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   1925篇
  免费   2篇
  国内免费   13篇
系统科学   113篇
丛书文集   3篇
教育与普及   8篇
理论与方法论   26篇
现状及发展   228篇
研究方法   300篇
综合类   1130篇
自然研究   132篇
  2021年   5篇
  2020年   13篇
  2019年   8篇
  2018年   9篇
  2017年   10篇
  2016年   17篇
  2015年   12篇
  2014年   18篇
  2013年   41篇
  2012年   122篇
  2011年   291篇
  2010年   46篇
  2009年   20篇
  2008年   141篇
  2007年   171篇
  2006年   147篇
  2005年   123篇
  2004年   129篇
  2003年   144篇
  2002年   169篇
  2001年   24篇
  2000年   21篇
  1999年   12篇
  1998年   8篇
  1997年   7篇
  1996年   6篇
  1995年   6篇
  1994年   11篇
  1993年   19篇
  1992年   17篇
  1991年   12篇
  1990年   19篇
  1989年   13篇
  1988年   9篇
  1987年   11篇
  1986年   5篇
  1985年   8篇
  1984年   7篇
  1983年   5篇
  1982年   5篇
  1978年   5篇
  1975年   5篇
  1974年   4篇
  1973年   4篇
  1971年   9篇
  1970年   11篇
  1969年   5篇
  1968年   4篇
  1967年   5篇
  1966年   5篇
排序方式: 共有1940条查询结果,搜索用时 31 毫秒
861.
Despite being a major organ of the immune system, the spleen's role in resisting, controlling or simply ameliorating nematode infections has been neglected. A review of both filarial and gastrointestinal nematodes suggests that though it is difficult to fully assess or quantify the organ's importance in vivo, the spleen is prominent in acting against nematode parasites in mammals. One manifestation of this is that transfer of lymphocytes from the spleen of immunised individuals can protect recipients against the disease. Expansion of splenic lymphoid tissue also alludes to its activity during nematode infection. There is a considerable need for investigation of the spleen under natural conditions as well as much more rigorously controlled experiments even in mammals besides birds and other vertebrates.  相似文献   
862.
In colour measurement ralated industry, reflectance spectrophotometer is the one of the popular measuring machine for measuring colour and quality control. Colour communications is frequently confusing. This is because the colour appearance is subject to the influence of at least three different phenomena: the light source, the object and the visual system. The variation in either the radiant quantity or the spectral distribution of the source can alter the observed colour. Because of this reason, the objective quantitative tool, colour measurement equipment and communication method; become more important in evaluating of the colour. In fact, based on the advanced in computer system and electronic device, the colour measurement becomes more and more accuracy, especially in spectrophotometer measurement. In this paper, we will focus on the review of modern spectrophotometers in coloration industries.  相似文献   
863.
Embryonal tumours of the central nervous system (CNS) represent a heterogeneous group of tumours about which little is known biologically, and whose diagnosis, on the basis of morphologic appearance alone, is controversial. Medulloblastomas, for example, are the most common malignant brain tumour of childhood, but their pathogenesis is unknown, their relationship to other embryonal CNS tumours is debated, and patients' response to therapy is difficult to predict. We approached these problems by developing a classification system based on DNA microarray gene expression data derived from 99 patient samples. Here we demonstrate that medulloblastomas are molecularly distinct from other brain tumours including primitive neuroectodermal tumours (PNETs), atypical teratoid/rhabdoid tumours (AT/RTs) and malignant gliomas. Previously unrecognized evidence supporting the derivation of medulloblastomas from cerebellar granule cells through activation of the Sonic Hedgehog (SHH) pathway was also revealed. We show further that the clinical outcome of children with medulloblastomas is highly predictable on the basis of the gene expression profiles of their tumours at diagnosis.  相似文献   
864.
This paper unpeels two influential contributions to modern systems thinking, Beer's viable system model (VSM) and Checkland's soft systems methodology (SSM), and finds inside a critical kernel. It is argued, indeed, that these approaches become coherent only when liberated from their regulative shells and interpreted from the critical position. So much does the critical kernel lie at the very heart of modern systems thinking.  相似文献   
865.
866.
Presented are estimates of demand equations and producer revenue projections for rural farm electricity consumers in the USA. Statistical tests include a Box-Cox comparison of functional form, a Koyck-distributed lag, and a contrast of average versus marginal prices. Producer revenue projections take account of the estimated demand equation and non-continuous rate schedules for each of the electricity distributors. Multiple price equilibria result from differences in administered price schedules between sellers. The ‘effective’ price elasticity based upon forecasts which take account of the market circumstances is lower than the simple demand point elasticity but higher than found in some previous studies.  相似文献   
867.
从我开始能够阅读有关科学内容书籍的时候起,我就希望将来可以成为一名科学家。由于对数学方面有着特殊的爱好,使我最终转向了理论物理学的学习。研究金属电子的性质听起来有些平常,但这是真事。事实上要进行这方面的研究,对量子理论知识有着很高的要求,因此我始终将这些原理当作从未被发  相似文献   
868.
869.
870.
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