全文获取类型
收费全文 | 97599篇 |
免费 | 2368篇 |
国内免费 | 1428篇 |
专业分类
电工技术 | 1718篇 |
技术理论 | 5篇 |
综合类 | 3358篇 |
化学工业 | 13544篇 |
金属工艺 | 5989篇 |
机械仪表 | 3879篇 |
建筑科学 | 3366篇 |
矿业工程 | 1026篇 |
能源动力 | 1475篇 |
轻工业 | 4499篇 |
水利工程 | 1615篇 |
石油天然气 | 1204篇 |
武器工业 | 155篇 |
无线电 | 10646篇 |
一般工业技术 | 17923篇 |
冶金工业 | 3359篇 |
原子能技术 | 451篇 |
自动化技术 | 27183篇 |
出版年
2024年 | 87篇 |
2023年 | 269篇 |
2022年 | 514篇 |
2021年 | 721篇 |
2020年 | 566篇 |
2019年 | 400篇 |
2018年 | 14837篇 |
2017年 | 13818篇 |
2016年 | 10366篇 |
2015年 | 1161篇 |
2014年 | 1005篇 |
2013年 | 1005篇 |
2012年 | 4036篇 |
2011年 | 10385篇 |
2010年 | 9062篇 |
2009年 | 6355篇 |
2008年 | 7584篇 |
2007年 | 8573篇 |
2006年 | 879篇 |
2005年 | 1868篇 |
2004年 | 1643篇 |
2003年 | 1682篇 |
2002年 | 1138篇 |
2001年 | 580篇 |
2000年 | 557篇 |
1999年 | 392篇 |
1998年 | 272篇 |
1997年 | 202篇 |
1996年 | 195篇 |
1995年 | 155篇 |
1994年 | 97篇 |
1993年 | 71篇 |
1992年 | 79篇 |
1991年 | 68篇 |
1990年 | 38篇 |
1989年 | 40篇 |
1988年 | 35篇 |
1969年 | 24篇 |
1968年 | 43篇 |
1967年 | 33篇 |
1966年 | 42篇 |
1965年 | 45篇 |
1963年 | 28篇 |
1960年 | 30篇 |
1959年 | 39篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Daniel Meister 《Theory of Computing Systems》2007,41(2):257-289
A finite recurrent system over sets of natural numbers of dimension n is a pair composed of n n-ary functions over sets of
natural numbers and an n-tuple of singleton sets of natural numbers. Every function is applied to the entries of the tuple
and computes a set of natural numbers, that may also be empty. The results are composed into another tuple, and the process
is started anew. Thus, a finite recurrent system defines an infinite sequence of n-tuples containing sets of natural numbers.
The last entry of a generated n-tuple is called the output of a step, and the union of the output sets of all steps is the
set defined by the finite recurrent system. Membership problems ask whether a given number is in a specified output set or
in some output set. We study membership problems for special finite recurrent systems, whose functions are built from the
set operations union, intersection and complementation and the arithmetical operations addition and multiplication. Sum and
product of two sets of natural numbers are defined elementwise. We restrict the set of operations from which functions are
built and determine the impact on the complexity of the membership problems. We focus on PSPACE-decidable membership problems
and show completeness results for the complexity classes NL, NP and PSPACE. 相似文献
42.
The multi-homogeneous Bezout number is a bound for the number of solutions of a system of multi-homogeneous polynomial equations,
in a suitable product of projective spaces. Given an arbitrary, not necessarily multi-homogeneous, system, one can ask for
the optimal multi-homogenization that would minimize the Bezout number. In this paper it is proved that the problem of computing,
or even estimating, the optimal multi-homogeneous Bezout number is actually NP-hard. In terms of approximation theory for
combinatorial optimization, the problem of computing the best multi-homogeneous structure does not belong to APX, unless P
= NP. Moreover, polynomial-time algorithms for estimating the minimal multi-homogeneous Bezout number up to a fixed factor
cannot exist even in a randomized setting, unless BPP ⫆ NP. 相似文献
43.
Compared to other fields of engineering, in mechanical engineering, the Discrete Element Method (DEM) is not yet a well known
method. Nevertheless, there is a variety of simulation problems where the method has obvious advantages due to its meshless
nature. For problems where several free bodies can collide and break after having been largely deformed, the DEM is the method
of choice. Neighborhood search and collision detection between bodies as well as the separation of large solids into smaller
particles are naturally incorporated in the method. The main DEM algorithm consists of a relatively simple loop that basically
contains the three substeps contact detection, force computation and integration. However, there exists a large variety of
different algorithms to choose the substeps to compose the optimal method for a given problem. In this contribution, we describe
the dynamics of particle systems together with appropriate numerical integration schemes and give an overview over different
types of particle interactions that can be composed to adapt the method to fit to a given simulation problem. Surface triangulations
are used to model complicated, non-convex bodies in contact with particle systems. The capabilities of the method are finally
demonstrated by means of application examples.
Commemorative Contribution. 相似文献
44.
45.
46.
47.
In this paper, we argue that successful integration of knowledge across work domains in the short-term can mask the generation of long-term consequences. We explore a setting, the introduction of environmental considerations into semiconductor manufacturing, where the eventual adoption of common measurement artifacts and associated practices enabled knowledge integration, but failed to address significant underlying consequences. Drawing from observational, interview, and archival data we develop an understanding of the work practices of the Tech and EnviroTech groups as structured by the material world and broader collective conventions. We introduce the concept of knowledge regime to outline the differences in knowledge across these work domains. More specifically, we find that differences in the causal specificity and developmental time horizon of knowledge and the measurement artifacts that result contribute to the relative power of one knowledge regime over another. Understanding these sources of incompatibility provides insight into the design requirements of information systems as boundary objects for knowledge integration, but also specifies the potential limits to any design effort. 相似文献
48.
49.
50.