全文获取类型
收费全文 | 164407篇 |
免费 | 863篇 |
国内免费 | 315篇 |
专业分类
化学 | 63754篇 |
晶体学 | 2600篇 |
力学 | 10403篇 |
综合类 | 8篇 |
数学 | 29983篇 |
物理学 | 58837篇 |
出版年
2022年 | 535篇 |
2021年 | 478篇 |
2020年 | 446篇 |
2019年 | 426篇 |
2018年 | 10073篇 |
2017年 | 10954篇 |
2016年 | 5243篇 |
2015年 | 1582篇 |
2014年 | 1119篇 |
2013年 | 2625篇 |
2012年 | 6896篇 |
2011年 | 16274篇 |
2010年 | 9841篇 |
2009年 | 9995篇 |
2008年 | 14071篇 |
2007年 | 17736篇 |
2006年 | 2667篇 |
2005年 | 9631篇 |
2004年 | 5747篇 |
2003年 | 5413篇 |
2002年 | 3163篇 |
2001年 | 3181篇 |
2000年 | 2479篇 |
1999年 | 1599篇 |
1998年 | 1129篇 |
1997年 | 1077篇 |
1996年 | 1071篇 |
1995年 | 889篇 |
1994年 | 796篇 |
1993年 | 705篇 |
1992年 | 1024篇 |
1991年 | 1022篇 |
1990年 | 923篇 |
1989年 | 834篇 |
1988年 | 829篇 |
1987年 | 887篇 |
1986年 | 737篇 |
1985年 | 980篇 |
1984年 | 941篇 |
1983年 | 669篇 |
1982年 | 659篇 |
1981年 | 627篇 |
1980年 | 591篇 |
1979年 | 736篇 |
1978年 | 772篇 |
1977年 | 693篇 |
1976年 | 646篇 |
1975年 | 536篇 |
1974年 | 574篇 |
1973年 | 547篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
991.
M. G. Voronkov S. G. D’yachkova I. P. Lebedeva A. V. Evart L. G. Shagun 《Russian Journal of General Chemistry》2007,77(4):604-606
Alkylthiochloroacetylenes react with geminal (in ether) and vicinal (in THF) dithiols to form, respectively, functional 1,3-dithiolenes (yield 43%) or 1,3-dithiolanes (yield 34%). 相似文献
992.
L. R. Gavrilov 《Acoustical Physics》2003,49(5):508-515
A numerical simulation and a comparative analysis of the acoustic fields produced by two-dimensional phased arrays intended for ultrasonic surgery are performed for the case of a multiple focus (in particular, 25 foci) generation. The calculations were conducted for arrays (with an operating frequency of 1.5 MHz) consisting of 256 elements 5 mm in diameter, which were positioned on the array surface both regularly and randomly. The array foci can be formed simultaneously, but, in this case, the intensity levels of the secondary peaks in the ultrasonic field can exceed the values that guarantee the safe application of this method in surgery. A much safer way is to synthesize many foci with the use of several configurations, each of which contains a smaller number of foci. The number of foci in individual configurations must be approximately the same. It is demonstrated that randomization of the element distribution over the array surface provides an opportunity to improve the array performance, to reduce the intensity levels of secondary peaks in the acoustic field, and to increase the array capability for multiple focus scanning off the array axis. 相似文献
993.
The HERA - B Collaboration I. Abt et al. 《The European Physical Journal C - Particles and Fields》2003,26(3):345-355
Using the HERA - B detector, the production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The production was tagged via inclusive bottom quark decays into by exploiting the longitudinal separation of decay vertices from the primary proton-nucleus interaction. Both e
+
e
-
and channels have been reconstructed and the combined analysis yields the cross section .
Received: 6 June 2002 / Revised version: 2 September 2002 / Published online: 8 November 2002 相似文献
994.
J. S. Yang Z. G. Chen Y. T. Hu S. N. Jiang S. H. Guo 《Archive of Applied Mechanics (Ingenieur Archiv)》2007,77(10):689-696
We study thickness-twist vibrations and waves in an unbounded, multi-sectioned piezoelectric plate of crystals with 6 mm symmetry
or polarized ceramics. An exact solution from the three-dimensional equations of piezoelectricity is obtained. Basic vibration
and wave propagation characteristics are calculated based on the solution. The results are useful in the understanding and
design of plate resonators, filters and acoustic wave sensors of ZnO, AlN and polarized ceramics. 相似文献
995.
Yu. I. Samoilenko 《Ukrainian Mathematical Journal》2007,59(11):1750-1767
We propose and investigate a mathematical model of an open bilinear control system for the conversion of heat energy in a
coherent form. We show that the use of a combinational parametric resonance formed by the control system in a one-temperature
ensemble of weakly dissipative elastic-gyroscopic subsystems enables one to obtain a positive energy output without using
any cooling device apart from the control system.
__________
Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, No. 11, pp. 1557–1573, November, 2007. 相似文献
996.
If M is an isoparametric hypersurface in a sphere S
n
with four distinct principal curvatures, then the principal curvatures κ1, . . . , κ4 can be ordered so that their multiplicities satisfy m
1 = m
2 and m
3 = m
4, and the cross-ratio r of the principal curvatures (the Lie curvature) equals −1. In this paper, we prove that if M is an irreducible connected proper Dupin hypersurface in R
n
(or S
n
) with four distinct principal curvatures with multiplicities m
1 = m
2 ≥ 1 and m
3 = m
4 = 1, and constant Lie curvature r = −1, then M is equivalent by Lie sphere transformation to an isoparametric hypersurface in a sphere. This result remains true if the
assumption of irreducibility is replaced by compactness and r is merely assumed to be constant.
相似文献
997.
In a seminal paper, Martin Clark (Communications Systems and Random Process Theory, Darlington, 1977, pp. 721–734, 1978) showed how the filtered dynamics giving the optimal estimate of a Markov chain observed in Gaussian noise can be expressed
using an ordinary differential equation. These results offer substantial benefits in filtering and in control, often simplifying
the analysis and an in some settings providing numerical benefits, see, for example Malcolm et al. (J. Appl. Math. Stoch.
Anal., 2007, to appear).
Clark’s method uses a gauge transformation and, in effect, solves the Wonham-Zakai equation using variation of constants.
In this article, we consider the optimal control of a partially observed Markov chain. This problem is discussed in Elliott
et al. (Hidden Markov Models Estimation and Control, Applications of Mathematics Series, vol. 29, 1995). The innovation in our results is that the robust dynamics of Clark are used to compute forward in time dynamics for a simplified
adjoint process. A stochastic minimum principle is established. 相似文献
998.
Wing-Sum Cheung Patricia J.Y. Wong 《Journal of Mathematical Analysis and Applications》2007,329(2):851-869
We consider a system of focal boundary value problems where the nonlinearities may be singular in the independent variable and may also be singular in the dependent arguments. Using Schauder fixed point theorem, we establish criteria such that the system of boundary value problems has at least one fixed-sign solution. 相似文献
999.
Considering a plane hyperbolic system with time-periodic coefficients, we construct a version of the direct Lyapunov method with the condition on the derivative of the Lyapunov functional along the trajectories of the system which is weakened by use of periodicity of the coefficients. We exhibit an illustrative example. 相似文献
1000.