全文获取类型
收费全文 | 5930篇 |
免费 | 622篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 1076篇 |
丛书文集 | 1篇 |
教育与普及 | 2篇 |
理论与方法论 | 297篇 |
现状及发展 | 1029篇 |
研究方法 | 71篇 |
综合类 | 4070篇 |
自然研究 | 8篇 |
出版年
2021年 | 4篇 |
2019年 | 4篇 |
2018年 | 729篇 |
2017年 | 734篇 |
2016年 | 437篇 |
2015年 | 33篇 |
2014年 | 14篇 |
2013年 | 24篇 |
2012年 | 285篇 |
2011年 | 992篇 |
2010年 | 831篇 |
2009年 | 458篇 |
2008年 | 535篇 |
2007年 | 793篇 |
2006年 | 42篇 |
2005年 | 71篇 |
2004年 | 151篇 |
2003年 | 165篇 |
2002年 | 76篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1994年 | 3篇 |
1991年 | 7篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 9篇 |
1974年 | 4篇 |
1973年 | 8篇 |
1972年 | 5篇 |
1971年 | 2篇 |
1970年 | 5篇 |
1969年 | 3篇 |
1968年 | 9篇 |
1967年 | 3篇 |
1966年 | 4篇 |
1964年 | 5篇 |
1961年 | 3篇 |
排序方式: 共有6554条查询结果,搜索用时 15 毫秒
171.
This paper considers the optimal investment strategy for an insurer under the criterion of mean-variance. The risk process
is a compound Poisson process and the insurer can invest in a risk-free asset and multiple risky assets. This paper obtains
the optimal investment policy using the stochastic linear quadratic (LQ) control theory with no-shorting constraint. Then
the efficient strategy (optimal investment strategy) and efficient frontier are derived explicitly by a verification theorem
with the viscosity solution of Hamilton-Jacobi-Bellman (HJB) equation. 相似文献
172.
This paper considers a class of stochastic variational inequality problems. As proposed by Jiang and Xu (2008), by using the
so-called regularized gap function, the authors formulate the problems as constrained optimization problems and then propose
a sample average approximation method for solving the problems. Under some moderate conditions, the authors investigate the
limiting behavior of the optimal values and the optimal solutions of the approximation problems. Finally, some numerical results
are reported to show efficiency of the proposed method. 相似文献
173.
In this paper, the dynamic behaviors of a discrete epidemic model with a nonlinear incidence rate obtained by Euler method
are discussed, which can exhibit the periodic motions and chaotic behaviors under the suitable system parameter conditions.
Codimension-two bifurcations of the discrete epidemic model, associated with 1:1 strong resonance, 1:2 strong resonance, 1:3
strong resonance and 1:4 strong resonance, are analyzed by using the bifurcation theorem and the normal form method of maps.
Moreover, in order to eliminate the chaotic behavior of the discrete epidemic model, a tracking controller is designed such
that the disease disappears gradually. Finally, numerical simulations are obtained by the phase portraits, the maximum Lyapunov
exponents diagrams for two different varying parameters in 3-dimension space, the bifurcation diagrams, the computations of
Lyapunov exponents and the dynamic response. They not only illustrate the validity of the proposed results, but also display
the interesting and complex dynamical behaviors. 相似文献
174.
J. D. R. de Raadt 《Systemic Practice and Action Research》2011,24(6):509-521
Starting from the crisis in our communities—research has indicated it to be widely spread—the author explores the scientific
tradition preceding the rise of modernism in order to draw out a more humane way of thinking that may help our contemporary
societies. He discerns between two types of Renaissance humanism, one characterised by its cleverness, the other by its compassion
and desire to restore dignity to the lives of people struggling to escape the clutches of medievalism. The father of the latter
is Erasmus of Rotterdam and we follow the development of his conception of a social humanism as it branches out, through the
work of his successors, into every scientific discipline, both natural and human. These disciplines are united by an embracing
systemic idea of philosophy that unites the mind with the heart and which Erasmus called “Philosophia Christi”. We examine
the two main pillars upon which this philosophy is built, love for our neighbour and education as the only legitimate instrument
to change society, and the extraordinary impact it had on science and on communities living in the seventeenth century. We
contrast this with the idea of power, the chosen instrument of modernity to transform society, and trace some of its tragic
outcomes. We conclude by discussing the incorporation of an Erasmian type of social humanism into systems education and the
future development of such programmes. 相似文献
175.
176.
