全文获取类型
收费全文 | 5053篇 |
免费 | 282篇 |
国内免费 | 547篇 |
专业分类
化学 | 1536篇 |
晶体学 | 12篇 |
力学 | 62篇 |
综合类 | 60篇 |
数学 | 3752篇 |
物理学 | 460篇 |
出版年
2024年 | 4篇 |
2023年 | 70篇 |
2022年 | 76篇 |
2021年 | 94篇 |
2020年 | 135篇 |
2019年 | 147篇 |
2018年 | 142篇 |
2017年 | 134篇 |
2016年 | 101篇 |
2015年 | 108篇 |
2014年 | 166篇 |
2013年 | 655篇 |
2012年 | 147篇 |
2011年 | 186篇 |
2010年 | 162篇 |
2009年 | 248篇 |
2008年 | 300篇 |
2007年 | 318篇 |
2006年 | 295篇 |
2005年 | 297篇 |
2004年 | 262篇 |
2003年 | 255篇 |
2002年 | 234篇 |
2001年 | 196篇 |
2000年 | 204篇 |
1999年 | 159篇 |
1998年 | 160篇 |
1997年 | 159篇 |
1996年 | 99篇 |
1995年 | 59篇 |
1994年 | 46篇 |
1993年 | 46篇 |
1992年 | 41篇 |
1991年 | 22篇 |
1990年 | 12篇 |
1989年 | 23篇 |
1988年 | 19篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 20篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有5882条查询结果,搜索用时 312 毫秒
991.
The Tobey-Simon (additivity) rule for aromatic groups which was devised about 40 years ago has been found to need revision. The rule shows an aromatic group attached to a C==C double bond as causing a downfield chemical shift of a cis-related vinylic proton and a small upfield shift of a trans-related proton. A search of data in the recent literature has shown that this rule should apply mainly to monosubstituted phenyl groups and some polynuclear aromatics. In contrast with them, 2,6-disubstituted phenyl, 2,4,6-trisubstituted phenyl and 9-anthracenyl groups cause cis-related vinylic protons to resonate upfield of comparable trans-related protons. Further, the current rule for o-substituted phenyl groups has been found to be inaccurate. In writing a rule for aromatic groups, therefore, greater attention needs to be given to the diversity of effects that these groups have on chemical shifts of vinylic protons. 相似文献
992.
Let G be a non-Engel group and let L(G) be the set of all left Engel elements of G. Associate with G a graph as follows: Take G L(G) as vertices of and join two distinct vertices x and y whenever [x,ky]≠1 and [y,kx]≠1 for all positive integers k. We call , the Engel graph of G. In this paper we study the graph theoretical properties of . 相似文献
993.
V. G. Safonov 《Siberian Mathematical Journal》2007,48(1):150-155
We prove that the finite length condition for the lattice of τ-closed totally saturated subformations of a τ-closed totally saturated formation is equivalent to the finite length condition for this lattice, and it is also equivalent for the formation to be soluble and one-generated. 相似文献
994.
Volodymyr Nekrashevych 《Geometriae Dedicata》2007,124(1):153-190
We construct and study a family of 3-generated groups parametrized by infinite binary sequences w. We show that two groups of the family are isomorphic if and only if the sequences are cofinal and that two groups cannot
be distinguished by finite sets of relations. We show a connection of the family with 2-dimensional holomorphic dynamics.
相似文献
995.
We consider cocycles over certain hyperbolic actions, , and show rigidity properties for cocycles with values in a Lie group or a diffeomorphism group, which are close to identity
on a set of generators, and are sufficiently smooth. The actions we consider are Cartan actions of or , for , and Γ torsion free cocompact lattice. The results in this paper rely on a technique developed recently by D. Damjanović
and A. Katok.
相似文献
996.
We study the pure braid groups of the real projective plane , and in particular the possible splitting of the Fadell–Neuwirth short exact sequence , where n ≥ 2 and m ≥ 1, and p
* is the homomorphism which corresponds geometrically to forgetting the last m strings. This problem is equivalent to that of the existence of a section for the associated fibration of configuration spaces. Van Buskirk proved (1966, Trans. Am. Math. Soc., 122:81–97) that p and p
* admit a section if n = 2 and m = 1. Our main result in this paper is to prove that there is no section if n ≥ 3. As a corollary, it follows that n = 2 and m = 1 are the only values for which a section exists. As part of the proof, we derive a presentation of : this appears to be the first time that such a presentation has been given in the literature.
相似文献
997.
This paper continues the study begun in [GEROLDINGER, A.: On non-unique factorizations into irreducible elements II, Colloq. Math. Soc. János Bolyai 51 (1987), 723–757] concerning factorization properties of block monoids of the form ℬ(ℤ
n
, S) where S =
(hereafter denoted ℬ
a
(n)). We introduce in Section 2 the notion of a Euclidean table and show in Theorem 2.8 how it can be used to identify the irreducible elements of ℬ
a
(n). In Section 3 we use the Euclidean table to compute the elasticity of ℬ
a
(n) (Theorem 3.4). Section 4 considers the problem, for a fixed value of n, of computing the complete set of elasticities of the ℬ
a
(n) monoids. When n = p is a prime integer, Proposition 4.12 computes the three smallest possible elasticities of the ℬ
a
(p).
Part of this work was completed while the second author was on an Academic Leave granted by the Trinity University Faculty
Development Committee. 相似文献
998.
999.
Synthetic organic chemists have long depended on protecting group manipulations when faced with the challenges of chemoselectivity and functional group incompatibility. Overcoming this dependence will improve the overall efficiency of chemical synthesis. By taking advantage of orthogonally reactive functional groups, amphoteric molecules can afford access not only to more efficient and strategic syntheses but also to the development of novel chemical transformations. 相似文献
1000.