全文获取类型
收费全文 | 202389篇 |
免费 | 1961篇 |
国内免费 | 504篇 |
专业分类
化学 | 110631篇 |
晶体学 | 3647篇 |
力学 | 8000篇 |
综合类 | 11篇 |
数学 | 19204篇 |
物理学 | 63361篇 |
出版年
2020年 | 1730篇 |
2019年 | 1921篇 |
2018年 | 2239篇 |
2017年 | 2358篇 |
2016年 | 3516篇 |
2015年 | 2193篇 |
2014年 | 3551篇 |
2013年 | 9083篇 |
2012年 | 6513篇 |
2011年 | 8014篇 |
2010年 | 5698篇 |
2009年 | 5658篇 |
2008年 | 7161篇 |
2007年 | 7076篇 |
2006年 | 6698篇 |
2005年 | 6057篇 |
2004年 | 5526篇 |
2003年 | 5105篇 |
2002年 | 4872篇 |
2001年 | 6145篇 |
2000年 | 4625篇 |
1999年 | 3521篇 |
1998年 | 2747篇 |
1997年 | 2702篇 |
1996年 | 2593篇 |
1995年 | 2440篇 |
1994年 | 2332篇 |
1993年 | 2166篇 |
1992年 | 2829篇 |
1991年 | 2732篇 |
1990年 | 2674篇 |
1989年 | 2693篇 |
1988年 | 2713篇 |
1987年 | 2721篇 |
1986年 | 2592篇 |
1985年 | 3304篇 |
1984年 | 3316篇 |
1983年 | 2599篇 |
1982年 | 2712篇 |
1981年 | 2764篇 |
1980年 | 2535篇 |
1979年 | 2875篇 |
1978年 | 2862篇 |
1977年 | 2980篇 |
1976年 | 2819篇 |
1975年 | 2555篇 |
1974年 | 2510篇 |
1973年 | 2466篇 |
1972年 | 1704篇 |
1968年 | 1703篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
331.
The last decade has seen a considerable growth in the use of Artificial Intelligence (AI) for operations management with the aim of finding solutions to problems that are increasing in complexity and scale. This paper begins by setting the context for the survey through a historical perspective of OR and AI. An extensive survey of applications of AI techniques for operations management, covering a total of over 1200 papers published from 1995 to 2004 is then presented. The survey utilizes Elsevier's ScienceDirect database as a source. Hence, the survey may not cover all the relevant journals but includes a sufficiently wide range of publications to make it representative of the research in the field. The papers are categorized into four areas of operations management: (a) design, (b) scheduling, (c) process planning and control and (d) quality, maintenance and fault diagnosis. Each of the four areas is categorized in terms of the AI techniques used: genetic algorithms, case-based reasoning, knowledge-based systems, fuzzy logic and hybrid techniques. The trends over the last decade are identified, discussed with respect to expected trends and directions for future work suggested. 相似文献
332.
We exhibit a characteristic structure of the class of all regular graphs of degree d that stems from the spectra of their adjacency matrices. The structure has a fractal threadlike appearance. Points with coordinates given by the mean and variance of the exponentials of graph eigenvalues cluster around a line segment that we call a filar. Zooming-in reveals that this cluster splits into smaller segments (filars) labeled by the number of triangles in graphs. Further zooming-in shows that the smaller filars split into subfilars labeled by the number of quadrangles in graphs, etc. We call this fractal structure, discovered in a numerical experiment, a multifilar structure. We also provide a mathematical explanation of this phenomenon based on the Ihara-Selberg trace formula, and compute the coordinates and slopes of all filars in terms of Bessel functions of the first kind. 相似文献
333.
334.
This paper considers two models of BASIC mode data transmission: When we transmit data with length l
- 1. (1) data are divided into N blocks and each block is transmitted individually
- 2. (2) data are divided into N sub-blocks and all sub-blocks are transmitted together.
335.
A finite volume solver for the 2D depth‐integrated harmonic hyperbolic formulation of the mild‐slope equation for wave propagation is presented and discussed. The solver is implemented on unstructured triangular meshes and the solution methodology is based upon a Godunov‐type second‐order finite volume scheme, whereby the numerical fluxes are computed using Roe's flux function. The eigensystem of the mild‐slope equations is derived and used for the construction of Roe's matrix. A formulation that updates the unknown variables in time implicitly is presented, which produces a more accurate and reliable scheme than hitherto available. Boundary conditions for different types of boundaries are also derived. The agreement of the computed results with analytical results for a range of wave propagation/transformation problems is very good, and the model is found to be virtually paraxiality‐free. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
336.
A new mechanism of pulsed laser oscillation based on ion-ion recombination is proposed. The advantages of utilizing this mechanism in a pulsed mode are considered, and the expected characteristics of the proposed lasers are estimated. 相似文献
337.
338.
Exponential growth is estimated for integral operators with continuous periodic kernels of exponential type. The result is
applied to the boundary-value problem for a general hyperbolic differential equation having periodic coefficients with Goursat
boundary conditions. It is proved that the exponential characteristic for such a boundary-value problem has canonical form.
__________
Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 38, Suzdal
Conference-2004, Part 3, 2006. 相似文献
339.
We construct a three-parameter family of contact metric structures on the unit tangent sphere bundle T
1
M of a Riemannian manifold M and we study some of their special properties related to the Levi-Civita connection. More precisely, we give the necessary
and sufficient conditions for a constructed contact metric structure to be K-contact, Sasakian, to satisfy some variational conditions or to define a strongly pseudo-convex CR-structure. The obtained
results generalize classical theorems on the standard contact metric structure of T
1
M. 相似文献
340.
Anuradha Sharma Gurmeet K. Bakshi Madhu Raka 《Finite Fields and Their Applications》2007,13(4):1071-1085
Polyadic codes constitute a special class of cyclic codes and are generalizations of quadratic residue codes, duadic codes, triadic codes, m-adic residue codes and split group codes, which have good error-correcting properties. In this paper, we give necessary and sufficient conditions for the existence of polyadic codes of prime power length. Examples of some good codes arising from the family of polyadic codes of prime power length are also given. 相似文献