全文获取类型
收费全文 | 130823篇 |
免费 | 3731篇 |
国内免费 | 1218篇 |
专业分类
化学 | 59692篇 |
晶体学 | 1080篇 |
力学 | 8473篇 |
综合类 | 47篇 |
数学 | 39157篇 |
物理学 | 27323篇 |
出版年
2023年 | 524篇 |
2022年 | 479篇 |
2021年 | 745篇 |
2020年 | 991篇 |
2019年 | 943篇 |
2018年 | 11243篇 |
2017年 | 10898篇 |
2016年 | 7684篇 |
2015年 | 2571篇 |
2014年 | 2131篇 |
2013年 | 3802篇 |
2012年 | 7704篇 |
2011年 | 14102篇 |
2010年 | 7933篇 |
2009年 | 7944篇 |
2008年 | 9743篇 |
2007年 | 11730篇 |
2006年 | 3246篇 |
2005年 | 4145篇 |
2004年 | 3962篇 |
2003年 | 3894篇 |
2002年 | 2733篇 |
2001年 | 1343篇 |
2000年 | 1175篇 |
1999年 | 839篇 |
1998年 | 733篇 |
1997年 | 655篇 |
1996年 | 800篇 |
1995年 | 514篇 |
1994年 | 532篇 |
1993年 | 530篇 |
1992年 | 482篇 |
1991年 | 437篇 |
1990年 | 419篇 |
1989年 | 401篇 |
1988年 | 331篇 |
1987年 | 328篇 |
1986年 | 305篇 |
1985年 | 445篇 |
1984年 | 402篇 |
1983年 | 288篇 |
1982年 | 375篇 |
1981年 | 359篇 |
1980年 | 331篇 |
1979年 | 320篇 |
1978年 | 302篇 |
1976年 | 289篇 |
1975年 | 263篇 |
1974年 | 268篇 |
1973年 | 267篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Let C be a family of n different unit circles in the plane. We show that C determines at least n - c intersection points,
where c is the number of connected components in ∪C ∈ CC. This proves a conjecture of A. Bezdek. 相似文献
992.
Discrete-time GI/Geo/1 queue with multiple working vacations 总被引:2,自引:0,他引:2
Consider the discrete time GI/Geo/1 queue with working vacations under EAS and LAS schemes. The server takes the original
work at the lower rate rather than completely stopping during the vacation period. Using the matrix-geometric solution method,
we obtain the steady-state distribution of the number of customers in the system and present the stochastic decomposition
property of the queue length. Furthermore, we find and verify the closed property of conditional probability for negative
binomial distributions. Using such property, we obtain the specific expression for the steady-state distribution of the waiting
time and explain its two conditional stochastic decomposition structures. Finally, two special models are presented.
相似文献
993.
This study uses data envelopment analysis to analyse the efficiency of educational systems in 31 countries. This type of evaluation is of interest both when formulating a model for analysis and when applying such a model empirically. The efficiency of an educational system must take into account the students' economic and social background, as this is an environmental factor that decisively influences their performance. This is a highly important aspect and so we propose a specific evaluative process for it. Secondly, we evaluate the efficiency of educational systems in different countries, an analysis that has few forerunners since the majority of previous research has focused on analysing a single country. The results suggest that, in general, the most efficient management of educational systems can be found in those countries with a Communist past. They also suggest that there is a series of developed countries, which, judging by the results obtained, could increase their students' performance with even fewer resources than those currently allocated to their educational systems. 相似文献
994.
O Berman 《The Journal of the Operational Research Society》2007,58(10):1396-1396
995.
Paolo Battocchio Francesco Menoncin Olivier Scaillet 《Annals of Operations Research》2007,152(1):141-165
In a financial market with one riskless asset and n risky assets whose prices are lognormal, we solve in a closed form the problem of a pension fund maximizing the expected
CRRA utility of its surplus till the (stochastic) death time of a representative agent. We consider a unique asset allocation
problem for both accumulation and decumulation phases. The optimal investment in the risky assets must decrease during the
first phase and increase during the second one. We accordingly suggest it is not optimal to manage the two phases separately,
and outsourcing of allocation decisions should be avoided in both phases.
