全文获取类型
收费全文 | 225篇 |
免费 | 2篇 |
国内免费 | 8篇 |
专业分类
理论与方法论 | 3篇 |
现状及发展 | 71篇 |
研究方法 | 47篇 |
综合类 | 105篇 |
自然研究 | 9篇 |
出版年
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 6篇 |
2014年 | 2篇 |
2013年 | 5篇 |
2012年 | 27篇 |
2011年 | 27篇 |
2010年 | 9篇 |
2009年 | 3篇 |
2008年 | 24篇 |
2007年 | 22篇 |
2006年 | 9篇 |
2005年 | 12篇 |
2004年 | 11篇 |
2003年 | 5篇 |
2002年 | 7篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1994年 | 3篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有235条查询结果,搜索用时 171 毫秒
31.
32.
33.
E. Berlin L. Hellgren O. Thulesius J. Vincent 《Cellular and molecular life sciences : CMLS》1980,36(2):197-198
Summary The present study demonstrates a powerful vasoconstrictor activity of prostaglandin-like substances (PLS), extracted fromP. acnes, on human blood vessels. PLS is about equipotent to PGE2 in its effect on human umbilical vessels, but the contractile response pattern is different. PLS therefore seems to have specific and different physiological characteristics. 相似文献
34.
35.
Summary The phenetic relationships between the families of the Hamamelididae subclass were studied by using the following multifactorial analytical methods: principal components analysis (PCA) and reciprocal averaging (RA). The results obtained agree appreciably with Takhtajan's classification system.
Remerciements. Nous remercions Mlle Sylvie Laliberté, Institut botanique de l'Université de Montréal, pour sa révision du manuscrit; M. Michel Labrecque, Institut botanique de l'Université de Montréal, pour avoir dessiné les figures; de même que M. André Bouchard, Jardin Botanique de Montréal et Institut botanique de l'Université de Montréal, de nous avoir permis d'utiliser les programmes dont il se sert pour ses travaux d'écologie. 相似文献
Remerciements. Nous remercions Mlle Sylvie Laliberté, Institut botanique de l'Université de Montréal, pour sa révision du manuscrit; M. Michel Labrecque, Institut botanique de l'Université de Montréal, pour avoir dessiné les figures; de même que M. André Bouchard, Jardin Botanique de Montréal et Institut botanique de l'Université de Montréal, de nous avoir permis d'utiliser les programmes dont il se sert pour ses travaux d'écologie. 相似文献
36.
J. D. Vincent B. Dufy J. M. A. Faure 《Cellular and molecular life sciences : CMLS》1970,26(11):1266-1267
Résumé Certains neurones hypothalamiques présentent dans la demi-heure qui suit une stimulation vaginale des modifications de leur décharge spontanée indépendantes de l'état de vigilance de l'animal. Ces variations pourraient être en rapport avec les mécanismes nerveux de l'ovulation provoquée chez la Lapine.
We are very grateful to Dr. J. N.Hayward, Department of Anatomy, University of California, Los Angeles, for his constructive criticisms. 相似文献
We are very grateful to Dr. J. N.Hayward, Department of Anatomy, University of California, Los Angeles, for his constructive criticisms. 相似文献
37.
38.
Snodgrass C Tubiana C Vincent JB Sierks H Hviid S Moissl R Boehnhardt H Barbieri C Koschny D Lamy P Rickman H Rodrigo R Carry B Lowry SC Laird RJ Weissman PR Fitzsimmons A Marchi S;OSIRIS team 《Nature》2010,467(7317):814-816
The peculiar object P/2010?A2 was discovered in January 2010 and given a cometary designation because of the presence of a trail of material, although there was no central condensation or coma. The appearance of this object, in an asteroidal orbit (small eccentricity and inclination) in the inner main asteroid belt attracted attention as a potential new member of the recently recognized class of main-belt comets. If confirmed, this new object would expand the range in heliocentric distance over which main-belt comets are found. Here we report observations of P/2010?A2 by the Rosetta spacecraft. We conclude that the trail arose from a single event, rather than a period of cometary activity, in agreement with independent results. The trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid. The shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately. We determine that this was an asteroid collision that occurred around 10 February 2009. 相似文献
39.
Sven Herrmann Katharina T. Huber Vincent Moulton Andreas Spillner 《Journal of Classification》2012,29(3):321-340
A k-dissimilarity D on a finite set X, |X|????k, is a map from the set of size k subsets of X to the real numbers. Such maps naturally arise from edgeweighted trees T with leaf-set X: Given a subset Y of X of size k, D(Y ) is defined to be the total length of the smallest subtree of T with leaf-set Y . In case k?=?2, it is well-known that 2-dissimilarities arising in this way can be characterized by the so-called ??4-point condition??. However, in case k?>?2 Pachter and Speyer (2004) recently posed the following question: Given an arbitrary k-dissimilarity, how do we test whether this map comes from a tree? In this paper, we provide an answer to this question, showing that for k????3 a k-dissimilarity on a set X arises from a tree if and only if its restriction to every 2?k-element subset of X arises from some tree, and that 2?k is the least possible subset size to ensure that this is the case. As a corollary, we show that there exists a polynomial-time algorithm to determine when a k-dissimilarity arises from a tree. We also give a 6-point condition for determining when a 3-dissimilarity arises from a tree, that is similar to the aforementioned 4-point condition. 相似文献
40.
Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots. Our work may offer new directions in terms of control and tunability for molecular spintronics. 相似文献