全文获取类型
收费全文 | 21421篇 |
免费 | 39篇 |
国内免费 | 86篇 |
专业分类
系统科学 | 133篇 |
丛书文集 | 306篇 |
教育与普及 | 48篇 |
理论与方法论 | 65篇 |
现状及发展 | 9319篇 |
研究方法 | 1048篇 |
综合类 | 10337篇 |
自然研究 | 290篇 |
出版年
2013年 | 140篇 |
2012年 | 367篇 |
2011年 | 646篇 |
2010年 | 120篇 |
2009年 | 116篇 |
2008年 | 386篇 |
2007年 | 444篇 |
2006年 | 445篇 |
2005年 | 443篇 |
2004年 | 468篇 |
2003年 | 387篇 |
2002年 | 398篇 |
2001年 | 738篇 |
2000年 | 662篇 |
1999年 | 465篇 |
1992年 | 408篇 |
1991年 | 313篇 |
1990年 | 339篇 |
1989年 | 302篇 |
1988年 | 298篇 |
1987年 | 342篇 |
1986年 | 330篇 |
1985年 | 408篇 |
1984年 | 345篇 |
1983年 | 289篇 |
1982年 | 233篇 |
1981年 | 244篇 |
1980年 | 301篇 |
1979年 | 716篇 |
1978年 | 558篇 |
1977年 | 521篇 |
1976年 | 416篇 |
1975年 | 458篇 |
1974年 | 642篇 |
1973年 | 547篇 |
1972年 | 538篇 |
1971年 | 653篇 |
1970年 | 859篇 |
1969年 | 611篇 |
1968年 | 539篇 |
1967年 | 596篇 |
1966年 | 518篇 |
1965年 | 376篇 |
1959年 | 197篇 |
1958年 | 329篇 |
1957年 | 252篇 |
1956年 | 213篇 |
1955年 | 175篇 |
1954年 | 193篇 |
1948年 | 151篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
一种适合于网络专家系统的通用理论(英文) 总被引:1,自引:0,他引:1
描述运用于工业自动化的一种自适应自学习方法。基于此方法,Aptronix公司开发出一套通用软件工具-STIMTM,并应用于不同工业领域。STIM可用于构造各类专家系统。基于因素空间理论,STIM具有一系列独特之处,比如自动化、自学习,以及自翻译。如果与因特网,嵌入式控制器以及可编程逻辑控制器等结合使用,STIM则成为一个十分有效的工具,可应用于远程连通与控制、模式识别、机器故障诊断,以及自动化加工过程中的传感器数据融合 相似文献
82.
A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean 总被引:70,自引:0,他引:70
Coale KH Johnson KS Fitzwater SE Gordon RM Tanner S Chavez FP Ferioli L Sakamoto C Rogers P Millero F Steinberg P Nightingale P Cooper D Cochlan WP Landry MR Constantinou J Rollwagen G Trasvina A Kudela R 《Nature》1996,383(6600):495-501
The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability. 相似文献
83.
Insect motion detectors matched to visual ecology 总被引:1,自引:0,他引:1
To detect motion, primates, birds and insects all use local detectors to correlate signals sampled at one location in the image with those sampled after a delay at adjacent locations. These detectors can adapt to high image velocities by shortening the delay. To investigate whether they use long delays for detecting low velocities, we compared motion-sensitive neurons in ten species of fast-flying insects, some of which encounter low velocities while hovering. Neurons of bee-flies and hawkmoths, which hover, are tuned to lower temporal frequencies than those of butterflies and bumblebees, which do not. Tuning to low frequencies indicates longer delays and extends sensitivity to lower velocities. Hoverflies retain fast temporal tuning but use their high spatial acuity for sensing low-velocity motion. Thus an unexpectedly wide range of spatio-temporal tuning matches motion detection to visual ecology. 相似文献
84.
S. F. Wang J. C. Braekman D. Daloze J. Pasteels P. Soetens N. V. Handjieva P. Kalushkov 《Cellular and molecular life sciences : CMLS》1996,52(6):628-630
The isolation of N-quinaldyl-L-arginine·HCl (1) from the CoccinellidaeSubcoccinella-24-punctata is reported. The structure, first established on the basis of the analysis of the spectral properties of1, has been confirmed by synthesis. The alkaloid is of endogenous origin and markedly deterrent to ants. 相似文献
85.
86.
E. A. Morguenstern M. I. Vacas M. I. Keller Sarmiento D. P. Cardinali 《Cellular and molecular life sciences : CMLS》1984,40(2):223-224
Summary Pinealectomy (Px) in adult male rats resulted in increased cyclic AMP accumulation by medial basal hypothalamic (MBH) explants 3 and 7 days after surgery. 24 h after superior cervical ganglionectomy (Gx) an augmented MBH cyclic AMP accumulation was observed. The effects of Px and Gx were additive, as revealed in animals subjected to Gx 3 days after Px.This study was supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (No 6638) and from the Fundación Alberto J. Roemmers, Argentina. 相似文献
87.
Summary The intensity of sound-induced convulsions in the genetically epilepsy-prone rat (GEPR) was reduced in a dose related fashion by intracerebroventricular administration of dobutamine, (1 agonist), terbutaline (2 agonist) or phenylephrine (1 agonist). BHT-920 (2 agonist) did not cause a dose-related decrease in sound-induced convulsion intensity. Binding studies showed that whole brain and receptor densities (Bmax) were normal while the Kd was increased for the ligand in GEPR brain.Acknowledgment. We are most grateful to Boehringer Ingelheim for generously supplying BHT 920. We are also indebted to Ciba-Geigy Corporation for the gift of terbutaline hydrochloride and phentolamine hydrochloride. The work was supported in part by NIH grant NS 16829. 相似文献
88.
F. M. Goñi J. M. Valpuesta M. C. Barbero E. Rial J. I. G. Gurtubay J. M. Macarulla 《Cellular and molecular life sciences : CMLS》1984,40(2):193-195
Summary The solubilizing effect of Triton X-100 on beef heart submitochondrial particles (ETPH) has been studied under various physiological conditions. Coupled, uncoupled and azide-inhibited ETPH particles have been studied. Quantitative and qualitative differences are found in the proteins solubilized by the detergent from ETPH particles under the various conditions tested.Acknowledgments: This work was supported in part by a grant from a Spanish Comisión Asesora para la Investigación Cientifica y Técnica. M.C. Barbero was recipient of a scholarship from the Basque Government. 相似文献
89.
Summary The leaf of the bitter oliveOlea europaea (Oleaceae) is heavily coated by almost pure oleanolic acid, which forms part of a multichemical defense against fungal attack.We thank Mr K. A. Hoelmer for taking the electron micrographs and also Professor T. Kamikawa for NMR measurements. 相似文献
90.