全文获取类型
收费全文 | 353085篇 |
免费 | 2474篇 |
国内免费 | 987篇 |
专业分类
化学 | 158504篇 |
晶体学 | 5223篇 |
力学 | 19560篇 |
综合类 | 4篇 |
数学 | 68677篇 |
物理学 | 104578篇 |
出版年
2018年 | 18360篇 |
2017年 | 18998篇 |
2016年 | 11026篇 |
2015年 | 3248篇 |
2014年 | 2770篇 |
2013年 | 8213篇 |
2012年 | 12244篇 |
2011年 | 26791篇 |
2010年 | 15959篇 |
2009年 | 16316篇 |
2008年 | 21573篇 |
2007年 | 26727篇 |
2006年 | 6282篇 |
2005年 | 13611篇 |
2004年 | 9784篇 |
2003年 | 9431篇 |
2002年 | 6515篇 |
2001年 | 6141篇 |
2000年 | 4928篇 |
1999年 | 3682篇 |
1998年 | 2955篇 |
1997年 | 2920篇 |
1996年 | 3023篇 |
1995年 | 2704篇 |
1994年 | 2533篇 |
1993年 | 2439篇 |
1992年 | 2883篇 |
1991年 | 2780篇 |
1990年 | 2721篇 |
1989年 | 2759篇 |
1988年 | 2714篇 |
1987年 | 2710篇 |
1986年 | 2522篇 |
1985年 | 3387篇 |
1984年 | 3411篇 |
1983年 | 2841篇 |
1982年 | 3032篇 |
1981年 | 2961篇 |
1980年 | 2876篇 |
1979年 | 3028篇 |
1978年 | 3277篇 |
1977年 | 3009篇 |
1976年 | 2965篇 |
1975年 | 2811篇 |
1974年 | 2754篇 |
1973年 | 2806篇 |
1972年 | 1807篇 |
1968年 | 2035篇 |
1967年 | 2173篇 |
1966年 | 1980篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
221.
We initiate a program to study certain recent problems in non-compact coset CFT by the BRST approach. We derive a reduction formula for the BRST cohomology by making use of a twisting by highest weight modules. As illustrations, we apply the formula to the bosonic string model and a rank one non-compact coset model [DPL]. Our formula provides a completely new approach to non-compact coset construction.Partially supported by NSF Grant DMS-8703581 相似文献
222.
M. B. Wright 《The Journal of the Operational Research Society》1989,40(2):187-192
This paper addresses a problem common to all railway networks. Given a fixed train timetable and locomotives (or other forms of traction) of various types, each train must be allocated a locomotive. This paper examines the use of stochastic algorithms for such a problem. Two types of algorithm are used—a simple ‘local improvement’ method, performed successively from randomly chosen starting points, and a ‘simulated annealing’ approach. Both are found to give considerably better results than a deterministic method in current use, and the annealing approach is probably the better stochastic method. 相似文献
223.
An applied cell mapping method for optimal control problems 总被引:1,自引:0,他引:1
W. H. Zhu 《Journal of Optimization Theory and Applications》1989,60(3):485-500
From the application point of view, a series of modifications are proposed for the cell mapping method discussed in Ref. 1 for the optimal control analysis of dynamical systems. The cell order around the target set is rearranged. A set of common discriminate principles is used for the selection of the optimal one among competing control strategies of the same cost. Inequality constraints of the system are taken into account. The number of elements in the set of allowable time intervals is not prescribed, but left open. These modifications seem to make the cell mapping method more efficient for analyzing feedback systems and for obtaining their global optimal control information. The algorithms presented in this paper could broaden the application of the cell mapping approach of Ref. 1 to a wider class of engineering problems. 相似文献
224.
In this paper we suggest a unique model for estimating the operating cost of each of three waste-collection systems. Under the traditional system, which is widely used, waste is typically collected in plastic bags and a three-man crew is needed on each vehicle. The other two systems require a one-man crew for vehicle collecting street containers. The side-loader system with fixed body automatically empties street containers into the vehicle body and empties the load at the disposal site. The side-loader system with demountable body allows the separation of the waste collection phase from transport to the disposal site, since the vehicle body can be demounted. We also present two case studies and show how the estimation of operating costs is a critical issue in decisions regarding the type of system to be used for waste collection. 相似文献
225.
Using Brown's construction (J. Algebra 15 (1970) 103) of an exact 6-term sequence for a fibration of groupoids we show how an exact 9-term sequence can be associated to a fibration of bigroupoids. Applications to topology and algebra are given. 相似文献
226.
The behaviour of correlation functions of d=1$ interacting fermionic systems is determined by a small number of critical indices. We prove that one of them is exactly
zero. As a consequence, the behavior of the Fourier transform of the density-density correlation at zero momentum is qualitatively
unaffected by the interaction, contrary to what happens at , if is the Fermi momentum. The result is obtained by implementing Ward identities in a Renormalization Group approach.
Received: 12 November 2001 / Accepted: 25 February 2002?Published online: 2 October 2002 相似文献
227.
The Boltzmann distribution used in the steady-state analysis of the simulated annealing algorithm gives rise to several scale invariant properties. Scale invariance is first presented in the context of parallel independent processors and then extended to an abstract form based on lumping states together to form new aggregate states. These lumped or aggregate states possess all of the mathematical characteristics, forms and relationships of states (solutions) in the original problem in both first and second moments. These scale invariance properties therefore permit new ways of relating objective function values, conditional expectation values, stationary probabilities, rates of change of stationary probabilities and conditional variances. Such properties therefore provide potential applications in analysis, statistical inference and optimization. Directions for future research that take advantage of scale invariance are also discussed. 相似文献
228.
A nonlinear, compressible, non-isothermal gravity wave model that involves photochemistry is used to study the effects of gravity wave on atmospheric chemical species distributions in this paper. The changes in the distributions of oxygen compound and hydrogen compound density induced by gravity wave propagation are simulated. The results indicate that when a gravity wave propagates through a mesopause region, even if it does not break, it can influence the background distributions of chemical species. The effect of gravity wave on chemical species at night is larger than in daytime. 相似文献
229.
The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu. 相似文献
230.
Electrical impedance measurements of Na3H(SO4)2 were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO4 tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na+ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ωs. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition. 相似文献