全文获取类型
收费全文 | 11600篇 |
免费 | 363篇 |
国内免费 | 366篇 |
专业分类
化学 | 2220篇 |
晶体学 | 60篇 |
力学 | 102篇 |
综合类 | 111篇 |
数学 | 8850篇 |
物理学 | 986篇 |
出版年
2023年 | 47篇 |
2022年 | 177篇 |
2021年 | 139篇 |
2020年 | 116篇 |
2019年 | 298篇 |
2018年 | 329篇 |
2017年 | 198篇 |
2016年 | 196篇 |
2015年 | 183篇 |
2014年 | 367篇 |
2013年 | 793篇 |
2012年 | 416篇 |
2011年 | 685篇 |
2010年 | 648篇 |
2009年 | 869篇 |
2008年 | 898篇 |
2007年 | 860篇 |
2006年 | 672篇 |
2005年 | 468篇 |
2004年 | 466篇 |
2003年 | 373篇 |
2002年 | 272篇 |
2001年 | 219篇 |
2000年 | 234篇 |
1999年 | 260篇 |
1998年 | 218篇 |
1997年 | 178篇 |
1996年 | 200篇 |
1995年 | 192篇 |
1994年 | 173篇 |
1993年 | 167篇 |
1992年 | 127篇 |
1991年 | 89篇 |
1990年 | 87篇 |
1989年 | 69篇 |
1988年 | 64篇 |
1987年 | 66篇 |
1986年 | 57篇 |
1985年 | 54篇 |
1984年 | 62篇 |
1983年 | 31篇 |
1982年 | 56篇 |
1981年 | 50篇 |
1980年 | 48篇 |
1979年 | 38篇 |
1978年 | 46篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1975年 | 5篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
For any finite dimensional control system with arbitrary cost, Pontryagin's Maximum Principle (PMP) [N. Bensalem, Localisation des courbes anormales et problème d'accessibilité sur un groupe de Lie hilbertien nilpotent de degré 2, Thèse de doctorat, Université de Savoie, 1998. [6]] gives necessary conditions for optimality of trajectories. In the infinite dimensional case, it is well known that these conditions are no more true in general. The purpose of this paper is to establish an “approached” version of PMP for infinite dimensional bilinear systems, with fixed final time and without constraints on the final state. Moreover, if the set of control is contained in a closed bounded convex subset with operators defining its dynamics are compact, or if it is contained in a finite dimensional space, we get an “exact” version of PMP. We also give two applications of these results. The first one deals with sub-Riemannian geometry on nilpotent Hilbertian Lie groups for which we can define a sub-Riemannian distance. The second one deals with heat equation for which we analyse the necessary conditions to give the optimal controls. 相似文献
42.
Danny E. Akin W. Herbert Morrison III Luanne L. Rigsby Franklin E. Barton II David S. Himmelsbach Kevin B. Hicks 《Applied biochemistry and biotechnology》2006,129(1-3):104-116
Information is presented on structure, composition, and response to enzymes of corn stover related to barriers for bioconversion
to ethanol. Aromatic compounds occurred in most tissue cell walls. Ferulic acid esterase treatment before cellulase treatment
significantly improved dry weight loss and release of phenolic acids and sugars in most fractions over cellulase alone. Leaf
fractions were considerably higher in dry weight loss and released sugars with esterase treatment, but stem pith cells gave
up the most phenolic acids. Results help identify plant fractions more appropriate for coproducts and bioconversion and those
more suitable as residues for soil erosion control. 相似文献
43.
The paper is concerned with a two-delay singular differential system with a twin parameter. Applying fixed-point index theory, we show the relationship between the asymptotic behaviors of nonlinearities (at zero and infinity) and the open regions (eigenvalue regions) of parameters, which are correlated with delays, such that the system has zero, one and two positive solution(s). 相似文献
44.
Jurie Conradie 《Positivity》2006,10(3):591-606
The possibility of characterizing the Mackey topology of a dual pair of vector spaces as a generalized inductive limit (or
mixed) topology is investigated. Positive answers are given for a wide range of dual pairs of Riesz spaces (vector lattices)
and non-commutative Banach function spaces (or symmetric operator spaces). 相似文献
45.
The oxide films formed on AISI 304L stainless steel at 300 °C in the oxidation time range between 2 and 4 h have been studied by photoelectrochemistry. Photocurrents were investigated as a function of the wavelength of the incident light and the electrode potential. The investigation allowed the determination of the semiconductive properties of the oxides. The oxide films showed n-type behaviour. A duplex structure of the oxide films has been suggested on the basis of the photocurrent spectra, with an internal oxide layer having an optical gap (Eg2 = 2.16-2.3 eV) depending on the applied potential and oxidation time, higher to that of the external oxide layer (Eg1 ≈ 1.9 eV). Significant variations in the amplitude of the photocurrent were detected as a function of the applied potential and the oxidation time. 相似文献
46.
Piotr Ko?cielniak 《Topology and its Applications》2007,154(9):1951-1955
Let M be a compact manifold with dimM?2. We prove that some iteration of the generic homeomorphism on M is semiconjugated to the shift map and has infinite topological entropy (Theorem 1.1). 相似文献
47.
Teruhisa Tsuda 《Letters in Mathematical Physics》2006,77(1):21-30
Starting from certain point sets in the projective plane, we construct a tropical (or subtraction-free birational) representation of Weyl groups over the field of τ-functions. In particular, our construction includes E
8
(1)
, E
7
(1)
, E
6
(1)
and D
5
(1)
as affine cases; each of them gives rise to the q-difference Painlevé equation. 相似文献
48.
Paul Fabel 《Topology and its Applications》2007,154(3):635-638
With a certain natural topology, the fundamental group of a locally path connected metric space X becomes discrete if and only if X is semilocally simply connected. We include some related material and offer a counterexample to a similar result. 相似文献
49.
In the present paper, a Lotka–Volterra type mutualism system with several delays is studied. Some new and interesting sufficient conditions are obtained for the global existence of positive periodic solutions of the mutualism system. Our method is based on Mawhin’s coincidence degree and novel estimation techniques for the a priori bounds of unknown solutions. Our results are different from the existing ones such as those in of Yang et al. [F. Yang, D. Jiang, A. Ying, Existence of positive solution of multidelays facultative mutualism system, J. Eng. Math. 3 (2002) 64–68] and Chen et al. [F. Chen, J. Shi, X. Chen, Periodicity in a Lotka–Volterra facultative mutualism system with several delays, J. Eng. Math. 21 (3) (2004) 403–409]. 相似文献
50.
We study the inverse scattering problem for the nonlinear Schrödinger equation and for the nonlinear Klein–Gordon equation with the generalized Hartree type nonlinearity. We reconstruct the nonlinearity from knowledge of the scattering operator, which improves the known results. 相似文献