全文获取类型
收费全文 | 8386篇 |
免费 | 632篇 |
国内免费 | 529篇 |
专业分类
化学 | 561篇 |
晶体学 | 10篇 |
力学 | 518篇 |
综合类 | 62篇 |
数学 | 1699篇 |
物理学 | 1113篇 |
综合类 | 5584篇 |
出版年
2024年 | 25篇 |
2023年 | 71篇 |
2022年 | 136篇 |
2021年 | 175篇 |
2020年 | 182篇 |
2019年 | 184篇 |
2018年 | 179篇 |
2017年 | 223篇 |
2016年 | 234篇 |
2015年 | 265篇 |
2014年 | 373篇 |
2013年 | 410篇 |
2012年 | 449篇 |
2011年 | 492篇 |
2010年 | 353篇 |
2009年 | 364篇 |
2008年 | 382篇 |
2007年 | 499篇 |
2006年 | 424篇 |
2005年 | 389篇 |
2004年 | 370篇 |
2003年 | 366篇 |
2002年 | 352篇 |
2001年 | 316篇 |
2000年 | 269篇 |
1999年 | 247篇 |
1998年 | 192篇 |
1997年 | 225篇 |
1996年 | 181篇 |
1995年 | 183篇 |
1994年 | 164篇 |
1993年 | 132篇 |
1992年 | 101篇 |
1991年 | 123篇 |
1990年 | 82篇 |
1989年 | 109篇 |
1988年 | 85篇 |
1987年 | 54篇 |
1986年 | 34篇 |
1985年 | 29篇 |
1984年 | 23篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 6篇 |
排序方式: 共有9547条查询结果,搜索用时 343 毫秒
31.
32.
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
33.
We propose iteration methods for solving the Dirichlet problem in domains with involved geometry. Such problems arise in relation to the problem of optimizing quantum dot and antidot infrared detectors. We estimate the deviation of an approximate solution from the exact solution. 相似文献
34.
This work is concerned with the maximum principle for optimal control problem governed by magnetohydrodynamic equations, which describe the motion of a viscous incompressible conducting fluid in a magnetic field and consist of a subtle coupling of the Navier-Stokes equation of viscous incompressible fluid flow and the Maxwell equation of electromagnetic field. An integral type state constraint is considered. 相似文献
35.
In this work, the NP-hard maximum clique problem on graphs is considered. Starting from basic greedy heuristics, modifications and improvements are proposed and combined in a two-phase heuristic procedure. In the first phase an improved greedy procedure is applied starting from each node of the graph; on the basis of the results of this phase a reduced subset of nodes is selected and an adaptive greedy algorithm is repeatedly started to build cliques around such nodes. In each restart the selection of nodes is biased by the maximal clique generated in the previous execution. Computational results are reported on the DIMACS benchmarks suite. Remarkably, the two-phase procedure successfully solves the difficult Brockington-Culberson instances, and is generally competitive with state-of-the-art much more complex heuristics. 相似文献
36.
Weaving is a matrix construction developed in 1990 for the purpose of obtaining new weighing matrices. Hadamard matrices obtained by weaving have the same orders as those obtained using the Kronecker product, but weaving affords greater control over the internal structure of matrices constructed, leading to many new Hadamard equivalence classes among these known orders. It is known that different classes of Hadamard matrices may have different maximum excess. We explain why those classes with smaller excess may be of interest, apply the method of weaving to explore this question, and obtain constructions for new Hadamard matrices with maximum excess in their respective classes. With this method, we are also able to construct Hadamard matrices of near‐maximal excess with ease, in orders too large for other by‐hand constructions to be of much value. We obtain new lower bounds for the maximum excess among Hadamard matrices in some orders by constructing candidates for the largest excess. For example, we construct a Hadamard matrix with excess 1408 in order 128, larger than all previously known values. We obtain classes of Hadamard matrices of order 96 with maximum excess 912 and 920, which demonstrates that the maximum excess for classes of that order may assume at least three different values. Since the excess of a woven Hadamard matrix is determined by the row sums of the matrices used to weave it, we also investigate the properties of row sums of Hadamard matrices and give lists of them in small orders. © 2004 Wiley Periodicals, Inc. J Combin Designs 12: 233–255, 2004. 相似文献
37.
LetB
1
be a ball of radiusr
1
inS
n
(ℍn), and letB
0
be a smaller ball of radiusr
0
such thatB
0
⊂B
1
. ForS
n
we considerr
1
π. Let u be a solution of the problem- δm = 1 in Ω :=B
1
/B
0
vanishing on the boundary. It is shown that the associated functionalJ (Ω) is minimal if and only if the balls are concentric. It is also shown that the first Dirichlet eigenvalue of the Laplacian
on Ω is maximal if and only if the balls are concentric. 相似文献
38.
杨云英 《云南民族大学学报(自然科学版)》2004,13(2):136-138
针对连铸生产中存在的问题,设计用变频调速系统取代直流调速系统.阐述了系统的构成和工作过程,实现PC对变频器的控制和两台变频器间的连锁控制,并给出了PC控制程序框图.实际运行结果表明:本系统安全可靠、故障率低、性能优于原直流系统. 相似文献
39.
再论从物理角度分析跳远成绩 总被引:1,自引:1,他引:0
陈德平 《曲阜师范大学学报》2004,30(2):91-92
建立2个坐标系,从力学原理分析起跳时的角度、初速度与跳远成绩的关系,同时与郑亚晨的文章的理论分析进行商榷。 相似文献
40.
Guang-hua DONG~ Yan-pei LIU Department of Mathematics School of Science Beijing Jiaotong University Beijing China 《中国科学A辑(英文版)》2007,50(11):1563-1570
In this paper,we provide a new class of up-embeddable graphs,and obtain a tight lower bound on the maximum genus of a class of 2-connected pseudographs of diameter 2 and of a class of diameter 4 multi-graphs.This extends a result of Skoviera. 相似文献