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351.
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
352.
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
353.
354.
长春花是重要的观赏和药用植物.为了探索适宜长春花种子萌发及幼苗生长的温度和光照条件,研究了温度(20℃、24℃、28℃和30℃)和光照条件(每日连续光照14h和全黑暗)对经自来水浸种24h后的长春花种子发芽和幼苗生长的影响.结果表明,种子的发芽率和发芽指数、幼苗的芽长和干重均受光照条件、温度以及两者间交互作用的显著影响(P0.01),种子的发芽势、幼苗的鲜重受光照条件以及光照条件和温度间交互作用的显著影响(P0.01),幼苗的根长仅受温度的显著影响(P0.01).各光照和温度条件组合间,种子在黑暗、20℃-28℃下的发芽率和发芽势均最高,在黑暗、24℃和28℃下的发芽指数最高;幼苗的根在光照和28℃以及黑暗和24℃下较长,芽在黑暗、24℃-30℃下最长,鲜重在光照和24℃以及黑暗和20℃-30℃下最大,干重在光照、24℃下最大.黑暗、24℃是长春花种子萌发和幼苗生长的最适条件. 相似文献
355.
356.
Enzyme-catalysed oxidations are some of the most common transformations in primary and secondary metabolism. The vancomycin biosynthetic enzyme DpgC belongs to a small class of oxygenation enzymes that are not dependent on an accessory cofactor or metal ion. The detailed mechanism of cofactor-independent oxygenases has not been established. Here we report the first structure of an enzyme of this oxygenase class in complex with a bound substrate mimic. The use of a designed, synthetic substrate analogue allows unique insights into the chemistry of oxygen activation. The structure confirms the absence of cofactors, and electron density consistent with molecular oxygen is present adjacent to the site of oxidation on the substrate. Molecular oxygen is bound in a small hydrophobic pocket and the substrate provides the reducing power to activate oxygen for downstream chemical steps. Our results resolve the unique and complex chemistry of DpgC, a key enzyme in the biosynthetic pathway of an important class of antibiotics. Furthermore, mechanistic parallels exist between DpgC and cofactor-dependent flavoenzymes, providing information regarding the general mechanism of enzymatic oxygen activation. 相似文献
357.
Parton LE Ye CP Coppari R Enriori PJ Choi B Zhang CY Xu C Vianna CR Balthasar N Lee CE Elmquist JK Cowley MA Lowell BB 《Nature》2007,449(7159):228-232
A subset of neurons in the brain, known as 'glucose-excited' neurons, depolarize and increase their firing rate in response to increases in extracellular glucose. Similar to insulin secretion by pancreatic beta-cells, glucose excitation of neurons is driven by ATP-mediated closure of ATP-sensitive potassium (K(ATP)) channels. Although beta-cell-like glucose sensing in neurons is well established, its physiological relevance and contribution to disease states such as type 2 diabetes remain unknown. To address these issues, we disrupted glucose sensing in glucose-excited pro-opiomelanocortin (POMC) neurons via transgenic expression of a mutant Kir6.2 subunit (encoded by the Kcnj11 gene) that prevents ATP-mediated closure of K(ATP) channels. Here we show that this genetic manipulation impaired the whole-body response to a systemic glucose load, demonstrating a role for glucose sensing by POMC neurons in the overall physiological control of blood glucose. We also found that glucose sensing by POMC neurons became defective in obese mice on a high-fat diet, suggesting that loss of glucose sensing by neurons has a role in the development of type 2 diabetes. The mechanism for obesity-induced loss of glucose sensing in POMC neurons involves uncoupling protein 2 (UCP2), a mitochondrial protein that impairs glucose-stimulated ATP production. UCP2 negatively regulates glucose sensing in POMC neurons. We found that genetic deletion of Ucp2 prevents obesity-induced loss of glucose sensing, and that acute pharmacological inhibition of UCP2 reverses loss of glucose sensing. We conclude that obesity-induced, UCP2-mediated loss of glucose sensing in glucose-excited neurons might have a pathogenic role in the development of type 2 diabetes. 相似文献
358.
The mechanisms that determine the lifespan of an organism are still largely a mystery. One goal of ageing research is to find drugs that would increase lifespan and vitality when given to an adult animal. To this end, we tested 88,000 chemicals for the ability to extend the lifespan of adult Caenorhabditis elegans nematodes. Here we report that a drug used as an antidepressant in humans increases C. elegans lifespan. In humans, this drug blocks neural signalling by the neurotransmitter serotonin. In C. elegans, the effect of the drug on lifespan is reduced or eradicated by mutations that affect serotonin synthesis, serotonin re-uptake at synapses, or either of two G-protein-coupled receptors: one that recognizes serotonin and the other that detects another neurotransmitter, octopamine. In vitro studies show that the drug acts as an antagonist at both receptors. Testing of the drug on dietary-restricted animals or animals with mutations that affect lifespan indicates that its effect on lifespan involves mechanisms associated with lifespan extension by dietary restriction. These studies indicate that lifespan can be extended by blocking certain types of neurotransmission implicated in food sensing in the adult animal, possibly leading to a state of perceived, although not real, starvation. 相似文献
359.
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
360.
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