全文获取类型
收费全文 | 29566篇 |
免费 | 2487篇 |
国内免费 | 1734篇 |
专业分类
化学 | 2676篇 |
晶体学 | 56篇 |
力学 | 3592篇 |
综合类 | 299篇 |
数学 | 2169篇 |
物理学 | 3702篇 |
综合类 | 21293篇 |
出版年
2024年 | 153篇 |
2023年 | 403篇 |
2022年 | 682篇 |
2021年 | 765篇 |
2020年 | 838篇 |
2019年 | 671篇 |
2018年 | 677篇 |
2017年 | 910篇 |
2016年 | 983篇 |
2015年 | 1081篇 |
2014年 | 1495篇 |
2013年 | 1581篇 |
2012年 | 1775篇 |
2011年 | 1803篇 |
2010年 | 1308篇 |
2009年 | 1462篇 |
2008年 | 1488篇 |
2007年 | 1743篇 |
2006年 | 1601篇 |
2005年 | 1515篇 |
2004年 | 1391篇 |
2003年 | 1277篇 |
2002年 | 1029篇 |
2001年 | 908篇 |
2000年 | 846篇 |
1999年 | 764篇 |
1998年 | 598篇 |
1997年 | 572篇 |
1996年 | 554篇 |
1995年 | 432篇 |
1994年 | 389篇 |
1993年 | 329篇 |
1992年 | 325篇 |
1991年 | 285篇 |
1990年 | 276篇 |
1989年 | 211篇 |
1988年 | 202篇 |
1987年 | 150篇 |
1986年 | 89篇 |
1985年 | 57篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 32篇 |
1981年 | 15篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1957年 | 14篇 |
1955年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 297 毫秒
21.
陈子国 《阜阳师范学院学报(自然科学版)》2002,19(1):59-61
本文重点介绍了集成有源负载放大器的分析方法,并以镜像电流源为例详细介绍、对比了两种方法(图解分析法和微变等效电路分析法)。此外,介绍了用厄雷电压计算 Av 的简便方法。 相似文献
22.
Papert's (1978) appeal to reconsider the power and possibilities of the aesthetic in mathematics learning is often ignored in mathematics education research. This paper begins with the premise, put forth by Dewey (1934), that the aesthetic structures many dimensions of inquiry and experience. In the same way that using particular paintings, musical compositions, or even everyday experiences has been instrumental to attempts by philosophers to understand the aesthetic dimensions of meaning and experience in artistic domains, I propose that analysing a particular encounter with mathematics may help reveal the nature and role of the often nebulous responses of elegance, beauty, and `fit' to which mathematicians lay claim in their mathematical activity. To achieve this, I draw on and adapt the defining features of the aesthetic character of experience set forth by the aesthetician Beardsley (1982). This, in turn, sheds light on the role thataesthetics can play in mathematical inquiry and experience, and provides initial categories and conjectures that can be used to investigate the potential roles of aesthetics in mathematics learning contexts.This revised version was published online in September 2005 with corrections to the Cover Date. 相似文献
23.
24.
26.
ON HYPERBOLIC TIME DISCOUNTING IN EXHAUSTIBLE RESOURCE MODELS: AN APPLICATION TO WORLD OIL RESOURCES
JOHN ROWSE 《Natural Resource Modeling》2006,19(2):243-277
ABSTRACT. Recent research on discounting in long term economic models involves hyperbolic discounting, in which the marginal discount rate shrinks as time passes. To investigate hyperbolic discounting and exhaustible resource allocation, this work develops a discrete‐time world oil model and model solution procedure, then uses the model to examine the consequences of adopting conventional (constant annual) discounting when hyperbolic discounting is appropriate, of adopting one hyperbolic discount rate path when a different hyperbolic path is appropriate, and of adopting hyperbolic discounting when conventional discounting is appropriate. Five conventional and two hyperbolic discount rate paths are considered. One hyperbolic path is that used by Nordhaus and Boyer [2000]; the other is that recommended by Weitzman [2001]. The generality of the findings is also assessed. 相似文献
27.
Dalila B. M. M. Fontes Eleni Hadjiconstantinou Nicos Christofides 《Journal of Global Optimization》2006,34(1):97-125
In this paper we obtain Lower Bounds (LBs) to concave cost network flow problems. The LBs are derived from state space relaxations
of a dynamic programming formulation, which involve the use of non-injective mapping functions guaranteing a reduction on
the cardinality of the state space. The general state space relaxation procedure is extended to address problems involving
transitions that go across several stages, as is the case of network flow problems. Applications for these LBs include: estimation
of the quality of heuristic solutions; local search methods that use information of the LB solution structure to find initial
solutions to restart the search (Fontes et al., 2003, Networks, 41, 221–228); and branch-and-bound (BB) methods having as
a bounding procedure a modified version of the LB algorithm developed here, (see Fontes et al., 2005a). These LBs are iteratively
improved by penalizing, in a Lagrangian fashion, customers not exactly satisfied or by performing state space modifications.
Both the penalties and the state space are updated by using the subgradient method. Additional constraints are developed to
improve further the LBs by reducing the searchable space. The computational results provided show that very good bounds can
be obtained for concave cost network flow problems, particularly for fixed-charge problems. 相似文献
28.
We consider the optimal service control of a multiclass M/G/1 queueing system in which customers are served nonpreemptively and the system cost rate is additive across classes and increasing convex in the numbers present in each class. Following Whittle's approach to a class of restless bandit problems, we develop a Langrangian relaxation of the service control problem which serves to motivate the development of a class of index heuristics. The index for a particular customer class is characterised as a fair charge for service of that class. The paper develops these indices and reports an extensive numerical investigation which exhibits strong performance of the index heuristics for both discounted and average costs. 相似文献
29.
Hong-Xing Zheng Dao-Yin Yu 《International Journal of Infrared and Millimeter Waves》2005,26(9):1343-1353
In this paper, a very useful numerical technique has been developed for analyzing the transient characteristics of a planar-spiral inductor on-chip. A locally conformal technique and an alternating-direction implicit scheme are applied to the finite-difference time-domain method. A formulation for solving three dimensional Maxwell’s equations is proposed. Using the proposed method, various parameters of the planar-spiral inductors have been analyzed and an equivalent circuit, which includes frequency-independent circuit elements, has been introduced. Highly computational efficiency is implemented. Numerical results show excellent agreement with the measured data over a wide frequency range. 相似文献
30.
The finite-difference method is a numerical technique for obtaining approximate solutions to differential equations. The main
objective of the present study is to give a new aspect to the finite-difference method by using a variational derivative.
By applying this formulation, accurate values of the buckling loads of beams and frames with various end supports are obtained.
The performance of this formulation is verified by comparison with numerical examples in the literature
__________
Published in Prikladnaya Mekhanika, Vol. 41, No. 7, pp. 139–144, July 2005. 相似文献