全文获取类型
收费全文 | 20447篇 |
免费 | 648篇 |
国内免费 | 717篇 |
专业分类
化学 | 4088篇 |
晶体学 | 858篇 |
力学 | 473篇 |
综合类 | 70篇 |
数学 | 13704篇 |
物理学 | 2619篇 |
出版年
2023年 | 76篇 |
2022年 | 166篇 |
2021年 | 163篇 |
2020年 | 165篇 |
2019年 | 463篇 |
2018年 | 516篇 |
2017年 | 359篇 |
2016年 | 353篇 |
2015年 | 351篇 |
2014年 | 658篇 |
2013年 | 1115篇 |
2012年 | 655篇 |
2011年 | 1312篇 |
2010年 | 1353篇 |
2009年 | 1550篇 |
2008年 | 1535篇 |
2007年 | 1572篇 |
2006年 | 1255篇 |
2005年 | 975篇 |
2004年 | 894篇 |
2003年 | 710篇 |
2002年 | 592篇 |
2001年 | 465篇 |
2000年 | 404篇 |
1999年 | 396篇 |
1998年 | 346篇 |
1997年 | 301篇 |
1996年 | 408篇 |
1995年 | 357篇 |
1994年 | 356篇 |
1993年 | 329篇 |
1992年 | 254篇 |
1991年 | 151篇 |
1990年 | 135篇 |
1989年 | 131篇 |
1988年 | 84篇 |
1987年 | 95篇 |
1986年 | 102篇 |
1985年 | 138篇 |
1984年 | 92篇 |
1983年 | 63篇 |
1982年 | 81篇 |
1981年 | 71篇 |
1980年 | 52篇 |
1979年 | 28篇 |
1978年 | 41篇 |
1977年 | 50篇 |
1976年 | 42篇 |
1974年 | 18篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
171.
A survey of the contributions of Aldo Cossu in finite geometry is given.
Dedicated to the memory of Professor Aldo Cossu 相似文献
172.
Martin Skutella 《Mathematical Programming》2002,91(3):493-514
In the single source unsplittable min-cost flow problem, commodities must be routed simultaneously from a common source vertex
to certain destination vertices in a given graph with edge capacities and costs; the demand of each commodity must be routed
along a single path so that the total flow through any edge is at most its capacity. Moreover, the total cost must not exceed
a given budget. This problem has been introduced by Kleinberg [7] and generalizes several NP-complete problems from various
areas in combinatorial optimization such as packing, partitioning, scheduling, load balancing, and virtual-circuit routing.
Kolliopoulos and Stein [9] and Dinitz, Garg, and Goemans [4] developed algorithms improving the first approximation results
of Kleinberg for the problem of minimizing the violation of edge capacities and for other variants. However, known techniques
do not seem to be capable of providing solutions without also violating the cost constraint. We give the first approximation
results with hard cost constraints. Moreover, all our results dominate the best known bicriteria approximations. Finally,
we provide results on the hardness of approximation for several variants of the problem.
Received: August 23, 2000 / Accepted: April 20, 2001?Published online October 2, 2001 相似文献
173.
J. -P. Kauthen 《Numerische Mathematik》1989,56(5):409-424
Summary Integral equations of mixed Volterra-Fredholm type arise in various physical and biological problems. In the present paper we study continuous time collocation, time discretization and their global and discrete convergence properties. 相似文献
174.
We give general conditions on a generator of a C0-semigroup (resp. of a C0-resolvent) on Lp(E,μ), p ≥ 1, where E is an arbitrary (Lusin) topological space and μ a σ-finite measure on its Borel σ-algebra, so that it generates a sufficiently
regular Markov process on E. We present a general method how these conditions can be checked in many situations. Applications to solve stochastic differential
equations on Hilbert space in the sense of a martingale problem are given.
Dedicated to Giuseppe Da Prato on the occasion of his 70th birthday 相似文献
175.
CuX(X=Al, Ga, In)分子的势能函数与稳定性的密度泛函研究 总被引:5,自引:3,他引:2
刘风丽 《原子与分子物理学报》2006,23(5):838-842
根据原子分子反应静力学原理导出了CuX(X=Al,Ga,In) 分子基态电子状态及其离解极限,并在B3LYP/LANL2DZ水平上计算了平衡几何、振动频率和解离能.利用Murrell-Sorbie 函数拟合出了解析势能函数,并计算出光谱参数和力常数.计算结果表明该分子体系是稳定存在的,其稳定性排序为 CuAl>CuGa>CuIn. 相似文献
176.
Z.X. Cheng S.J. ZhangF. Song H.C. GuoJ.R. Han H.C. Chen 《Journal of Physics and Chemistry of Solids》2002,63(11):2011-2017
Erbium and ytterbium codoped double tungstates NaY(WO4)2 crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er3+ ions in NaY(WO4)2 crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb3+ to Er3+ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels. 相似文献
177.
Shigeru Yamagami 《Journal of Pure and Applied Algebra》2002,167(1):83-128
A duality for orbifolds is presented as an application of group extensions in tensor categories. 相似文献
178.
In this paper, we establish some new nonlinear integral inequalities of the Gronwall–Bellman–Ou-Iang-type in two variables. These on the one hand generalizes and on the other hand furnish a handy tool for the study of qualitative as well as quantitative properties of solutions of differential equations. We illustrate this by applying our new results to certain boundary value problem. 相似文献
179.
We prove that a for a mapping f of finite distortion , the -Hausdorff measure of any point preimage is zero provided is integrable, with , and the multiplicity function of f is essentially bounded. As a consequence for we obtain that the mapping is then open and discrete.
Received: 18 June 2001 / Revised version: 31 January 2002 / Published online: 27 June 2002 相似文献
180.
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