and with the 'kinetic' and thermodynamic stabilities of the monotropic nematic phases are discussed. Among these is the orientation between the rings of the core groups in I and in the more common mesogens, the trans-4-alkyl-1-(4-cyanophenyl) cyclohexanes, II. 相似文献
47.
48.
Christopher K. Yee‐Chan Robert C. Scogna Richard A. Register 《Journal of Polymer Science.Polymer Physics》2007,45(10):1198-1204
In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (Tg) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine Tg in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. Tg is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007 相似文献
49.
Partially fluorinated alcohols F(CF2)m(CHOH)(CH2)nH (where m = 7, n = 8 and m = 9, n = 10) exhibit highly ordered smectic liquid-crystalline phases, as confirmed by optical microscopy and differential scanning calorimetry. Miscibility studies show that the smectic phases of the two alcohols are not of the same type. The related partially fluorinated ketones F(CF2)mCO(CH2)nH do not form a detectable mesophase. This is rather surprising since the (more polar) corresponding alcohols and the (less polar) analogous iodides and n-alkanes do. A brief discussion of the results and a comparison among partially fluorinated alkanes with various functional groups are presented. 相似文献
50.
Guantao Chen Ronald J. Gould Michael S. Jacobson Richard H. Schelp 《Journal of Graph Theory》1994,18(7):663-672
For several years, the study of neighborhood unions of graphs has given rise to important structural consequences of graphs. In particular, neighborhood conditions that give rise to hamiltonian cycles have been considered in depth. In this paper we generalize these approaches to give a bound on the smallest number of cycles in G containing all the vertices of G. We show that if for all x, y ? V(G), |N(x) ∩ N(y)| ≧ 2n/5 + 1, then V(G) is coverable by at most two cycles. Several related results and extensions to t cycles are also given. 相似文献