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   4880篇
  免费   519篇
  国内免费   351篇
  2024年   15篇
  2023年   177篇
  2022年   130篇
  2021年   206篇
  2020年   235篇
  2019年   276篇
  2018年   236篇
  2017年   233篇
  2016年   197篇
  2015年   199篇
  2014年   249篇
  2013年   307篇
  2012年   187篇
  2011年   185篇
  2010年   159篇
  2009年   215篇
  2008年   221篇
  2007年   236篇
  2006年   200篇
  2005年   194篇
  2004年   190篇
  2003年   170篇
  2002年   159篇
  2001年   140篇
  2000年   136篇
  1999年   101篇
  1998年   67篇
  1997年   62篇
  1996年   46篇
  1995年   41篇
  1994年   47篇
  1993年   48篇
  1992年   48篇
  1991年   40篇
  1990年   29篇
  1989年   24篇
  1988年   39篇
  1987年   45篇
  1986年   22篇
  1985年   33篇
  1984年   19篇
  1983年   31篇
  1982年   24篇
  1981年   27篇
  1980年   23篇
  1979年   16篇
  1978年   14篇
  1977年   12篇
  1976年   12篇
  1975年   7篇
排序方式: 共有5750条查询结果,搜索用时 78 毫秒
41.
湖北菱科的数量分类研究[Ⅱ]   总被引:2,自引:0,他引:2  
引言菱科(Trapaceae)植物为一年生水生浮叶草本,常见于湖泊,池塘及沟渠中。全世界约有70余种,主要分布于旧大陆的温带、亚热带地区,亦见于热带地区,如印度,东南亚及非洲,后来被引进美洲大陆和澳大利亚。中国有5至10种及其栽培种。菱科为一单属科。自林奈时期以来,菱科的分类学研究一直是一个富有探索性的问  相似文献   
42.
Computers, the human mind, and social systems have common problems of inadequate memory and insufficient data manipulation speed. In each of these domains, information compression techniques have evolved to reduce storage and processing needs. Among the techniques for information compression, coding of information in procedures stands out as exceptionally powerful. Procedural information coding also gives rise to behavior that may be defined as intelligent. It is found in the human mind, in machines and in social systems. Its use in human thought is aided by language development which promotes regular review of abstract procedures. A practical consequence of better understanding of procedural information coding is the possibility of training people to exhibit greater mental capacity, a controversial possibility. This paper explores the impact of data processing resource limitations, data compression and procedural thinking in men and machines.  相似文献   
43.
44.
Patterns of food evacuation in fishes: a critical review   总被引:5,自引:0,他引:5  
  相似文献   
45.
Principal component analysis of compositional data   总被引:7,自引:0,他引:7  
AITCHISON  J. 《Biometrika》1983,70(1):57-65
  相似文献   
46.
Examination of dispersional characteristics of Pratylenchus scribneri and Hoplolaimus galeatus indicated that there were patches within soybean fields in which both survival and reproduction wexe enhanced in spite of apparent homogeneity of soil type and topography. Treatment with carbofuran reduced the patchiness (or increased the dispersion) for H. galeatus while it had the opposite effect for P. scribneri. P. scribneri was less highly dispersed in conventional tillage plots than in the zero tillage plots. Populations from quadrats contained entirely within the patches could be described by the normal distribution (in the case of P. scribneri) or by the Poisson distribution (in the case of H. galeatus), while populations from quadrats contained entirely outside the patches could be described by the Poisson distribution for both nematodes. None of the distributions tested (Poisson, normal, negative binomial, Neyman''s) gave an adequate fit when populations from both inside and outside the patches were considered together. In all instances, log₁₀ and ln transformations reduced the goodness of fit of the data to all of the distributions tested. Even with logarithmic transformations, parametric statistics were not appropriate for analysis of data in most instances.  相似文献   
47.
48.
Attempts to demonstrate low-dimensional attractor behaviour in the analysis of electroencephalographic (EEG) signals meet with difficulties that in part stem from a departure from single-system dynamics. In order to address this problem, the -waves can be extracted by digital filtering or by wave separation; these two techniques are compared in order to specify the conditions in which finite impulse response (FIR) bandpass filters can be used. The comparison was made using 18 EEG records of 3 min duration under resting conditions (6 subjects, 3 records per subject: prior to apomorphine administration, then 90 min and 150 min post-treatment). No presence of low-dimensional dynamic episodes in -signals was observed without digital processing. Sixty 5 s sections showing attractor behaviour were found after filtering and twenty five 5 s sections after wave separation. The mean correlation dimension was calculated for each experimental condition and for 4 subjects, in order to observe the temporal profile of the drug. When attractors were found after wave separation, bandpass filtering then also showed attractor behaviour, with the same temporal profile. However, the reverse is not true: attractors were found after bandpass filtering that were not present after wave separation; in this case the results deserve confirmation, although the temporal profiles for all cases in which attractors were found after filtering remained comparable.  相似文献   
49.
The antitumor effect of the combined administration with recombinant human interleukin-2 (rIL-2) and sizofiran (SPG), a single glucan of Shizophyllum commune Fries, was studied in vivo in C57BL/6 mice intraperitoneally inoculated with EL-4 lymphoma. The effect was evaluated by a) comparing the survival time of the mice, b) analysis of the intraperitoneal cell population in Giemsa-stained specimens, c) surface marker analysis of peritoneal exudative cells with flow cytometry, d) cytotoxic assay of cells against EL-4 and Yac-1 lymphoma, and e) elimination of some cell populations by monoclonal antibodies, to identify the antitumor-effector cells showing cytotoxic activity. The survival of mice given both rIL-2 and SPG was significantly longer than the control mice or those given SPG alone or rIL-2 alone. It was demonstrated that the administration of SPG and/or rIL-2 to the EL-4 lymphoma-bearing mice activated immune-response cells in the peritoneal cavity such as T lymphocytes, NK cells, or macrophages, which might be effective in reducing lymphoma cells. The combination of rIL-2 and SPG administration appears to activate the antitumor- immune response at the tumor site more effectively than when either agent was administered alone.  相似文献   
50.
植物资源与化学数据库系统(DPRC)的功能与应用   总被引:4,自引:0,他引:4  
由中国科学院昆植物研究所和云南省机械研究设计院的植物资源化学数据库系统是一个将植物资源,植物化学成分及化学结构的多种信息、数据和图象融为一体的、开和式多功能软件系统、,包括植资源检索系统、植物化学成分检索系统及天然产物化学结构检索系统等三个子系统。  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