全文获取类型
收费全文 | 247665篇 |
免费 | 1360篇 |
国内免费 | 426篇 |
专业分类
化学 | 114345篇 |
晶体学 | 3829篇 |
力学 | 13036篇 |
综合类 | 3篇 |
数学 | 38141篇 |
物理学 | 80097篇 |
出版年
2021年 | 1049篇 |
2020年 | 1221篇 |
2019年 | 1263篇 |
2018年 | 10486篇 |
2017年 | 11343篇 |
2016年 | 6625篇 |
2015年 | 2454篇 |
2014年 | 2529篇 |
2013年 | 6235篇 |
2012年 | 9230篇 |
2011年 | 18561篇 |
2010年 | 11405篇 |
2009年 | 11490篇 |
2008年 | 16330篇 |
2007年 | 19766篇 |
2006年 | 5668篇 |
2005年 | 11807篇 |
2004年 | 8000篇 |
2003年 | 7609篇 |
2002年 | 5523篇 |
2001年 | 5784篇 |
2000年 | 4366篇 |
1999年 | 3156篇 |
1998年 | 2195篇 |
1997年 | 2255篇 |
1996年 | 2298篇 |
1995年 | 2052篇 |
1994年 | 2047篇 |
1993年 | 1833篇 |
1992年 | 2327篇 |
1991年 | 2403篇 |
1990年 | 2243篇 |
1989年 | 2204篇 |
1988年 | 2196篇 |
1987年 | 2205篇 |
1986年 | 2018篇 |
1985年 | 2499篇 |
1984年 | 2524篇 |
1983年 | 2023篇 |
1982年 | 2037篇 |
1981年 | 1865篇 |
1980年 | 1849篇 |
1979年 | 2159篇 |
1978年 | 2244篇 |
1977年 | 2095篇 |
1976年 | 2063篇 |
1975年 | 1930篇 |
1974年 | 1909篇 |
1973年 | 1989篇 |
1972年 | 1293篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
J.-Q. Shen H.-Y. Zhu P. Chen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(2):305-313
There exist a number of typical and interesting systems and/or models, which possess three-generator Lie-algebraic structure,
in atomic physics, quantum optics, nuclear physics and laser physics. The well-known fact that all simple 3-generator algebras
are either isomorphic to the algebra sl (2, C) or to one of its real forms enables us to treat these time-dependent quantum systems in a unified way. By making use of
both the Lewis-Riesenfeld invariant theory and the invariant-related unitary transformation formulation, the present paper
obtains exact solutions of the time-dependent Schr?dinger equations governing various three-generator Lie-algebraic quantum
systems. For some quantum systems whose time-dependent Hamiltonians have no quasialgebraic structures, it is shown that the exact solutions can also be obtained by working in a
sub-Hilbert-space corresponding to a particular eigenvalue of the conserved generator (i.e., the time-independent invariant that commutes with the time-dependent Hamiltonian). The topological property of geometric phase factors and its adiabatic limit in time-dependent systems is briefly
discussed.
Received 6 July 2002 / Received in final form 21 October 2002 Published online 11 February 2003 相似文献
993.
I T Vieira P R Harper A K Shahani V De Senna 《The Journal of the Operational Research Society》2003,54(7):713-722
Mother-to-child transmission (MTCT) of HIV is the most significant source of HIV infection in children below the age of 15 years. In 2000 alone, about 600 000 new infections occurred, the vast majority from mothers living in developing countries who were not aware of their HIV-positive status. To date, at least 4.3 million children have died from AIDS. In this paper, we discuss the development of an operational model that can be used to evaluate intervention options for the prevention of MTCT of HIV. The problem was approached by defining suitable pregnancy risk groups, prevention options and the required model outputs to evaluate different intervention strategies. The method chosen to approach the problem was a discrete-event, three-phase simulation, built in Visual Basic, with a stochastic semi-Markov structure. The developed model takes individual pregnant women through each stage of their pregnancy, labour and birth. Different intervention strategies may be modelled at any time, including short-course antiretroviral drugs and cessation of breastfeeding. The model is demonstrated with data from Botswana, which has one of the highest HIV-infection rates in the world. 相似文献
994.
