全文获取类型
收费全文 | 1057篇 |
免费 | 110篇 |
国内免费 | 94篇 |
专业分类
化学 | 653篇 |
晶体学 | 3篇 |
力学 | 12篇 |
综合类 | 8篇 |
数学 | 175篇 |
物理学 | 130篇 |
综合类 | 280篇 |
出版年
2024年 | 4篇 |
2023年 | 10篇 |
2022年 | 25篇 |
2021年 | 17篇 |
2020年 | 23篇 |
2019年 | 28篇 |
2018年 | 24篇 |
2017年 | 17篇 |
2016年 | 42篇 |
2015年 | 42篇 |
2014年 | 48篇 |
2013年 | 80篇 |
2012年 | 46篇 |
2011年 | 54篇 |
2010年 | 54篇 |
2009年 | 66篇 |
2008年 | 65篇 |
2007年 | 75篇 |
2006年 | 75篇 |
2005年 | 78篇 |
2004年 | 55篇 |
2003年 | 65篇 |
2002年 | 53篇 |
2001年 | 44篇 |
2000年 | 25篇 |
1999年 | 19篇 |
1998年 | 24篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1995年 | 20篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有1261条查询结果,搜索用时 9 毫秒
91.
We consider a space of Chebyshev splines whose left and right derivatives
satisfy linear constraints that are given by arbitrary nonsingular connection matrices.
We show that for almost all knot sequences such spline spaces have basis functions
whose support is equal to the support of the ordinary B-splines with the same knots.
Consequently, there are knot insertion and evaluation algorithms analogous to de Boors
algorithm for ordinary splines. 相似文献
92.
The Volume of Hyperbolic Alternating Link Complements 总被引:1,自引:0,他引:1
If a hyperbolic link has a prime alternating diagram D, thenwe show that the link complement's volume can be estimated directlyfrom D. We define a very elementary invariant of the diagramD, its twist number t(D), and show that the volume lies betweenv3(t(D) 2)/2 and v3(10t(D) 10), where v3 isthe volume of a regular hyperbolic ideal 3-simplex. As a consequence,the set of all hyperbolic alternating and augmented alternatinglink complements is a closed subset of the space of all completefinite-volume hyperbolic 3-manifolds, in the geometric topology.2000 Mathematics Subject Classification 57M25, 57N10. 相似文献
93.
Masamichi Takase 《Topology》2004,43(6):1425-1447
Haefliger has shown that a smooth embedding of the (4k−1)-sphere in the 6k-sphere can be knotted in the smooth sense. In this paper, we give a formula with which we can detect the isotopy class of such a Haefliger knot. The formula is expressed in terms of the geometric characteristics of an extension, analogous to a Seifert surface, of the given embedding. In particular, the Hopf invariant associated to the extension plays a crucial role. This leads us to a new characterisation of Haefliger knots. 相似文献
94.
Let G be a graph. An independent set Y in G is called an essential independent set (or essential set for simplicity) if there is {y1,y2} (?) Y such that dist (y1,y2) = 2. In this paper, we use the technique of the vertex insertion on l-connected (l = k or k + 1, k≥ 2) graphs to provide a unified proof for G to be hamiltonian, or hamiltonian-connected. The sufficient conditions are expressed 相似文献
95.
Greg Friedman 《Proceedings of the American Mathematical Society》2004,132(10):3103-3109
Frame-spun knots are constructed by spinning a knot of lower dimension about a framed submanifold of . We show that all frame-spun knots are slice (null-cobordant).
96.
97.
Vincent Blanloeil Osamu Saeki 《Transactions of the American Mathematical Society》2002,354(10):3955-3971
Consider a closed connected oriented 3-manifold embedded in the -sphere, which is called a -knot in this paper. For two such knots, we say that their Seifert forms are spin concordant, if they are algebraically concordant with respect to a diffeomorphism between the 3-manifolds which preserves their spin structures. Then we show that two simple fibered 3-knots are geometrically concordant if and only if they have spin concordant Seifert forms, provided that they have torsion free first homology groups. Some related results are also obtained.
98.
Joao Marcelo Ribeiro 《Molecular physics》2015,113(13-14):1865-1872
Ab initio and density functional calculations have been performed to elucidate the mechanism of CH radical insertion into methane. The results show that the reaction can be viewed to occur via two stages. On the first stage, the CH radical approaches methane without large structural changes to acquire proper positioning for the subsequent stage, where H-migration occurs from CH4 to CH, along with a C–C bond formation. Where the first stage ends and the second begins, a tight transition state was located using the B3LYP/6-311G(d,p) and MP4(SDQ)/6-311++G(d,p) methods. Using a rigid rotor – harmonic oscillator approach within transition state theory, we show that at the MP5/6-311++G(d,p)//MP4(SDQ)/6-311++G(d,p) level the calculated rate constants are in a reasonably good agreement with experiment in a broad temperature range of 145–581 K. Even at low temperatures, the insertion reaction bottleneck is found about the location of the tight transition state, rather than at long separations between the CH and CH4 reactants. In addition, high level CCSD(T)-F12/CBS calculations of the remainder of the C2H5 potential energy surface predict the CH+CH4 reaction to proceed via the initial insertion step to the ethyl radical which then can emit a hydrogen atom to form highly exothermic C2H4+H products. 相似文献
99.
Caiyun Xiong Xiaolin Nie Yixue Peng Xun Zhou Yangtao Fan Hu Chen Yanhui Liu 《理论物理通讯》2021,73(7):75602
Knots are discovered in biophysical systems, such as DNA and proteins. Knotted portions in knotted DNA are significantly bent and their corresponding bending angles are comparable with or larger than the sharp bending angle resulting in flexible defects. The role of flexible defects in the interplay of supercoiling and knotting of circular DNA were predicted by a Monte Carlo simulation. In knotted DNA with a particular knot type, a flexible defect noticeably enhances the supercoiling of the knotted DNA and the decreasing excitation energy makes the knotted portion more compact. A reduction in twist rigidity and unwinding of flexible defects are incorporated into the numerical simulations, so that interplay of supercoiling and knotting of circular DNA is studied under torsional conditions. Increasing unwinding not only results in a wider linking number distribution, but also leads to a drift of the distribution to lower values. A flexible defect has obvious effects on knotting probability. The summation of equilibrium distribution probability for nontrivial knotted DNA with different contour length does not change with excitation energy monotonically and has a maximum at an intermediate value of excitation energy around 5 kBT. In the phase space of knot length and gyration radius of knotted DNA, knot length does not anticorrelate with its gyration radius, which is attributed to the flexible defect in the knotted portion, which leads to the release of bending energy and inhibited the competition between entropy and bending energy. 相似文献
100.
This paper studies the topological properties of knotted solitons in
the (3+1)-dimensional Aratyn--Ferreira--Zimerman (AFZ) model.
Topologically, these solitons are characterized by the Hopf
invariant I, which is an integral class in the homotopy group
π3(S3)=Z. By making use of the decomposition of U(1) gauge
potential theory and Duan's topological current theory, it is shown
that the invariant is just the total sum of all the self-linking and
linking numbers of the knot family while only linking
numbers are considered in other papers. Furthermore, it is pointed out that this
invariant is preserved in the branch processes (splitting, merging
and intersection) of these knot vortex lines. 相似文献