首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4456篇
  免费   91篇
  国内免费   297篇
系统科学   49篇
丛书文集   130篇
教育与普及   33篇
理论与方法论   36篇
现状及发展   116篇
研究方法   1篇
综合类   4469篇
自然研究   10篇
  2024年   16篇
  2023年   11篇
  2022年   25篇
  2021年   30篇
  2020年   43篇
  2019年   53篇
  2018年   31篇
  2017年   57篇
  2016年   50篇
  2015年   119篇
  2014年   450篇
  2013年   283篇
  2012年   600篇
  2011年   440篇
  2010年   452篇
  2009年   302篇
  2008年   332篇
  2007年   86篇
  2006年   79篇
  2005年   97篇
  2004年   60篇
  2003年   97篇
  2002年   67篇
  2001年   51篇
  2000年   48篇
  1999年   30篇
  1998年   28篇
  1997年   33篇
  1996年   363篇
  1995年   253篇
  1994年   167篇
  1993年   20篇
  1992年   8篇
  1991年   9篇
  1990年   19篇
  1989年   21篇
  1988年   3篇
  1987年   6篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
排序方式: 共有4844条查询结果,搜索用时 15 毫秒
41.
大型的汉字显示日益普及,而显示内容,如新闻提要等却千变万化,以往的单片机、单板机等用造字的方法来进行显示已不能满足需要。本文提出一种无需中文操作系统管理,只在西文状态下直接用微机中软汉字库,将任何已编辑好的中文文本在X-Y寻址DC-PDP上进行显示的方法。  相似文献   
42.
为了保证桥梁的安全,对现役桥梁进行检测就显得尤为重要.结合邕江大桥的检测情况,对大桥出现的病害加以分析,可以为以后邕江大桥评估与加固提供依据.最后结合病害分析对桥梁的设计和施工提出一些改进建议.  相似文献   
43.
Apolipoprotein E (apoE) ɛ4 allele is a genetic risk factor for late-onset familial and sporadic Alzheimer’s disease (AD). In the central nervous system, apoE is secreted mainly by astrocytes as a constituent of high-density lipoproteins. A recent study using apoE knockout mice provided strong evidence that apoE promotes cerebral deposition of amyloid β protein (Aβ). However, no clear explanation of the pathogenesis of apoE-induced AD has been provided. Here we discuss two possible mechanisms by which apoE might enhance Aβ deposition. One is the intracellular pathway in which apoE is internalized by neurons and induces lysosomal accumulation of Aβ and amyloidogenic APP (amyloid precursor protein) fragments, leading to neuronal death. The other is the extracellular pathway in which apoE-containing lipoproteins are trapped by Aβ1–42 deposits mobilizing soluble Aβ peptides and consequently enlarge amyloid plaques. These two mechanisms may operate at different stages of AD pathogenesis and suggest a chaperone-like function for the apoE molecule. Received 4 February 1999; received after revision 9 April 1999; accepted 23 April 1999  相似文献   
44.
七绝,本为来自民间的七言诗,经永明体而为近体诗之一种诗体;竹枝词,本为巴渝民歌,自刘禹锡以降.历代文人多有仿作,所仿作者为文人竹枝词;而客家山歌,则是自明清客家民系形成以来,至今犹传承于客地的鲜活民歌。从这三种诗歌中,可发现雅俗文学之间的交融、碰撞而嬗变的文学雅俗互动的有趣轨迹。  相似文献   
45.
陈燕 《韶关学院学报》2007,28(10):102-104
古代中日两国吟咏爱情的诗歌中,“秋风”一词时有所见。从日本最早的古典和歌大成《万叶集》中所记载的、由女歌人额田王所咪的“思近江天皇咏秋风歌”可知,它与中国六朝闺怨诗既有联系又有区别,“秋风”语象在当时的中日诗歌中具有不同的意蕴。  相似文献   
46.
We distinguish two orientations in Weyl's analysis of the fundamental role played by the notion of symmetry in physics, namely an orientation inspired by Klein's Erlangen program and a phenomenological-transcendental orientation. By privileging the former to the detriment of the latter, we sketch a group(oid)-theoretical program—that we call the Klein-Weyl program—for the interpretation of both gauge theories and quantum mechanics in a single conceptual framework. This program is based on Weyl's notion of a “structure-endowed entity” equipped with a “group of automorphisms”. First, we analyze what Weyl calls the “problem of relativity” in the frameworks provided by special relativity, general relativity, and Yang-Mills theories. We argue that both general relativity and Yang-Mills theories can be understood in terms of a localization of Klein's Erlangen program: while the latter describes the group-theoretical automorphisms of a single structure (such as homogenous geometries), local gauge symmetries and the corresponding gauge fields (Ehresmann connections) can be naturally understood in terms of the groupoid-theoretical isomorphisms in a family of identical structures. Second, we argue that quantum mechanics can be understood in terms of a linearization of Klein's Erlangen program. This stance leads us to an interpretation of the fact that quantum numbers are “indices characterizing representations of groups” ((Weyl, 1931a), p.xxi) in terms of a correspondence between the ontological categories of identity and determinateness.  相似文献   
47.
