全文获取类型
收费全文 | 45715篇 |
免费 | 3212篇 |
国内免费 | 2649篇 |
专业分类
化学 | 22678篇 |
晶体学 | 459篇 |
力学 | 2628篇 |
综合类 | 171篇 |
数学 | 10711篇 |
物理学 | 14929篇 |
出版年
2023年 | 366篇 |
2022年 | 543篇 |
2021年 | 607篇 |
2020年 | 755篇 |
2019年 | 652篇 |
2018年 | 1735篇 |
2017年 | 1930篇 |
2016年 | 1488篇 |
2015年 | 1315篇 |
2014年 | 1420篇 |
2013年 | 1797篇 |
2012年 | 4672篇 |
2011年 | 3905篇 |
2010年 | 2868篇 |
2009年 | 2537篇 |
2008年 | 1701篇 |
2007年 | 1594篇 |
2006年 | 1619篇 |
2005年 | 5260篇 |
2004年 | 4551篇 |
2003年 | 2836篇 |
2002年 | 1030篇 |
2001年 | 747篇 |
2000年 | 477篇 |
1999年 | 618篇 |
1998年 | 422篇 |
1997年 | 359篇 |
1996年 | 378篇 |
1995年 | 325篇 |
1994年 | 312篇 |
1993年 | 221篇 |
1992年 | 358篇 |
1991年 | 307篇 |
1990年 | 267篇 |
1989年 | 218篇 |
1988年 | 204篇 |
1987年 | 122篇 |
1986年 | 101篇 |
1985年 | 79篇 |
1984年 | 59篇 |
1983年 | 52篇 |
1981年 | 36篇 |
1976年 | 79篇 |
1975年 | 37篇 |
1974年 | 40篇 |
1973年 | 48篇 |
1972年 | 39篇 |
1969年 | 37篇 |
1968年 | 35篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
有界连通区域上Dirichlet空间及其算子 总被引:1,自引:0,他引:1
本文主要讨论了有界连通区域Dirichlet空间上Toeplitz算子的Fredholm性质,计算了符号在C1中的Toeplitz算子的本性谱和Fredholm指标. 相似文献
82.
A tree T is arbitrarily vertex decomposable if for any sequence τ of positive integers adding up to the order of T there is a sequence of vertex-disjoint subtrees of T whose orders are given by τ. An on-line version of the problem of characterizing arbitrarily vertex decomposable trees is completely solved here. 相似文献
83.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 1016 m−3, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 1021 m−3 for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy. 相似文献
84.
85.
Kj?rand Iversen Per Nilsson 《International Journal of Computers for Mathematical Learning》2007,12(2):113-133
This paper focuses on the different ways in which students in lower secondary school (14–16 year olds) experience compound
random events, presented to them in the form of combined junctions. A carefully designed ICT environment was developed enabling
the students to interact with different representations of such structures. Data for the analysis was gathered from two interview
sessions. The analysis of the interaction is based on constructivist principles on learning; i.e. we adopted a student-oriented
perspective, taking into consideration the different ways students try to make sense of chance encounters.
Our results show how some students give priority to geometrical and physical concerns, and we discuss how seeking causal explanations
of random phenomena may have encouraged this. With respect to numerically oriented models a division strategy appears to stand
out as the preferred one. 相似文献
86.
87.
Jü rgen Herzog Takayuki Hibi Xinxian Zheng 《Transactions of the American Mathematical Society》2006,358(9):4119-4134
Squarefree monomial ideals arising from finite meet-semilattices and their free resolutions are studied. For the squarefree monomial ideals corresponding to poset ideals in a distributive lattice, the Alexander dual is computed.
88.
The nonradiative recombination effect on carrier dynamics in GalnNAs/GaAs quantum wells is studied by timeresolved photoluminescence (TRPL) and polarization-dependent TRPL at various excitation intensities. It is found that both recombination dynamics and spin relaxation dynamics strongly depend on the excitation intensity. Under moderate excitation intensities the PL decay curves exhibit unusual non-exponential behaviour. This result is well simulated by a rate equation involving both the radiative and non-radiative recombinations via the introduction of a new parameter of the effective concentration of nonradiative recombination centres in the rate equation. In the spin dynamics study, the spin relaxation also shows strong excitation power dependence. Under the high excitation power an increase of spin polarization degree with time is observed. This new finding provides a useful hint that the spin process can be controlled by excitation power in GaInNAs systems. 相似文献
89.
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
90.
Covering numbers of precompact symmetric convex subsets of Hilbert spaces are investigated. Lower bounds are derived for sets containing orthogonal subsets with norms of their elements converging to zero sufficiently slowly. When these sets are convex hulls of sets with power-type covering numbers, the bounds are tight. The arguments exploit properties of generalized Hadamard matrices. The results are illustrated by examples from machine learning, neurocomputing, and nonlinear approximation. 相似文献