全文获取类型
收费全文 | 5688篇 |
免费 | 514篇 |
国内免费 | 650篇 |
专业分类
化学 | 1739篇 |
晶体学 | 25篇 |
力学 | 215篇 |
综合类 | 127篇 |
数学 | 3929篇 |
物理学 | 817篇 |
出版年
2023年 | 53篇 |
2022年 | 92篇 |
2021年 | 113篇 |
2020年 | 151篇 |
2019年 | 189篇 |
2018年 | 174篇 |
2017年 | 172篇 |
2016年 | 210篇 |
2015年 | 158篇 |
2014年 | 268篇 |
2013年 | 437篇 |
2012年 | 246篇 |
2011年 | 326篇 |
2010年 | 287篇 |
2009年 | 376篇 |
2008年 | 401篇 |
2007年 | 350篇 |
2006年 | 345篇 |
2005年 | 260篇 |
2004年 | 237篇 |
2003年 | 264篇 |
2002年 | 236篇 |
2001年 | 154篇 |
2000年 | 182篇 |
1999年 | 141篇 |
1998年 | 105篇 |
1997年 | 142篇 |
1996年 | 104篇 |
1995年 | 75篇 |
1994年 | 60篇 |
1993年 | 49篇 |
1992年 | 39篇 |
1991年 | 34篇 |
1990年 | 22篇 |
1989年 | 10篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1985年 | 31篇 |
1984年 | 38篇 |
1983年 | 22篇 |
1982年 | 20篇 |
1981年 | 50篇 |
1980年 | 27篇 |
1979年 | 13篇 |
1978年 | 27篇 |
1977年 | 31篇 |
1976年 | 25篇 |
1975年 | 17篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有6852条查询结果,搜索用时 15 毫秒
1.
A bounded linear operator T acting on a Hilbert space is called Coburn operator if ker(T ? λ) = {0} or ker(T ? λ)*= {0} for each λ ∈ C. In this paper, the authors define other Coburn type properties for Hilbert space operators and investigate the compact perturbations of operators with Coburn type properties. They characterize those operators for which has arbitrarily small compact perturbation to have some fixed Coburn property.Moreover, they study the stability of these properties under small compact perturbations. 相似文献
2.
3.
利用Galerkin方法,研究了一类具有结构阻尼的kirchhoff型波动方程,方程是截面弹性杆运动的模型.通过各种不等式技巧及算子半群理论,证明了方程的解半群具有全局吸引子. 相似文献
4.
5.
Geetika Wadhwa Kowthavarapu Venkata Krishna Rajeev Taliyan Neeraj Tandon Satyapal Singh Yadav Dipankar Banerjee Avinash Narwaria Chandrakant Katiyar Sunil Kumar Dubey 《Biomedical chromatography : BMC》2022,36(2):e5275
Trigonelline (TR), 4-hydroxyisoleucine (4-HI), and diosgenin (DG) are the main bioactives of the purified standardized extract of the popular plant Trigonella foenum-graecum L. (TFG), and it has been proven effective for the treatment of various diseases. However, to the best of our knowledge, no study has investigated the pharmacokinetic parameters of purified standardized T. foenum-graecum extract in normal and diabetic Wistar rats. The present study has developed and validated a rapid, reliable, and sensitive simultaneous ultra-performance liquid chromatography MS method to estimate these bioactives. The chromatographic separation was achieved using methanol, acetonitrile, and 0.1% formic acid with the ideal gradient flow system on a BEH Shield RP 18 column. A positive electrospray ionization mode was selected to estimate m/z values of TR (138.14 > 94.63), 4-HI (148.19 > 74.08), and DG (415.54 > 271.33). The method was robust and reproducible over the linearity range of 60–5000, 6–5000, and 15–5000 ng/mL for TR, 4-HI, and DG, respectively. Using this novel validated method, we investigated the pharmacokinetic parameters of bioactives using Phoenix WinNonlin version 8.0 (Certera) in normal and diabetic rats. The assay was successfully applied for the estimation of pharmacokinetic parameters using noncompartmental analysis. This investigation shows that the absorption rate increased, whereas distribution and elimination processes slowed down in diabetic rats compared with normal rats. 相似文献
6.
《Arabian Journal of Chemistry》2022,15(11):104272
Yinlan lipid regulatory capsule (YL) is a composite traditional Chinese medicine (TCM) new drug to alleviate hyperlipidemia, while its therapeutic mechanism in vivo was not clarified with nontargeted metabolomics investigation. An animal model was established in rats fed a high-fat diet, and their body weights, body mass index (BMI) and blood cholesterol levels were measured. Serum, liver and kidney tissue samples were also extracted for PXR-CYP3A4-ABCB1-FXR signaling pathway research using PCR and UHPLC–MS. The obtained plasma samples were analyzed by UHPLC-Q-TOF-MS metabolomic investigation, which revealed PXR-CYP3A4-related metabolites and changes induced by YL. Finally, the key metabolites were chosen as index components, and their levels in the serum, liver, small intestine and bile were used for simultaneous UHPLC–MS-MS determination. The results indicated that YL was effective in rebalancing blood TG and TC levels (compared to controls). With respect to the PXR-CYP3A4-ABCB1 pathway, as a result of YL’s effect, gene expression or activity of the two targets decreased significantly in both the liver and kidney. The same trend was observed in the serum samples mentioned above. Metabolomics screening and data revealed that 44 metabolites can be regarded as biomarkers related to hyperlipidemia, fatty acids synthesis, and body energy consumption, as well as synthesis, transportation and exertion of cholesterol. YL’s treatment focused on 26 of them, primarily bile acids, indicating that the antihyperlipidemic effect of this drug lies in its inhibitory activity of cholesterol metabolism. Subsequent analysis of those in vivo components revealed that significant increases (compared to the model group) occurred in the blood, liver, small intestine and bile in groups that received medium and high doses of YL (while the low dose was relatively unchanged). Those target components exhibit a close relationship with PXR and/or CYP3A4. The use of YL repressed PXR expression and subsequently decreased CYP3A4 activity. As a result, synthesis of related bile acids increased, while cholesterol levels decreased, consequently leading to the attenuation of hyperlipidemia. This study comprehensively investigated the antihyperlipidemia mechanism of YL based on its repression of PXR-CYP3A4 activity and related metabolite yield, establishing an accurate method for evaluating the therapeutic effect of YL. 相似文献
7.
学科大概念向下统领学科内核心知识,向上构成学科核心素养的基本框架,指向学科结构的中心,因此它不只是一个聚合概念,也是连接学科知识与学科核心素养的工具和媒介[1,2].以"机械能守恒定律"教学为例探讨基于大情境,促进还原与下沉,激活学科大概念;基于大问题,促进探究与整合,建构学科大概念;基于大反思,促进反思与上浮,迁移学科大概念. 相似文献
8.
9.
10.