基于QFD的网站用户需求分析   总被引:2,自引:0,他引:2  
以最终用户满意为驱动力,基于质量功能展开原理,提出一种将网站用户需求转化为对网站各个环节的要求的多层次演绎分析方法,即网站用户需求展开,为将用户满意理念贯穿于网站的设计与管理之中提供了理论框架与操作方法。  相似文献   
48.
The amyloid β-peptide (Aβ) is a 4-kDa species derived from the amyloid precursor protein, which accumulates in the brains of patients with Alzheimer’s disease. Although we lack full understanding of the etiology and pathogenesis of selective neuron death, considerable data do imply roles for both the toxic Aβ and increased oxidative stress. Another significant observation is the accumulation of abnormal, ubiquitin-conjugated proteins in affected neurons, suggesting dysfunction of the proteasome proteolytic system in these cells. Recent reports have indicated that Aβ can bind and inhibit the proteasome, the major cytoslic protease for degrading damaged and ubiquitin-conjugated proteins. Earlier results from our laboratory showed that moderately oxidized proteins are preferentially recognized and degraded by the proteasome; however, severely oxidized proteins cannot be easily degraded and, instead, inhibit the proteasome. We hypothesized that oxidatively modified Aβ might have a stronger (or weaker) inhibitory effect on the proteasome than does native Aβ. We therefore also investigated the proteasome inhibitory action of Aβ 1–40 (a peptide comprising the first 40 residues of Aβ) modified by the intracellular oxidant hydrogen peroxide, and by the lipid peroxidation product 4-hydroxynonenal (HNE). H2O2 modification of Aβ 1–40 generates a progressively poorer inhibitor of the purified human 20S proteasome. In contrast, HNE modification of Aβ 1–40 generates a progressively more selective and efficient inhibitor of the degradation of fluorogenic peptides and oxidized protein substrates by human 20S proteasome. This interaction may contribute to certain pathological manifestations of Alzheimer’s disease Received 26 September 2000; accepted 26 September 2000  相似文献   
49.
A.V.Bitsadze在文[1]中提出和研究了二阶一致线性双曲型方程uxx-uyy+aux+by+cu+d=0(A)的第一类和第二类Darboux问题.本文的目的是讨论二阶退化双曲型方程第二类广义Darboux问题和斜微商问题解的表示式,并证明这些问题解的存在唯一性。本文使用不同于[1]中的方法,但类似于[1]中的方程(A),根据本文中的结果,我们可以解决广义Chaplygin方程在一般区域上的Frankl问题.  相似文献   
50.
目的通过胰岛素和磷脂酰肌醇-3激酶(P13K)抑制剂渥曼青霉素(wortmannin)对P13K/丝氨酸苏氨酸蛋白激酶(P13K/Akt)信号通路的激活和抑制作用,观察P13K/Akt信号通路对海马神经元β-淀粉样前体蛋白裂解酶1(BACEl)mRNA水平表达的影响。方法20只sD大鼠随机分为空白对照组、假手术组、胰岛素组和渥曼青霉素组,海马立体定向注射胰岛素和P13K抑制剂渥曼青霉素。逆转录一聚合酶链反应(RT-PCR)检测P13K/Akt信号传导下游蛋白Akt以及BACEImRNA水平。结果注射胰岛素的海马P13K信号通路下游信号分子:AktmRNA表达上调(分别较空白和阴性对照组P=0.047,P=0.002),而BACElmRNA表达下调(分别较空白和阴性对照组P=0.004,P=0.01)。渥曼青霉素组的P13K下游信号分子AktmRNA表达明显被抑制(分别较空白和阴性对照组P=0.002,P=0.039),同时BACEImRNA的表达较对照组上调(分别较空白和阴性对照组P=0.039,P=0.018)。结论胰岛素信号通路P13K/AM可以调节BACEl的转录水平参与阿尔茨海默病的发病机制。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号