全文获取类型
收费全文 | 48667篇 |
免费 | 4797篇 |
国内免费 | 7809篇 |
专业分类
化学 | 36801篇 |
晶体学 | 1984篇 |
力学 | 2785篇 |
综合类 | 447篇 |
数学 | 5903篇 |
物理学 | 13353篇 |
出版年
2024年 | 102篇 |
2023年 | 419篇 |
2022年 | 942篇 |
2021年 | 981篇 |
2020年 | 1312篇 |
2019年 | 1194篇 |
2018年 | 1037篇 |
2017年 | 1277篇 |
2016年 | 1714篇 |
2015年 | 1634篇 |
2014年 | 2038篇 |
2013年 | 3371篇 |
2012年 | 3603篇 |
2011年 | 2683篇 |
2010年 | 2359篇 |
2009年 | 3299篇 |
2008年 | 3511篇 |
2007年 | 3451篇 |
2006年 | 3228篇 |
2005年 | 2770篇 |
2004年 | 2652篇 |
2003年 | 2315篇 |
2002年 | 2456篇 |
2001年 | 1624篇 |
2000年 | 1610篇 |
1999年 | 1321篇 |
1998年 | 1197篇 |
1997年 | 949篇 |
1996年 | 922篇 |
1995年 | 791篇 |
1994年 | 773篇 |
1993年 | 572篇 |
1992年 | 585篇 |
1991年 | 373篇 |
1990年 | 299篇 |
1989年 | 226篇 |
1988年 | 227篇 |
1987年 | 162篇 |
1986年 | 146篇 |
1985年 | 155篇 |
1984年 | 141篇 |
1983年 | 104篇 |
1982年 | 111篇 |
1981年 | 94篇 |
1980年 | 84篇 |
1979年 | 90篇 |
1978年 | 68篇 |
1977年 | 68篇 |
1976年 | 55篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
We obtain upper bounds for the tail distribution of the first nonnegative sum of a random walk and for the moments of the overshoot over an arbitrary nonnegative level if the expectation of jumps is positive and close to zero. In addition, we find an estimate for the expectation of the first ladder epoch. 相似文献
52.
我国上市公司资本结构影响因素实证分析 总被引:17,自引:0,他引:17
本文选取了可能影响企业资本结构的多个指标变量进行分析 ,利用主成分分析提供的方法将变量综合成彼此互不相关的少数几个主成分。再用主成分 (作为回归自变量 )对企业的 5种资本负债比 (作为回归因变量 )进行多元回归分析 ,得出了影响企业资本结构的主要因素 ,以及这些因素与企业资本结构之间的关系 ,为企业确定资本结构提供参考依据 ,为企业财务决策提供支持。 相似文献
53.
Under idealized conditions, electroconvection in a nematic liquid crystal sets in at a well-defined threshold of the driving ac voltage. Fluctuating convection rolls of a small amplitude below that threshold have been observed recently. The measurement of the amplitude of these fluctuations is described in detail in this paper. It is based on averaging the light intensity signals using the structure function and a quantitative analysis of the light deflection. 相似文献
54.
本文采用求解Schroedinger方程和数值计算方法,研究了Ⅴ-型三能级原子与双模奇偶纠缠相干光场相互作用系统的光子统计性质,结果表明:此性质与双模奇偶纠缠相干光场的纠缠程度、失谐量、原子的初态以及双模光的平均光子数相关联. 相似文献
55.
基于密度泛函理论(DFT),用完全势能线性缀加平面波方法(FPLAPW)计算模拟了MnO2的同质多相变体(金红石型结构、CaCl2型结构、黄铁矿型结构),通过计算,预测了β-MnO2在5 GPa时从金红石型结构转变为CaCl2型结构,在20 GPa时进一步转变为黄铁矿型结构。另外,还总结和比较了几种金红石型结构二氧化物的压致相变特点,得出第Ⅳ主族元素的金红石型结构氧化物有很好的规律性,发生的相变序列基本一致;并随着金属阳离子半径的增大,发生相变的压力值也相应地递减,各氧化物的体积模量值相应地减小。 相似文献
56.
R. S. Martynov Yu. M. Nechepurenko 《Computational Mathematics and Mathematical Physics》2006,46(7):1155-1167
For a discrete linear stochastic dynamical system, computation of the response matrix to the external action from a subspace using given observational data is examined. An algorithm is proposed and substantiated that makes it possible to improve the numerical accuracy and to reduce the amount of observational data compared to the general case where an arbitrary external action is allowed. As an illustration, a discrete system arising in the analysis of a linear stochastic dynamical continuous-time system is considered more thoroughly. Some numerical results are presented. 相似文献
57.
As borders between different entities, lines are an important element of natural images. Indeed, the neurons of the mammalian
visual cortex are tuned to respond best to lines of a given orientation. This preferred orientation varies continuously across
most of the cortex, but also has vortex-like singularities known as pinwheels. In attempting to describe such patterns of
orientation preference, we are led to consider underlying rotation symmetries: Oriented segments in natural images tend to
be collinear; neurons are more likely to be connected if their preferred orientations are aligned to their topographic separation.
These are indications of a reduced symmetry requiring joint rotations of both orientation preference and the underlying topography.
This is verified by direct statistical tests in both natural images and in cortical maps. Using the statistics of natural
scenes we construct filters that are best suited to extracting information from such images, and find qualitative similarities
to mammalian vision.
PACS84.35+i 89.70.+c 87.57.Nk 相似文献
58.
It is long known that the Fokker-Planck equation with prescribed constant coefficients of diffusion and linear friction describes
the ensemble average of the stochastic evolutions in velocity space of a Brownian test particle immersed in a heat bath of
fixed temperature. Apparently, it is not so well known that the same partial differential equation, but now with constant
coefficients which are functionals of the solution itself rather than being prescribed, describes the kinetic evolution (in
the N→∞ limit) of an
isolated
N-particle system with certain stochastic interactions. Here we discuss in detail this recently discovered interpretation.
An erratum to this article can be found at 相似文献
59.
Quenching of the eutectoidal composition of the shape memory alloys (SMA’s) allowed us to display the gradual transition of
the martensitic structure as a function of the composition. Indeed, we have verified that for low Al and Ni percentages, the
martensite structure obtained is β′, while, for high Al and Ni (Al<13.3 mass%) contents, the martensite structure is essentially
of β1′ type. Elsewhere, we have observed that a nickel addition (<4.3 mass%) operates simultaneously on the composition of the
eutectoidal pointand the domain of the different martensitic structures types. During reheating of the quenched structure,
the transformation sequences of the martensite to the parent phase (β1), then the decomposition of the later phase into the
equilibrium phases, and finally, the redissolution of all phases to form the β phase at high temperature, are rather similar.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
60.