Resolution of acute inflammation is an active process essential for appropriate host responses, tissue protection and the return to homeostasis. During resolution, specific omega-3 polyunsaturated fatty-acid-derived mediators are generated within resolving exudates, including resolvin E1 (RvE1) and protectin D1 (PD1). It is thus important to pinpoint specific actions of RvE1 and PD1 in regulating tissue resolution. Here we report that RvE1 and PD1 in nanogram quantities promote phagocyte removal during acute inflammation by regulating leukocyte infiltration, increasing macrophage ingestion of apoptotic polymorphonuclear neutrophils in vivo and in vitro, and enhancing the appearance of phagocytes carrying engulfed zymosan in lymph nodes and spleen. In this tissue terrain, inhibition of either cyclooxygenase or lipoxygenases--pivotal enzymes in the temporal generation of both pro-inflammatory and pro-resolving mediators--caused a 'resolution deficit' that was rescued by RvE1, PD1 or aspirin-triggered lipoxin A4 analogue. Also, new resolution routes were identified that involve phagocytes traversing perinodal adipose tissues and non-apoptotic polymorphonuclear neutrophils carrying engulfed zymosan to lymph nodes. Together, these results identify new active components for postexudate resolution traffic, and demonstrate that RvE1 and PD1 are potent agonists for resolution of inflamed tissues. 相似文献
177.
Research in semiconductor spintronics aims to extend the scope of conventional electronics by using the spin degree of freedom of an electron in addition to its charge. Significant scientific advances in this area have been reported, such as the development of diluted ferromagnetic semiconductors, spin injection into semiconductors from ferromagnetic metals and discoveries of new physical phenomena involving electron spin. Yet no viable means of developing spintronics in semiconductors has been presented. Here we report a theoretical design that is a conceptual step forward-spin accumulation is used as the basis of a semiconductor computer circuit. Although the giant magnetoresistance effect in metals has already been commercially exploited, it does not extend to semiconductor/ferromagnet systems, because the effect is too weak for logic operations. We overcome this obstacle by using spin accumulation rather than spin flow. The basic element in our design is a logic gate that consists of a semiconductor structure with multiple magnetic contacts; this serves to perform fast and reprogrammable logic operations in a noisy, room-temperature environment. We then introduce a method to interconnect a large number of these gates to form a 'spin computer'. As the shrinking of conventional complementary metal-oxide-semiconductor (CMOS) transistors reaches its intrinsic limit, greater computational capability will mean an increase in both circuit area and power dissipation. Our spin-based approach may provide wide margins for further scaling and also greater computational capability per gate. 相似文献
178.
CONG YanQing WU ZuCheng LI YuQiong 《科学通报(英文版)》2007,52(10):1432-1435
The most reactive hydroxyl radical (OH) was generated by electrochemical approach with safe water as the complete atom source. The direct evidence for OH formation was obtained by electron spin resonance method. The powerful OH electrochemically generated could effectively degrade organic pollutants and reduce the toxicity of wastewater. Electrochemical disinfection by OH was considerably efficient even without the aid of active chlorine. Bacteria inactivation of 99.99% was achieved for contact time of 30 min and current density of 5 mA·cm^-2, In comparison with active chlorine, OH is rather attractive as a promising environmentally benign disinfectant and opens a new route for microbial inactivation. 相似文献
179.
ZHANG Xuejun 《武汉大学学报:自然科学英文版》2007,12(1):5-8
T6 et al presented a bilinear-map-based traitor tracing scheme(TSZ scheme) with revocation, but it is a symmetric scheme because it does not provide non-repudiation. In this paper, an improved TSZ scheme was proposed by using oblivious polynomial evaluation (OPE) protocol and service parameters. Under the recondition of general sameness capabilities of both TSZ and improved TSZ scheme, the new scheme adds some advantages such as providing multi-service capability, user's non-repudiation and data provider's no-framing innocent users. Furthermore, it is also proved to be semantically secure under the decisional bilinear Diffie-Hellman (DBDH problem) assumption. 相似文献
180.
JIANG Huiming ZHANG Shubiao JIN Rongchao MA Yunhua 《武汉大学学报:自然科学英文版》2007,12(6):1099-1104
Piperidine absorbs CO2 and H2O in air to form a molecular complex: piperidium-l-piperidinecarboxylate-H2O. The structure of the complex was characterized by X-ray single crystal diffraction. The crystal structure was determined to be triclinic, space group P1^-with a=0.648 6(8) nm, b=0.809 200) nm, c= 1.357 1(16) nm, a=96.96706)°, β =102.506(15)°,γ=104.202 05)°, Z=2. The complex is stabilized via five hydrogen bonds between the three components, N-O electrostatic interaction and O-O interaction (electron transfer) betweenl-piperidinecarboxylate and H2O. Due to electron transference of carbamate ion, the oxygen atom in water molecule is strongly negatively charged and the O-H bond is considerably shorter than that of the free molecule of water. The formation of the molecular complex is a reversible process and will decompose upon heating. The mechanism of formation and stabilization is further investigated herein. 相似文献