全文获取类型
收费全文 | 469786篇 |
免费 | 3457篇 |
国内免费 | 1568篇 |
专业分类
化学 | 223578篇 |
晶体学 | 7290篇 |
力学 | 25972篇 |
综合类 | 11篇 |
数学 | 75849篇 |
物理学 | 142111篇 |
出版年
2020年 | 3451篇 |
2019年 | 4109篇 |
2018年 | 15599篇 |
2017年 | 15342篇 |
2016年 | 13597篇 |
2015年 | 5111篇 |
2014年 | 7704篇 |
2013年 | 17417篇 |
2012年 | 16584篇 |
2011年 | 25703篇 |
2010年 | 17175篇 |
2009年 | 17689篇 |
2008年 | 20977篇 |
2007年 | 22956篇 |
2006年 | 13676篇 |
2005年 | 13224篇 |
2004年 | 12831篇 |
2003年 | 12082篇 |
2002年 | 11061篇 |
2001年 | 11705篇 |
2000年 | 8969篇 |
1999年 | 6863篇 |
1998年 | 5846篇 |
1997年 | 5816篇 |
1996年 | 5528篇 |
1995年 | 4933篇 |
1994年 | 4924篇 |
1993年 | 4895篇 |
1992年 | 5271篇 |
1991年 | 5480篇 |
1990年 | 5187篇 |
1989年 | 5213篇 |
1988年 | 5166篇 |
1987年 | 5043篇 |
1986年 | 4832篇 |
1985年 | 6331篇 |
1984年 | 6593篇 |
1983年 | 5339篇 |
1982年 | 5565篇 |
1981年 | 5440篇 |
1980年 | 5207篇 |
1979年 | 5590篇 |
1978年 | 5620篇 |
1977年 | 5678篇 |
1976年 | 5772篇 |
1975年 | 5301篇 |
1974年 | 5266篇 |
1973年 | 5410篇 |
1972年 | 3951篇 |
1971年 | 3433篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models. 相似文献
32.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 815–819, May, 1991. 相似文献
33.
A new geometrical method to determine the surface damage threshold for ultrashort pulses is presented. It consists in the formation of a surface damage profile by a movement of the sample across the laser beam focus. A single measurement of the maximum transversal dimension of this damage profile, which depends solely on the laser beam power, is used to calculate the local damage threshold intensity. It is also theoretically shown that the damage in the transverse dimension can be controlled under the diffraction-limited spot. 相似文献
34.
B. P. Harlamov 《Journal of Mathematical Sciences》2006,133(3):1371-1377
An inverse process with independent positive increments is considered. For such a process, the first hitting time τx of level x as a function of x ≥ 0 is a proper process with independent positive increments. In terms of first hitting times
and their Levy measures, multidemensional distribution densities and Laplace transformations are derived. Stationary distributions
of increments of the process are investigated. Bibliography: 8 titles.
__________
Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 311, 2004, pp. 286–297. 相似文献
35.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
36.
The excitation of eigen surface waves by tubular electron beams in cylindrical discharge devices is studied. The influence of the wave‐field azimuthal structure on the excitation efficiency and nonlinear stage of the plasmabeam instability is investigated both numerically and analytically. Analytical expressions for the saturation amplitude and excitation efficiency of the wave under study are derived. They are found to agree well with results obtained by numerical modelling of the plasma‐beam interaction presented in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
37.
P.A. Ramachandran 《Numerical Methods for Partial Differential Equations》2006,22(4):831-846
Time‐dependent differential equations can be solved using the concept of method of lines (MOL) together with the boundary element (BE) representation for the spatial linear part of the equation. The BE method alleviates the need for spatial discretization and casts the problem in an integral format. Hence errors associated with the numerical approximation of the spatial derivatives are totally eliminated. An element level local cubic approximation is used for the variable at each time step to facilitate the time marching and the nonlinear terms are represented in a semi‐implicit manner by a local linearization at each time step. The accuracy of the method has been illustrated on a number of test problems of engineering significance. © 2005 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2006 相似文献
38.
39.
E. P. Gordov 《Russian Physics Journal》1981,24(11):1039-1041
An evolution operatior is described, which acts on the operators of a problem to take it into the solutions of the corresponding problem of semiclassical electrodynamics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 66–69, November, 1981. 相似文献
40.
A. R. Volkov B. V. Shul'gin T. I. Polupanova V. N. Lebedev A. A. Nagornyi V. L. Petrov Yu. F. Kargin 《Journal of Applied Spectroscopy》1991,54(6):585-590
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991. 相似文献