全文获取类型
收费全文 | 286篇 |
免费 | 3篇 |
国内免费 | 2篇 |
专业分类
化学 | 152篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 81篇 |
物理学 | 52篇 |
出版年
2022年 | 7篇 |
2021年 | 10篇 |
2019年 | 3篇 |
2018年 | 6篇 |
2016年 | 6篇 |
2015年 | 6篇 |
2014年 | 8篇 |
2013年 | 13篇 |
2012年 | 10篇 |
2011年 | 12篇 |
2010年 | 15篇 |
2009年 | 17篇 |
2008年 | 18篇 |
2007年 | 15篇 |
2006年 | 10篇 |
2005年 | 18篇 |
2004年 | 14篇 |
2003年 | 10篇 |
2002年 | 5篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1964年 | 1篇 |
1961年 | 1篇 |
1958年 | 1篇 |
1935年 | 1篇 |
排序方式: 共有291条查询结果,搜索用时 15 毫秒
101.
Complete classification of formally real fields with 8 square classes with respect to the behaviour of quadratic forms is given. Two fields F and K are equivalent with respect to quadratic forms if the quadratic form schemes of the two fields are isomorphic or in other words, if the Witt rings W(F) and W(K) are isomorphic. It is shown here that for formally real fields with 8 square classes there are exactly seven possible quadratic form schemes and for each of the seven schemes a formally real field with 8 square classes and the given scheme is constructed. 相似文献
102.
The aim of the present paper is to find a necessary and sufficient condition for the existence of the operator of the total number of particles in a representation of canonical commutation relations. The result is formulated by means of generating functionals of representations. It is shown that an irreducible representation possesses a (generalized) number operator if and only if the representation obtained by averaging its generating functional over the group of phase transformations is a factor representation of type I. 相似文献
103.
Jan Florek 《Journal of Graph Theory》2014,75(4):355-357
Isaak posed the following problem. Suppose T is a tournament having a minimum feedback arc set, which induces an acyclic digraph with a hamiltonian path. Is it true that the maximum number of arc‐disjoint cycles in T equals the cardinality of minimum feedback arc set of T? We prove that the answer to the problem is in the negative. 相似文献
104.
105.
106.
The paper applies generalized power series to determine effectively the lower critical hydrostatic pressure for imperfect cylindrical shells. The basic equations were assumed in the form derived by W.A. Nash. Two variants as regards the number of circumferential half-wavelengths are discussed: m constant during buckling, thus corresponding to upper critical pressure, and m furnishing minimal value of lower critical pressure. It is found that the dependence of lower critical pressure on an imperfection parameter V is linear, i.e. smaller than in the case of the upper critical pressure which decreases with . 相似文献
107.
Kazimierz Chmurski Antony W. Coleman Janusz Jurczak 《Journal of carbohydrate chemistry》2013,32(7):787-796
ABSTRACT The clean one step synthesis of the amphiphilic α-, β-, and γ-cyclodextrins starting from per-(6-bromo-6-deoxy)-α-, -β-, and -γ-cyclodextrins is described. The role of the lipophilic tail is played by various aryl groups (phenyl, p-bromophenyl, p-O-butoxyphenyl, p-pentylphenyl, and o-, m-, and p-nitrophenyl) linked by a thioether bridge to the position C-6 of each glucopyranose unit. The yields of the S-alkylation reactions were very high (85-95%). 相似文献
108.
Alkali metal cation extraction behaviour for two series of 1,3-alternate, mono-ionisable calix[4]arene-benzocrown-6 compounds is examined. In Series 1, the proton-ionisable group (PIG) is a substituent on the benzo group of the polyether ring that directs it away from the crown ether cavity. In Series 2, the PIG is attached to one para position in the calixarene framework, thereby positioning it over the crown ether ring. Competitive solvent extraction of alkali metal cations from aqueous solutions into chloroform shows high Cs+ efficiency and selectivity. Single-species extraction pH profiles of Cs+ for Series 1 and 2 ligands with the same PIG are very similar. Thus, association of Cs+ with the calixcrown ring is more important than the position of the PIG relative to the crown ether cavity. Solid-state structures of two unionised ligands from Series 2 are presented. Also described is a crystal containing two different ionised ligand–Cs+ complexes. 相似文献
109.
Krystyna Skalicka‐Woźniak Magdalena Walasek Agnieszka Ludwiczuk Kazimierz Głowniak 《Journal of separation science》2013,36(16):2611-2614
High‐performance counter‐current chromatography was successfully used for the isolation and purification of terpenoid compounds from the essential oil of Pimpinella anisum L. A two‐phase solvent system composed of n‐heptane/methanol/ethyl acetate/water (5:2:5:2, v/v/v/v) was suitable for the purification of linalool, terpinen‐4‐ol, α‐terpineol, p‐anisaldehyde, while n‐heptane/methanol (1:1, v/v) was used for the isolation of anethole and foeniculin. A scale‐up process from analytical to preparative was developed. Additionally, a stepwise gradient elution was applied and instead of two different runs, 40 min each, one 80 min separation was performed; although the time of separation remains the same, it was possible to repeat the efficiency even if the water‐containing mobile phase was changed to a nonaqueous system. The obtained essential oil, as well as purified compounds, was analyzed by GC. A total of 0.64 mg of linalool, 0.52 mg of terpinen‐4‐ol, 0.10 mg of α‐terpineol, 0.62 mg of p‐anisaldehyde, 15 mg of anethole, and 2.12 mg of foeniculin were obtained from 210 mg of the essential oil of P. anisum L. in a short time with purities of 99, 98, 94, 93.54, 93, and 93.6%, respectively. 相似文献
110.
Eunice Yanez Barrientos Kazimierz Wrobel Adolfo Lopez Torres Felix Gutiérrez Corona Katarzyna Wrobel 《Analytical and bioanalytical chemistry》2013,405(7):2397-2404
In the present work, application of the previously established reversed-phase liquid chromatography procedure based on fluorescent labeling of cytosine and methylcytosine moieties with 2-bromoacetophenone (HPLC-FLD) is presented for simultaneous evaluation of global DNA and total RNA methylation at cytosine carbon 5. The need for such analysis was comprehended from the recent advances in the field of epigenetics that highlight the importance of non-coding RNAs in DNA methylation and suggest that RNA methylation might play a similar role in the modulation of genetic information, as previously demonstrated for DNA. In order to adopt HPLC-FLD procedure for DNA and RNA methylation analysis in a single biomass extract, two extraction procedures with different selectivity toward nucleic acids were examined, and a simplified calibration was designed allowing for evaluation of methylation percentage based on the ratio of chromatographic peak areas: cytidine/5-methylcytidine for RNA and 2´-deoxycytidine/5-methyl-2′-deoxycytidine for DNA. As a proof of concept, global DNA and total RNA methylation were determined in Lepidium sativum hydroponically grown in the presence of different Cd(II) or Se(IV) concentrations, expecting that plant exposure to abiotic stress might affect not only global DNA but also total RNA methylation. The results obtained showed the increase of DNA methylation in the treated plants up to concentration levels 2 mg?L?1 Cd and 1 mg?L?1 Se in the growth medium. For higher stressors’ concentration, global DNA methylation tended to decrease. Most importantly, an inverse correlation was found between DNA and RNA methylation levels (r?=??0.6788, p?=?0.031), calling for further studies of this particular modification of nucleic acids in epigenetic context. 相似文献