全文获取类型
收费全文 | 31148篇 |
免费 | 207篇 |
国内免费 | 279篇 |
专业分类
化学 | 12153篇 |
晶体学 | 262篇 |
力学 | 1413篇 |
综合类 | 12篇 |
数学 | 9320篇 |
物理学 | 8474篇 |
出版年
2021年 | 62篇 |
2020年 | 74篇 |
2019年 | 77篇 |
2018年 | 1216篇 |
2017年 | 1471篇 |
2016年 | 779篇 |
2015年 | 647篇 |
2014年 | 557篇 |
2013年 | 784篇 |
2012年 | 3307篇 |
2011年 | 2480篇 |
2010年 | 1895篇 |
2009年 | 1628篇 |
2008年 | 668篇 |
2007年 | 728篇 |
2006年 | 738篇 |
2005年 | 4547篇 |
2004年 | 4046篇 |
2003年 | 2396篇 |
2002年 | 560篇 |
2001年 | 334篇 |
2000年 | 116篇 |
1999年 | 204篇 |
1998年 | 147篇 |
1997年 | 100篇 |
1996年 | 63篇 |
1995年 | 70篇 |
1994年 | 77篇 |
1993年 | 38篇 |
1992年 | 193篇 |
1991年 | 173篇 |
1990年 | 145篇 |
1989年 | 125篇 |
1988年 | 106篇 |
1987年 | 64篇 |
1986年 | 45篇 |
1985年 | 43篇 |
1984年 | 35篇 |
1979年 | 53篇 |
1978年 | 36篇 |
1976年 | 87篇 |
1975年 | 41篇 |
1974年 | 46篇 |
1973年 | 59篇 |
1972年 | 41篇 |
1970年 | 38篇 |
1969年 | 40篇 |
1968年 | 35篇 |
1967年 | 32篇 |
1966年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
23.
We present a systematic comparison of the correlation contribution at the level of the second-order polarization propagator approximation (SOPPA ) and MP 2 to the static dipole polarizability of (1) Be, BeH?, BH, CH+, MgH?, AIH, SiH+, and GeH+; (2) BH3, CH4, NH3, H2O, HF, BF, and F2; and (3) N2, CO, CN?, HCN, C2H2, and HCHO . Fairly extended basis sets were used in the calculations. We find that the agreement with experimental values is improved in SOPPA and MP .2 over the results at the SCF level. The signs and magnitudes of the correlation contribution in SOPPA are similar to those obtained in analytical derivative MP 2 calculations. However, it is not possible to say, in general, which method gives the largest correlation contribution or the best agreement with experiment, nor is it possible to make a priori prediction of the sign of the correlation contribution. For the first group of molecules, which have a quasi-degenerate ground state, additional CCDPPA and CCSDPPA calculations were performed and compared with polarizabilities obtained as analytical/numerical derivatives of the CCD and CCSD energies. The CCSDPPA results were found to be in better agreement with other calculations than were the SOPPA results, demonstrating the necessity of using methods based on infinite-order perturbation theory for these systems. © 1994 John Wiley & Sons, Inc. 相似文献
24.
We prove that RANDOM EDGE, the simplex algorithm that always chooses a random improving edge to proceed on, can take a mildly exponential number of steps in the model of abstract objective functions (introduced by Williamson Hoke [Completely unimodal numberings of a simple polytope, Discrete Appl. Math. 20 (1988) 69-81.] and by Kalai [A simple way to tell a simple polytope from its graph, J. Combin. Theory Ser. A 49(2) (1988) 381-383.] under different names). We define an abstract objective function on the n-dimensional cube for which the algorithm, started at a random vertex, needs at least exp(const·n1/3) steps with high probability. The best previous lower bound was quadratic. So in order for RANDOM EDGE to succeed in polynomial time, geometry must help. 相似文献
25.
This paper establishes a link between a generalized matrix Matsumoto-Yor (MY) property and the Wishart distribution. This link highlights certain conditional independence properties within blocks of the Wishart and leads to a new characterization of the Wishart distribution similar to the one recently obtained by Geiger and Heckerman but involving independences for only three pairs of block partitionings of the random matrix.In the process, we obtain two other main results. The first one is an extension of the MY independence property to random matrices of different dimensions. The second result is its converse. It extends previous characterizations of the matrix generalized inverse Gaussian and Wishart seen as a couple of distributions.We present two proofs for the generalized MY property. The first proof relies on a new version of Herz's identity for Bessel functions of matrix arguments. The second proof uses a representation of the MY property through the structure of the Wishart. 相似文献
26.
Daniel Pa?ca 《Journal of Mathematical Analysis and Applications》2007,325(1):90-100
Some existence results are obtained for periodic solutions of nonautonomous second-order differential inclusions systems with p-Laplacian. 相似文献
27.
Goran Pavi? 《Applied Acoustics》2006,67(9):864-881
A method is formulated for the identification of an unknown physical parameter of a fluid-filled pipe using the measurement of sound speed in the pipe. The method uses a simple formula which provides the relationship between the sound speed and a few physical parameters of the pipe: thickness, diameter, wall material constants and fluid constants. Once the sound speed in the pipe is measured, the simple formula can be used to extract one of the unknown parameters providing the remaining ones are known.The sound speed in the pipe is measured using a 3-transducer array. In order to demonstrate the potential of the technique the results of several measurements obtained in a water-filled steel pipe are presented.The required accuracy of the measurement of sound speed and of the specification of known parameters is analysed. The accuracy depends on the unknown parameter which is to be identified. For example, if the pipe thickness is the unknown parameter, the other parameters have to be known within a very narrow margin of error. On the contrary, if the fluid properties have to be identified the needed accuracy of known parameters gets much lower. 相似文献
28.
近年来,声散射的模拟成为室内声场计算机模拟研究的重点。现有的方法一般是依据壁面性质(结构、粗糙度等),利用经验确定散射系数,并基于该系数来模拟室内散射声能的分布。这种方法在低频情况下的精度较差,主要原因是忽略了在低频声场中起重要作用的波动现象。为此,本文提出一种新的计算壁面散射的模型,该模型既可考虑壁面上产生的散射声能,又可计算因壁面边缘衍射而产生的散射声能。文中通过模拟和实测数据的对比,分析验证了该模型的有效性,并给出了表面散射系数的取值规则。 相似文献
29.
A tree T is arbitrarily vertex decomposable if for any sequence τ of positive integers adding up to the order of T there is a sequence of vertex-disjoint subtrees of T whose orders are given by τ. An on-line version of the problem of characterizing arbitrarily vertex decomposable trees is completely solved here. 相似文献
30.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 1016 m−3, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 1021 m−3 for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy. 相似文献