Stephen P. Humphries 《Israel Journal of Mathematics》2004,143(1):189-222
We prove that (with two possible exceptions) the Hurwitz braid group action on the sequence of standard generators of an irreducible
Artin group has a finite orbit if and only if the Artin group is of finite type (i.e., the corresponding Coxeter group is
finite). 相似文献
995.
We study computable trees with distinguished initial subtree (briefly, I-trees). It is proved that all I-trees of infinite height are computably categorical, and moreover, they all have effectively infinite computable dimension. 相似文献
996.
Let A be a Weil algebra. The bijection between all natural operators lifting vector fields from m-manifolds to the bundle functor K
A
of Weil contact elements and the subalgebra of fixed elements SA of the Weil algebra A is determined and the bijection between all natural affinors on K
A
and SA is deduced. Furthermore, the rigidity of the functor K
A
is proved. Requisite results about the structure of SA are obtained by a purely algebraic approach, namely the existence of nontrivial SA is discussed. 相似文献
997.
In this paper, we present a new algorithm to estimate a regression function in a fixed design regression model, by piecewise
(standard and trigonometric) polynomials computed with an automatic choice of the knots of the subdivision and of the degrees
of the polynomials on each sub-interval. First we give the theoretical background underlying the method: the theoretical performances
of our penalized least-squares estimator are based on non-asymptotic evaluations of a mean-square type risk. Then we explain
how the algorithm is built and possibly accelerated (to face the case when the number of observations is great), how the penalty
term is chosen and why it contains some constants requiring an empirical calibration. Lastly, a comparison with some well-known
or recent wavelet methods is made: this brings out that our algorithm behaves in a very competitive way in term of denoising
and of compression. 相似文献
998.
Using the full-potential linearized augmented plane wave (FP-LAPW) method, we have studied the effect of chemistry on the average intercalation voltage (AIV) caused by the Na ions intercalating into transition metal oxides. The effect of transition metal was systematically studied by varying M=Co, Ni and Mn in NaMO2 and fixing the α-NaFeO2 layered structure. The effect of the guest atoms into the host material is discussed in terms of the structural and electronic properties. Comparatively to Li intercalation, a significant electron transfer towards transition metal was found. This observation suggests that the transition metal contribute to the AIV determination and confirms the common assumption that intercalated electron reduces M4+ to M3+. 相似文献
999.
Nonlinear processes caused by the propagation of low-frequency and high-frequency acoustic pulses in an unbounded medium and the propagation of continuous waves in a ring resonator are theoretically studied on the basis of two hysteretic equations of state for media with imperfect elasticity. The profiles and parameters of pulses, the resonance curve and the Q factor of the resonator, and the ratio of the nonlinear resonance frequency shift to the nonlinear damping decrement are determined. For nonlinear wave processes in such media, the distinctive features that allow one to choose an appropriate hysteretic equation of state for analytically describing the experimental data are revealed. 相似文献
1000.
T. Drglin 《Accreditation and quality assurance》2003,8(3-4):130-133
A validation procedure based on the ISO/IEC 17025 standard was used to demonstrate the long-term stability of a calibration
process and to assess the measurement uncertainty of a standard test method for optical emission vacuum spectrometric analysis
of carbon and low-alloy steel (ASTM E 415–99a). The validation was used to provide documented evidence that the selected method
fulfils the requirements and that the method is ”fit for purpose”. A test for drift was applied to determine statistically
whether the analytical results vary systematically with time. The accuracy and traceability of the optimised method were tested
by an analysis of closely matched matrix certified reference materials (CRMs). The measurement uncertainty estimations took
account of the precision study, the bias and its uncertainty, and the qualification of uncertainties not considered in the
overall performance studies.
Received: 2 November 2002 Accepted: 2 January 2003
Acknowledgement The author expresses gratitude to Dr. Aleš Fajgelj for helpful discussions during the 3rd Central European Conference on Reference Materials and Measurements.
Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials
in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia
Correspondence to T. Drglin 相似文献