JEL: G23, G11
MSC 2000: 62P05, 91B28, 91B30, 91B70, 93E20 相似文献
996.
K. Ren Z.Y. Li X. Ren B. Cheng D. Zhang 《Applied Physics A: Materials Science & Processing》2007,87(2):181-185
This paper describes a simple hydrothermal procedure for high-yield synthesis of single-crystalline ZnO hexagonal nanoplates in a surfactant-free system at 70 °C. The structures and morphologies of the synthesized ZnO nanoplates are derived from characterisation by X-ray diffraction, and scanning and transmission electron microscopy. Their optical properties are recorded by Raman and photoluminescence spectroscopy. These ZnO hexagonal nanoplates exhibit the enhanced photocatalytic activity of phenol photodecomposition, suggesting that they could be served as an active system for the treatment of the waste water, in addition to their common applications. PACS 81.10.Dn; 61.10.Nz; 68.37.Hk; 78.55.Hx 相似文献
997.
This paper addresses a kind of risk decision-making problem existing widely in public administration and business management, which is characterized by (1) occurrence probabilities of states of nature can be estimated by analysing historical observations, but historical observations of different objects are unhomogeneous, (2) the relation between observations and occurrence probabilities of states of nature are affected by some qualitative and quantitative indicators, (3) it is a real-time decision-making problem, that is, there are many decisions for different objects to be made in a limited time, (4) considering decision's execution, impact of resource constrains is an important issue in decision-making process. In this paper, we develop a rule-based approach to address the problem. In the proposed approach, a two-step clustering method is employed to classify objects into categories, and observations in each category can be approximately viewed as homogeneous. For objects in each category, occurrence probabilities of states of nature are estimated by logistic regression, and the decision rule is obtained through solving an optimization model, which is to minimize the total decision risks while satisfying resource constrains. Effect and efficacy of our approach are illustrated through its application to China's customs inspection decision. 相似文献
998.
Abraham Mehrez Zilla Sinuany-Stern Alan Stulman 《The Journal of the Operational Research Society》1986,37(10):971-977
In this note, we examine the algorithm of Drezner and Wesolowsky for locating a facility with maximin rectilinear distance to a given set of demand points. The concept of the closest-point constraint is introduced as a technique for increasing the efficiency of the algorithm. This potential increase in efficiency is demonstrated by an example. 相似文献
999.
J D Griffiths Z F Lawson J E Williams 《The Journal of the Operational Research Society》2006,57(12):1413-1424
Advances in recent treatments for HIV/AIDS patients have shown dramatic outcomes in extending the incubation period and AIDS survival time, while also providing significant improvements in the quality of patients' lives. A compartmental model is proposed to analyse the effects of the various treatment regimens which have been introduced. The results produced are in good agreement with routinely collected data relating to levels of HIV/AIDS incidence and prevalence in the UK homosexual population. Some parameter values within the model are obtained from surveys, census results, etc, but others are derived using a maximum likelihood estimation procedure. Finally, the model is used to project levels of incidence and prevalence over the next few years, and to investigate several possible scenarios. 相似文献
1000.
J. Balogh D. Kaptás L. F. Kiss T. Kemény L. Bujdosó I. Vincze 《Hyperfine Interactions》2006,169(1-3):1343-1347
Magnetic multilayers of 57Fe with nominal thickness, T
nom, between 0.4 and 1.0 nm separated by 3.0 nm Al spacer layers were prepared by alternate deposition of the constituents in
high vacuum. The samples were investigated at 4.2 K in external magnetic field. A fraction of Fe atoms corresponding to about
0.3 nm equivalent Fe-thickness was found to mix into the Al spacer. The extremely strong magnetic anisotropy observed for
T
nom < 0.8 nm is attributed to Fe layers of approximately two atomic planes thick. The anisotropy decreases considerably after
the building up of the third Fe atomic layer starts at T
nom = 0.8 nm, but full saturation was not achieved even for T
nom = 1 nm and 3 T magnetic field applied perpendicularly to the sample plane. 相似文献