全文获取类型
收费全文 | 195340篇 |
免费 | 1922篇 |
国内免费 | 616篇 |
专业分类
化学 | 108111篇 |
晶体学 | 3295篇 |
力学 | 7822篇 |
综合类 | 4篇 |
数学 | 18361篇 |
物理学 | 60285篇 |
出版年
2016年 | 2269篇 |
2015年 | 1683篇 |
2014年 | 2399篇 |
2013年 | 7835篇 |
2012年 | 5264篇 |
2011年 | 6626篇 |
2010年 | 4440篇 |
2009年 | 4370篇 |
2008年 | 6008篇 |
2007年 | 6176篇 |
2006年 | 5937篇 |
2005年 | 5436篇 |
2004年 | 4971篇 |
2003年 | 4431篇 |
2002年 | 4312篇 |
2001年 | 5787篇 |
2000年 | 4400篇 |
1999年 | 3520篇 |
1998年 | 2745篇 |
1997年 | 2781篇 |
1996年 | 2737篇 |
1995年 | 2521篇 |
1994年 | 2397篇 |
1993年 | 2290篇 |
1992年 | 2789篇 |
1991年 | 2672篇 |
1990年 | 2620篇 |
1989年 | 2641篇 |
1988年 | 2586篇 |
1987年 | 2589篇 |
1986年 | 2409篇 |
1985年 | 3271篇 |
1984年 | 3291篇 |
1983年 | 2717篇 |
1982年 | 2915篇 |
1981年 | 2851篇 |
1980年 | 2757篇 |
1979年 | 2900篇 |
1978年 | 3145篇 |
1977年 | 2963篇 |
1976年 | 2897篇 |
1975年 | 2760篇 |
1974年 | 2712篇 |
1973年 | 2730篇 |
1972年 | 1765篇 |
1971年 | 1468篇 |
1968年 | 1932篇 |
1967年 | 2129篇 |
1966年 | 1922篇 |
1965年 | 1500篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
371.
Ronald H. Nickel Igor Mikolic-Torreira Jon W. Tolle 《Computational Optimization and Applications》2006,35(1):109-126
Deployed US Navy aircraft carriers must stock a large number of spare parts to support the various types of aircraft embarked
on the ship. The sparing policy determines the spares that will be stocked on the ship to keep the embarked aircraft ready
to fly. Given a fleet of ten or more aircraft carriers and a cost of approximately 50 million dollars per carrier plus the
cost of spares maintained in warehouses in the United States, the sparing problem constitutes a significant portion of the
Navy’s resources. The objective of this work is to find a minimum-cost sparing policy that meets the readiness requirements
of the embarked aircraft. This is a very large, nonlinear, integer optimization problem. The cost function is piecewise linear
and convex while the constraint mapping is highly nonlinear. The distinguishing characteristics of this problem from an optimization
viewpoint are that a large number of decision variables are required to be integer and that the nonlinear constraint functions
are essentially “black box” functions; that is, they are very difficult (and expensive) to evaluate and their derivatives
are not available. Moreover, they are not convex. Integer programming problems with a large number of variables are difficult
to solve in general and most successful approaches to solving nonlinear integer problems have involved linear approximation
and relaxation techniques that, because of the complexity of the constraint functions, are inappropriate for attacking this
problem. We instead employ a pattern search method to each iteration of an interior point-type algorithm to solve the relaxed
version of the problem. From the solution found by the pattern search on each interior point iteration, we begin another pattern
search on the integer lattice to find a good integer solution. The best integer solution found across all interations is returned
as the optimal solution. The pattern searches are distributed across a local area network of non-dedicated, heterogeneous
computers in an office environment, thus, drastically reducing the time required to find the solution. 相似文献
372.
G. Moortgat-Pick H. Fraas 《The European Physical Journal C - Particles and Fields》2002,25(2):189-197
The consequences of CP and CPT invariance for production and subsequent decay of Dirac and Majorana fermions in polarized fermion-antifermion annihilation
are analytically studied. We derive general symmetry relations for the production spin density matrix and for the three-particle
decay matrices and obtain constraints for the polarization and the spin-spin correlations of Dirac and Majorana fermions.
We prove that only for Majorana fermions the energy and opening angle distribution factorizes exactly into contributions from
production and decay if CP is conserved.
Received: 6 November 2001 / Revised version: 23 April 2002 / Published online: 12 July 2002 相似文献
373.
374.
Differential algebraic equations with after-effect 总被引:4,自引:0,他引:4
C. T. H. Baker C. A. H. Paul H. Tian 《Journal of Computational and Applied Mathematics》2002,140(1-2):63-80
In this paper, we are concerned with the solution of delay differential algebraic equations. These are differential algebraic equations with after-effect, or constrained delay differential equations. The general semi-explicit form of the problem consists of a set of delay differential equations combined with a set of constraints that may involve retarded arguments. Even simply stated problems of this type can give rise to difficult analytical and numerical problems. The more tractable examples can be shown to be equivalent to systems of delay or neutral delay differential equations. Our purpose is to highlight some of the complexities and obstacles that can arise when solving these problems, and to indicate problems that require further research. 相似文献
375.
The first-order harmonic balance method via first Fourier coefficient is used to construct an approximate frequency-amplitude relation for a Duffing-harmonic oscillator. This relation is in agreement with the result obtained by the Ritz procedure. 相似文献
376.
Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr2O3 using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr2O3). A second phase, namely θ-(Al,Cr)2O3, was revealed by XRD in the sample containing 9.60 wt% Cr2O3. The structure of mullites was refined by the Rietveld method, location of Cr3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr2O3 content less than ∼5 wt%) Cr3+ ions were substituted for Al3+ in the AlO6 octahedra of the mullite structure (M1 site). For higher doping level, Cr3+ ions were additionally substituted for Al3+ in the AlO6 octahedra of the second phase [θ-(Al,Cr)2O3 at 1400 °C, or α-(Al,Cr)2O3 at 1600 °C] which segregated in the system. Substitution of Cr3+ for Al3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O6 octahedron and decrease of average distances in T*O4 tetrahedron, while average distances in TO4 tetrahedron stayed almost constant. 相似文献
377.
378.
High-average-power and high-energy lasers with good beam quality are fundamental tools for pumping high-peak and high-average-power ultrafast Ti:sapphire laser systems. Laser-diode (LD) pumped solid slate lasers with phase conjugation offer a unique combination of efficiency, reliability, and compactness. We present the design details and performance characteristics of a 360-W 1-kHz LD pumped Nd:YAG phase-conjugated laser. We also discuss the basic design aspects and present our preliminary experimental investigations on ultrashort-pulse laser systems. 相似文献
379.
A file ofn records can be sorted in linear time givenO(log(n)) processors. Four such algorithms are presented and analyzed. All of them have reasonable hardware requirements; no memory access conflicts are generated; a constant number of communication lines per processor are needed (except for one case); and the space requirements areO(n) orO(n log(log(n))). 相似文献
380.