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   794篇
  免费   32篇
  国内免费   3篇
化学   587篇
晶体学   2篇
力学   10篇
数学   151篇
物理学   79篇
  2023年   9篇
  2022年   9篇
  2021年   26篇
  2020年   30篇
  2019年   27篇
  2018年   11篇
  2017年   13篇
  2016年   27篇
  2015年   27篇
  2014年   27篇
  2013年   43篇
  2012年   68篇
  2011年   56篇
  2010年   32篇
  2009年   32篇
  2008年   38篇
  2007年   41篇
  2006年   42篇
  2005年   43篇
  2004年   39篇
  2003年   30篇
  2002年   23篇
  2001年   7篇
  2000年   6篇
  1998年   5篇
  1997年   11篇
  1996年   8篇
  1995年   5篇
  1994年   8篇
  1993年   4篇
  1991年   3篇
  1990年   3篇
  1989年   4篇
  1987年   4篇
  1985年   5篇
  1983年   2篇
  1982年   4篇
  1981年   2篇
  1978年   4篇
  1973年   3篇
  1967年   2篇
  1939年   2篇
  1935年   2篇
  1933年   2篇
  1931年   2篇
  1925年   2篇
  1923年   2篇
  1918年   3篇
  1917年   2篇
  1915年   2篇
排序方式: 共有829条查询结果,搜索用时 15 毫秒
81.
A new linker cis-5-aminopent-3-enoic acid (cis-Apa) was prepared for the synthesis of cyclic pseudopeptides by cyclization-cleavage by using ring-closing methatesis (RCM). We developed a new synthetic pathway for the preparation of the cis-Apa linker that was tested in the cyclization-cleavage process of different RGD peptide sequences. Different macrocyclic peptidomimetics were prepared by using this integrated microwave-assisted method, showing that the readily available cis-Apa amino acid is well adapted as a linker in the cyclization-cleavage process.  相似文献   
82.
The binding of seven multivalent glycoconjugates displaying linear or antenna-like structures and different electronic environments were evaluated towards PA-IL on a DNA-based carbohydrate microarray. The affinity can be modulated by the charge and the topology of the galactosylated derivatives.  相似文献   
83.
84.
In this lecture notes, we will discuss the classical Aleksandrov-Fenchel inequalities for quermassintegrals on convex domains and pose the problem of how to extend the inequalities to non-convex domains. We will survey some recent progress on the problem, then report some of our joint work [9] in which we generalize the k-th stage of the inequalities to a class of (k + 1)-convex domains. Our proof, following the earlier work of Castillion [8] for k =  1 case of the inequalities, uses the method of optimal transport.  相似文献   
85.
Twenty-two novel, variously substituted nitroazetidines were designed as both sulfonamide and urethane vinylogs possibly endowed with antimicrobial activity. The compounds under study were obtained following a general procedure recently developed, starting from 4-nitropentadienoates deriving from a common β-nitrothiophenic precursor. While being devoid of any activity against fungi and Gram-negative bacteria, most of the title compounds performed as potent antibacterial agents on Gram-positive bacteria (E. faecalis and three strains of S. aureus), with the most potent congener being the 1-(4-chlorobenzyl)-3-nitro-4-(p-tolyl)azetidine 22, which displayed potency close to that of norfloxacin, the reference antibiotic (minimum inhibitory concentration values 4 and 1–2 μg/mL, respectively). Since 22 combines a relatively efficient activity against Gram-positive bacteria and a cytotoxicity on eucharyotic cells only at 4-times higher concentrations (inhibiting concentration on 50% of the cultured eukaryotic cells: 36 ± 10 μM, MIC: 8.6 μM), it may be considered as a promising hit compound for the development of a new series of antibacterials selectively active on Gram-positive pathogens. The relatively concise synthetic route described herein, based on widely available starting materials, could feed further structure–activity relationship studies, thus allowing for the fine investigation and optimization of the toxico-pharmacological profile.  相似文献   
86.
An efficient Heck coupling of 2-hydroxy-3-iodo-naphthoquinone with a series of electron-deficient alkenes in aqueous solution has been accomplished. The method is characterized by simple conditions and facile work-up to isolate the products in good to excellent yields. The products contain the motif present in several naphthoquinone pigments but with enhanced polarity.  相似文献   
87.
This paper presents a mixed-integer programming formulation to find optimal solutions for the block layout problem with unequal departmental areas arranged in flexible bays. The nonlinear department area constraints are modeled in a continuous plane without using any surrogate constraints. The formulation is extensively tested on problems from the literature.  相似文献   
88.
We give a detailed analysis of the proportion of elements in the symmetric group on n points whose order divides m, for n sufficiently large and mn with m=O(n).   相似文献   
89.
The objective is to investigate the presence of dysphonic symptoms in multiple sclerosis (MS) patients and to compare quantitative acoustic parameters in multiple sclerosis patients and normal individuals. The method of study was an 8-month controlled cross-sectional that was carried out with 106 individuals (30 MS, 76 controls). Both groups included males and females from 20 to 55 years. Exclusion criteria were prior vocal disorder, laryngeal microsurgery, recent endotracheal intubation, tumors, laryngeal, lung or mediastinal metastases, respiratory disease, and other associated neurological diagnoses. For dysphonic symptoms (qualitative variables), associations were assessed using Mantel-Haenszel's chi2 test, with Yates correction or the Fisher exact test when necessary. Statistical significance was set at p< or =0.05. Dysphonia was observed in 70% of MS individuals versus 33% of controls (p=0.01). Association was found between MS and dysphonia (OR: 2.2, CI 95%: 1.13-4.25). Fundamental frequency was higher among MS patients (p=0.01). Fundamental frequency deviation was significantly higher in MS women (but not men) than controls (p=0.00). Jitter was higher in MS men than in all other groups (p=0.00). Results suggest that evaluation and treatment of MS patients should be revised, evaluating voice alterations in relation to other signs. MS seems to intensify gender effect on fundamental frequency deviation, noise, and jitter, with MS women presenting fewer voice variations than men.  相似文献   
90.
One of the main problems in phylogenetics is to find good approximations of metrics by weighted trees. As an aid to solving this problem, it could be tempting to consider optimal realizations of metrics—the guiding principle being that, the (necessarily unique) optimal realization of a tree metric is the weighted tree that realizes this metric. And, although optimal realizations of arbitrary metrics are, in general, not trees, but rather weighted networks, one could still hope to obtain a phylogenetically informative representation of a given metric, maybe even more informative than the best approximating tree. However, optimal realizations are not only difficult to compute, they may also be non-unique. Here we focus on one possible way out of this dilemma: hereditarily optimal realizations. These are essentially unique, and can be described in a rather explicit way. In this paper, we recall what a hereditarily optimal realization of a metric is and how it is related to the 1-skeleton of the tight span of that metric, and we investigate under what conditions it coincides with this 1-skeleton. As a consequence, we will show that hereditarily optimal realizations for consistent metrics, a large class of phylogentically relevant metrics, can be computed in a straight-forward fashion. Received August 26, 2004  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