全文获取类型
收费全文 | 653378篇 |
免费 | 5290篇 |
国内免费 | 1806篇 |
专业分类
化学 | 337709篇 |
晶体学 | 9212篇 |
力学 | 31425篇 |
综合类 | 21篇 |
数学 | 85667篇 |
物理学 | 196440篇 |
出版年
2021年 | 5741篇 |
2020年 | 6336篇 |
2019年 | 7109篇 |
2018年 | 9570篇 |
2017年 | 9658篇 |
2016年 | 13432篇 |
2015年 | 7415篇 |
2014年 | 12238篇 |
2013年 | 28728篇 |
2012年 | 21973篇 |
2011年 | 26253篇 |
2010年 | 19475篇 |
2009年 | 19250篇 |
2008年 | 25041篇 |
2007年 | 24914篇 |
2006年 | 22876篇 |
2005年 | 20567篇 |
2004年 | 18867篇 |
2003年 | 17068篇 |
2002年 | 16943篇 |
2001年 | 17855篇 |
2000年 | 13989篇 |
1999年 | 10769篇 |
1998年 | 9331篇 |
1997年 | 9249篇 |
1996年 | 8660篇 |
1995年 | 7705篇 |
1994年 | 7708篇 |
1993年 | 7532篇 |
1992年 | 7996篇 |
1991年 | 8481篇 |
1990年 | 8127篇 |
1989年 | 8016篇 |
1988年 | 7846篇 |
1987年 | 7615篇 |
1986年 | 7339篇 |
1985年 | 9448篇 |
1984年 | 9881篇 |
1983年 | 8200篇 |
1982年 | 8682篇 |
1981年 | 8104篇 |
1980年 | 7705篇 |
1979年 | 8259篇 |
1978年 | 8680篇 |
1977年 | 8546篇 |
1976年 | 8491篇 |
1975年 | 8073篇 |
1974年 | 7958篇 |
1973年 | 8305篇 |
1972年 | 5959篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
E. O. Carew P. Townsend M. F. Webster 《Numerical Methods for Partial Differential Equations》1994,10(2):171-190
Coupled and decoupled Taylor-Galerkin algorithms are considered for viscoelastic flow and a model problem—transient startup Poiseuille flow in a channel under a fixed pressure gradient. All algorithms reproduce the steady-state solutions and are stable at high elasticity numbers (E). For a fixed mesh, the coupled and decoupled versions (TGC and TGD) give exceptional time-accuracy at low elasticity numbers [to within O(1%) at E = 1] and reasonable accuracy at high elasticity numbers [to within O(10%) at E = 10, 100]. By definition, the decoupled false-transient scheme (TGF), which uses different time scales for velocity and stress time stepping, provides a poor transient history. Where the main requirement is to compute a steady-state algorithm efficiency is crucial. The TGF scheme attains a steady state between six to eight times faster than does the TGC scheme, and the latter is over twice as fast as the TGD form. © 1994 John Wiley & Sons, Inc. 相似文献
952.
953.
954.
Product integration rules generalizing the Fej?r, Clenshaw-Curtis,and Filippi quadrature rules respectively are derived for integralswith trigonometric and hyperbolic weight factors. The Chebyshevmoments of the weight functions are found to be given by well-conditionedexpressions, in terms of hypergeometric functions 0F1. An a priori error estimator is discussed which is shown bothto avoid wasteful invocation of the integration rule and toincrease significantly the robustness of the automatic quadratureprocedure. Then, specializing to extended Clenshaw-Curtis (ECC) rules,three types of a posteriori error estimates are considered andthe existence of a great risk of their failure is demonstratedby large scale validation tests. An empirical error estimator,superseding them for slowly varying integrands, is found toresult in a spectacular increase in the output reliability. Finally, enhancements in the control of the interval subdivisionstrategy aiming at increasing code robustness is discussed.Comparison with the code DQAWO of QUADPACK, with about a hundredthousand solved integrals, is illustrative of the increasedrobustness and error estimate reliability of our computer codeimplementation of the ECC rules. 相似文献
955.
956.
Amie E. Moch 《Results in Mathematics》2002,42(1-2):122-127
In this paper we determine the forcing linearity numbers for semisimple modules over integral domains. 相似文献
957.
958.
S. V. Bulyarskii N. A. Butylkina N. S. Grushko A. E. Luk'yanov M. V. Nazarov I. O. Stepin 《Russian Physics Journal》1991,34(4):339-342
Latent macroscopic defects in silicon are detected by electrical and electron microscope measurements. They lead to anomalous temperature dependence of the Fermi level position and growth in the hole capture coefficient. A level with energy of 0.55 eV measured from the conduction zone controls the recombination process. It is proposed that macroscopic defects develop upon association of oxygen-silicon vacancy complexes. Action of an electron beam leads to reversible changes which increase upon multiple scanning, affecting the value of the diffusion length.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 71–75, April, 1991. 相似文献
959.
A nonperturbative method is suggested for calculating functional integrals. Its efficiency is demonstrated for the quantum-mechanical anharmonic oscillator. A quantity we are interested in is represented by a series, a finite number of terms of which describes not only the region of small coupling constants but well reproduces the strong coupling limit. The method is formulated only in terms of the Gaussian functional quadratures and diagrams are used of the conventional perturbation theory. 相似文献
960.
J. Feldhaus E. L. Saldin J. R. Schneider E. A. Schneidmiller M. V. Yurkov 《Optics Communications》1997,140(4-6):341-352
A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ 5 × 10−5) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 1024 photons/(s × mrad2 × mm2 × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design. 相似文献