全文获取类型
收费全文 | 21232篇 |
免费 | 1098篇 |
国内免费 | 1469篇 |
专业分类
化学 | 16681篇 |
晶体学 | 169篇 |
力学 | 443篇 |
综合类 | 10篇 |
数学 | 2768篇 |
物理学 | 3728篇 |
出版年
2023年 | 145篇 |
2022年 | 402篇 |
2021年 | 501篇 |
2020年 | 382篇 |
2019年 | 421篇 |
2018年 | 332篇 |
2017年 | 322篇 |
2016年 | 744篇 |
2015年 | 623篇 |
2014年 | 718篇 |
2013年 | 1467篇 |
2012年 | 1649篇 |
2011年 | 1819篇 |
2010年 | 994篇 |
2009年 | 1188篇 |
2008年 | 1352篇 |
2007年 | 1315篇 |
2006年 | 1160篇 |
2005年 | 1149篇 |
2004年 | 1385篇 |
2003年 | 1018篇 |
2002年 | 911篇 |
2001年 | 410篇 |
2000年 | 340篇 |
1999年 | 268篇 |
1998年 | 248篇 |
1997年 | 229篇 |
1996年 | 249篇 |
1995年 | 192篇 |
1994年 | 145篇 |
1993年 | 163篇 |
1992年 | 139篇 |
1991年 | 104篇 |
1990年 | 105篇 |
1989年 | 79篇 |
1988年 | 79篇 |
1987年 | 69篇 |
1986年 | 73篇 |
1985年 | 112篇 |
1984年 | 118篇 |
1983年 | 78篇 |
1982年 | 72篇 |
1981年 | 72篇 |
1980年 | 70篇 |
1979年 | 42篇 |
1978年 | 44篇 |
1977年 | 47篇 |
1976年 | 39篇 |
1975年 | 32篇 |
1974年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
We continue the studies on the so–called genuine Bernstein–Durrmeyer operators U
n
by establishing a recurrence formula for the moments and by investigating the semigroup T(t) approximated by U
n
. Moreover, for sufficiently smooth functions the degree of this convergence is estimated. We also determine the eigenstructure
of U
n
, compute the moments of T(t) and establish asymptotic formulas.
Received: January 26, 2007. 相似文献
82.
Louis Block James Keesling Shihai Li Kevin Peterson 《Journal of statistical physics》1989,55(5-6):929-939
A new algorithm is presented for computing the topological entropy of a unimodal map of the interval. The accuracy of the algorithm is discussed and some graphs of the topological entropy which are obtained using the algorithm are displayed. 相似文献
83.
M.A. Domínguez-Crespo L. Díaz-García A.M. Torres-Huerta 《Applied Surface Science》2006,253(3):1205-1214
Four NiMo catalyst supported on Al2O3 with different textural properties have been studied in the hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodearomatization (HDA) of a Mexican straight run gasoil (SRGO). All reactions were carried out at three different temperatures 613, 633, and 653 K. Alumina supports were analysed by pyridine FTIR-TPD and nitrogen physisorption in order to determine their surface acidity and textural properties, respectively. TPR studies of the NiMo catalysts were analysed to correlate their hydrogenating properties. Metallic particles were characterized (after sulfidation) using transmission electron microscopy (TEM). Catalytic activities are discussed in relation to the physicochemical properties of NiMo catalysts. The importance of textural properties on coke deposition has been emphasized. The results of catalytic activity of these materials varied depending on dispersed MoS particles and pore distribution in final catalysts. The optimum pore diameter was found around 80 Å for HDS and HDN. 相似文献
84.
Kevin Hisler 《Tetrahedron letters》2006,47(35):6293-6295
Aldehydes are prepared in excellent yield by Wittig reactions of phosphoranes on the Weinreb amide of formic acid followed by in situ hydrolysis. 相似文献
85.
F. García 《Topology and its Applications》2006,153(15):2886-2892
In this paper we characterize the class of uniform Eberlein compact spaces through a network condition and also in terms of covering properties for the square of the space. 相似文献
86.
Parking Capacity and Pricing in Park'n Ride Trips: A Continuous Equilibrium Network Design Problem 总被引:2,自引:0,他引:2
In this paper we consider the problem of designing parking facilities for park'n ride trips. We present a new continuous equilibrium network design problem to decide the capacity and fare of these parking lots at a tactical level. We assume that the parking facilities have already been located and other topological decisions have already been taken.The modeling approach proposed is mathematical programming with equilibrium constraints. In the outer optimization problem, a central Authority evaluates the performance of the transport network for each network design decision. In the inner problem a multimodal traffic assignment with combined modes, formulated as a variational inequality problem, generates the share demand for modes of transportation, and for parking facilities as a function of the design variables of the parking lots. The objective is to make optimal parking investment and pricing decisions in order to minimize the total travel cost in a subnetwork of the multimodal transportation system.We present a new development in model formulation based on the use of generalized parking link cost as a design variable.The bilevel model is solved by a simulated annealing algorithm applied to the continuous and non-negative design decision variables. Numerical tests are reported in order to illustrate the use of the model, and the ability of the approach to solve applications of moderate size. 相似文献
87.
88.
The aim of this article is to derive stable generalized sampling in a shift-invariant space by using some special dual frames in L2(0,1). These sampling formulas involve samples of filtered versions of the functions in the shift-invariant space. The involved samples are expressed as the frame coefficients of an appropriate function in L2(0,1) with respect to some particular frame in L2(0,1). Since any shift-invariant space with stable generator is the image of L2(0,1) by means of a bounded invertible operator, our generalized sampling is derived from some dual frame expansions in L2(0,1). 相似文献
89.
90.
Gianluigi Luppi Rodrigo J. Corrêa Angelo C. Pinto Quirinus B. Broxterman Claudia Tomasini 《Tetrahedron》2006,62(51):12017-12024
The first total synthesis of (R)-convolutamydine A has been achieved by the organocatalytic addition of acetone to 4,6-dibromoisatin. The absolute configuration was determined by single crystal X-ray diffraction. DFT studies were used to model the transition states for the aldol reaction and equilibrium geometries of the post-aldol reaction intermediates. The DFT study revealed that the aldol bond forming reaction was considerably endothermic. 相似文献