全文获取类型
收费全文 | 6983篇 |
免费 | 450篇 |
国内免费 | 470篇 |
专业分类
化学 | 2126篇 |
晶体学 | 16篇 |
力学 | 847篇 |
综合类 | 98篇 |
数学 | 3163篇 |
物理学 | 1653篇 |
出版年
2023年 | 66篇 |
2022年 | 110篇 |
2021年 | 140篇 |
2020年 | 156篇 |
2019年 | 159篇 |
2018年 | 153篇 |
2017年 | 169篇 |
2016年 | 200篇 |
2015年 | 120篇 |
2014年 | 289篇 |
2013年 | 411篇 |
2012年 | 308篇 |
2011年 | 344篇 |
2010年 | 361篇 |
2009年 | 432篇 |
2008年 | 426篇 |
2007年 | 428篇 |
2006年 | 425篇 |
2005年 | 383篇 |
2004年 | 294篇 |
2003年 | 319篇 |
2002年 | 343篇 |
2001年 | 208篇 |
2000年 | 211篇 |
1999年 | 172篇 |
1998年 | 169篇 |
1997年 | 127篇 |
1996年 | 104篇 |
1995年 | 70篇 |
1994年 | 118篇 |
1993年 | 75篇 |
1992年 | 93篇 |
1991年 | 72篇 |
1990年 | 59篇 |
1989年 | 48篇 |
1988年 | 41篇 |
1987年 | 39篇 |
1986年 | 38篇 |
1985年 | 42篇 |
1984年 | 31篇 |
1983年 | 12篇 |
1982年 | 29篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1974年 | 5篇 |
1971年 | 6篇 |
排序方式: 共有7903条查询结果,搜索用时 15 毫秒
41.
In this paper, the authors deal with bifunctions defined on complete metric spaces and with values in locally convex spaces ordered by closed convex cones. The aim is to provide a vector version of Ekeland’s theorem related to equilibrium problems. To prove this principle, a weak notion of continuity of a vector-valued function is considered, and some of its properties are presented. Via the vector Ekeland’s principle, existence results for vector equilibria are proved in both compact and noncompact domains. 相似文献
42.
43.
Using an infinite-horizon two-player differential game, we derive and compare Bertrand and Cournot equilibria for a differentiated duopoly engaging in the process of R&D competition. The main findings of this study are as follows. First, Bertrand competition is more efficient if either R&D productivity is low or products are very different. Second, Cournot competition is more efficient provided that R&D productivity is high, products are close substitutes, and spillovers are not close to zero. This last result is different from what has been obtained in the literature. Hence, this shows that considering a dynamic model and more general investment costs does have an impact on the efficiency results. 相似文献
44.
Bounds are given on the size of the parameter-space decomposition induced by multiple sequence alignment problems where phylogenetic information may be given or inferred. It is shown that many of the usual formulations of these problems fall within the same integer parametric framework, implying that the number of distinct optima obtained as the parameters are varied across their ranges is polynomially bounded in the length and number of sequences. 相似文献
45.
We show that there is a threshold in energy for the onset of chaos in cosmology for the Universe described as a dynamical system derived from the Einstein equations of General Relativity (GR). In the case of the mixmaster model (homogeneous and anisotropic cosmology with a Bianchi IX metric), the chaos occurs precisely at the prescribed necessary value H
vac=0 of the GR for the energy of the Universe while the system is found to be regular for H<0 and chaotic for H>0 with respect to its pure vacuum part. In the case of generalized scalar tensor theories within the Bianchi IX model, we show using the ADM formalism and a conformal transformation that the energy of the dynamical system as compared to vacuum lies below the zero energy threshold. The system is thus not exhibiting chaos and the conclusion still holds in the presence of ordinary matter as well. The suppression of chaos occurs in a similar way for stiff matter alone. 相似文献
46.
J. L. Hernández–Pastora O. V. Manko V. S. Manko J. Martín E. Ruiz 《General Relativity and Gravitation》2004,36(4):781-797
The extended quadruple–Kerr metric is used to consider equilibrium states of four collinear Kerr particles. We explain our previous failure to solve numerically the full set of the balance equations, and we derive a self–consistent system of the axis conditions leading to the equilibrium of all four constituents which can be black holes or hyperextreme objects. The equilibrium configurations obtained in this paper exhibit similar features with those occurring in the systems of two Kerr particles, for instance, the balance of four Kerr black holes with positive masses does not seem possible. Equilibrium states of two identical compound Kerr objects are also discussed. 相似文献
47.
Optimized prefactored compact schemes 总被引:1,自引:0,他引:1
The numerical simulation of aeroacoustic phenomena requires high-order accurate numerical schemes with low dispersion and dissipation errors. In this paper we describe a strategy for developing high-order accurate prefactored compact schemes, requiring very small stencil support. These schemes require fewer boundary stencils and offer simpler boundary condition implementation than existing compact schemes. The prefactorization strategy splits the central implicit schemes into forward and backward biased operators. Using Fourier analysis, we show it is possible to select the coefficients of the biased operators such that their dispersion characteristics match those of the original central compact scheme and their numerical wavenumbers have equal and opposite imaginary components. This ensures that when the forward and backward stencils are added, the original central compact scheme is recovered. To extend the resolution characteristic of the schemes, an optimization strategy is employed in which formal order of accuracy is sacrificed in preference to enhanced resolution characteristics across the range of wavenumbers realizable on a given mesh. The resulting optimized schemes yield improved dispersion characteristics compared to the standard sixth- and eighth-order compact schemes making them more suitable for high-resolution numerical simulations in gas dynamics and computational aeroacoustics. The efficiency, accuracy and convergence characteristics of the new optimized prefactored compact schemes are demonstrated by their application to several test problems. 相似文献
48.
49.
燃煤烟气中AsSePb的形态分布及SCl元素对其形态分布的影响 《燃料化学学报》2003,48(11):1298-1309
基于化学热力学平衡分析方法,计算分析了燃煤烟气中重金属As、Se、Pb的形态分布规律,研究了S、Cl等元素对As、Se、Pb的形态分布规律的影响。结果表明,氧化性气氛下,As以As2O5、As4O6、AsO等氧化物的形式存在;Se主要以SeO2形式存在;Pb在1000 K以下主要是固态PbSO4,1200 K以上为气态PbO。还原性气氛下,As在较低温度时为固态As2S2,900-1400 K以As2、AsS、AsN气体共存,2000 K以上全部转化为气态AsO。Se在1100 K以下主要以气态H2Se存在,1100 K开始生成SeS和Se2气体,1800 K时主要是气态Se和少量气态SeO;Pb在中低温时主要是PbS,1800 K以上气态Pb为主要存在形态。S在还原性气氛下增大了AsS(g)、PbS(g)、SeS(g)的比例,氧化性气氛下对As、Se、Pb形态分布基本无影响;Cl无论在氧化还是还原气氛下对As、Se影响均较小,但对Pb的形态分布影响较大。 相似文献
50.
Packing two disks into a polygonal environment 总被引:1,自引:0,他引:1
We consider the following problem. Given a polygon P, possibly with holes, and having n vertices, compute a pair of equal radius disks that do not intersect each other, are contained in P, and whose radius is maximized. Our main result is a simple randomized algorithm whose expected running time, on any input, is O(nlogn). This is optimal in the algebraic decision tree model of computation. 相似文献