全文获取类型
收费全文 | 296篇 |
免费 | 4篇 |
专业分类
化学 | 194篇 |
力学 | 1篇 |
数学 | 84篇 |
物理学 | 21篇 |
出版年
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 11篇 |
2015年 | 6篇 |
2014年 | 6篇 |
2013年 | 21篇 |
2012年 | 14篇 |
2011年 | 18篇 |
2010年 | 9篇 |
2009年 | 12篇 |
2008年 | 16篇 |
2007年 | 11篇 |
2006年 | 16篇 |
2005年 | 12篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1922年 | 2篇 |
排序方式: 共有300条查询结果,搜索用时 171 毫秒
211.
Rolando Cavazos-Cadena 《Applied Mathematics and Optimization》1992,26(2):171-194
We consider discrete-timeaverage reward Markov decision processes with denumerable state space andbounded reward function. Under structural restrictions on the model the existence of an optimal stationary policy is proved; both the lim inf and lim sup average criteria are considered. In contrast to the usual approach our results donot rely on the average regard optimality equation. Rather, the arguments are based on well-known facts fromRenewal Theory.This research was supported in part by the Consejo Nacional de Ciencia y Tecnologia (CONACYT) under Grants PCEXCNA 040640 and 050156, and by SEMAC under Grant 89-1/00ifn$. 相似文献
212.
In previous papers [MS 1, 2], we considered stationary critical points of solutions of the initial-boundary value problems
for the heat equation on bounded domains in ℝN,N ≧ 2. In [MS 1], we showed that a solutionu has a stationary critical pointO if and only ifu satisfies a certain balance law with respect toO for any time. Furthermore, we proved necessary and sufficient conditions relating the symmetry of the domain to the initial
datau
0; in this way, we gave a characterization of the ball in ℝN([MS 1]) and of centrosymmetric domains ([MS 2]). In the present paper, we consider a rotationA
dby an angle 2π/d,d ≧ 2 for planar domains and give some necessary and some sufficient conditions onu
0 which relate to domains invariant underA
d. We also establish some conjectures.
This research was partially supported by a Grant-in-Aid for Scientific Research (C) (# 10640175) and (B) (# 12440042) of the
Japan Society for the Promotion of Science. The first author was supported also by the Italian MURST. 相似文献
213.
The effect of specifically adsorbed halides and pseudohalides upon the kinetics of BrO4? electroreduction, already investigated by the authors and Cozzi [7] and, more recently, by de Levie and Nemes [8], is further examined. The inadequacy of a simple diffuse-layer correction for the kinetics of BrO4? electroreduction and the “direct” effect of the charge density qi at the inner Helmholtz plane (qi-effect) upon such kinetics are stressed. An alternative method for estimating the qi-effect, based on the use of corrected Tafel plots at constant qi, is proposed. 相似文献
214.
Rolando Cavazos-Cadena 《Applied Mathematics and Optimization》1989,19(1):97-112
An average-reward Markov decision process (MDP) with discretetime parameter, denumerable state space, and bounded reward function is considered. With such a model, we associate a family of MDPs. Then, we determinenecessary conditions for the existence of a bounded solution to the optimality equation for each one of the models in the family. Moreover,necessary andsufficient conditions are given so that the optimality equations have a bounded solution with an additional property.This research was supported in part by the Consejo Nacional de Ciencia y Tecnología (CONACYT) under Grant No. PCEXCNA-040640.Dedicated to Professor Eutimio Alberto Cuéllar-Goríbar on the occasion of his eightieth birthday 相似文献
215.
Central limit theorems for local martingales 总被引:4,自引:0,他引:4
Rolando Rebolledo 《Probability Theory and Related Fields》1980,51(3):269-286
216.
We describe a biomimetic approach to (Z)-β-fluoroallyl alcohols based on the two carbon homologation of aromatic aldehydes to α-fluorocinnamic thioesters by Horner–Wadsworth–Emmons reaction with 2-(diethoxyphosphinyl)-2-fluoro-ethanethioic acid, S-ethyl ester, followed by reduction with sodium borohydride in mild conditions. The α-fluorothioesters were obtained in a good yield by condensing the aldehydes with the lithium anion of 2-(diethoxyphosphinyl)-2-fluoro-ethanethioic acid, S-ethyl ester in THF at ?78 °C. The (E,Z)-α-fluorocinnamic thioester mixtures were then cleanly reduced with double bond isomerisation to the corresponding (Z)-β-fluoroallyl alcohols by NaBH4 at room temperature. This methodology may be applied to highly functionalized aldehydes as exemplified by the straightforward access to (Z)-β-fluoroconiferin, a strong inhibitor of lignin polymerization from O-glucosylated vanillin. 相似文献
217.
Lozada-Garcia RR Ceponkus J Chevalier M Chin W Mestdagh JM Crépin C 《Angewandte Chemie (International ed. in English)》2012,51(28):6947-6950
A noteworthy example of a molecule with coupled large-amplitude motions is provided by acetylacetone (methyl group torsions and intramolecular hydrogen bonds). The molecule was trapped in solid parahydrogen to investigate the complex proton tunneling processes. Nuclear spin conversion in methyl groups is observed and, combined with IR spectra, documents the coupling between high frequency modes and large amplitude motions. 相似文献
218.
Rolando Magnanini 《Applicable analysis》2013,92(2-3):185-192
We study the regularity up to the boundary of solutions to the boundary value problem:[math001] in D, ∣?u∣= g on &;pardD, where D is the unit disc. This problem finds its application in the study of geophysical and geomagnetic surveys. If g?C,[math001](D) and is strictly positive, we prove that uis in the Holder class C1,α(D). An example shows that this is no longer true if g has some zeroes on ?D. In this case u isproved to be of class C1(D) 相似文献
219.
Rolando Luna-García Victor Barba Hiram I. Beltrán 《Journal of organometallic chemistry》2009,694(24):3965-3972
Reaction of R2SnCl2 (R = Me, nBu, Ph) and the potassium salts of salenN3H3 (N,N′-bis(salicylidene)diethylenetriamine) and saleanN3H5 (N,N′-bis(o-hydroxybenzyl)diethylenetriamine) provided diorganotin(IV) complexes of the composition [Me2Sn(salenN3H)]·solvate (solvate = 2.5H2O, MeOH or DMSO), [nBu2Sn(salenN3H)]·H2O, [Ph2Sn(salenN3H)]·2EtOH and [Me2Sn(saleanN3H3)]·2.5H2O. In all compounds the tin atoms are seven-coordinate and have pentagonal-bipyramidal coordination environments, in which the organic substituents attached to the tin atoms occupy the axial positions. This occurs both in solution and the solid state; however, in solution the molecules are involved in conformational equilibria that require the presence of intermediates, in which the N → Sn bonds are dissociated. Although the [saleanN3H3]2− ligand is more flexible and basic, a very similar complexing behavior to that of [salenN3H]2− has been found, and there is evidence that it is even a weaker ligand. Both ligands show the tendency to adopt a curved conformation within the complex, thus indicating that the dynamic process resembles the flapping of butterfly wings. However, the folding is reduced with increasing steric bulk of the organic substitutents attached to the tin atoms. The six-membered heterocyclic rings in the [R2Sn(salenN3H)] derivatives have envelope conformation, while those in [Me2Sn(saleanN3H3)] have distorted boat-conformation. Thus, small changes in the hybridization and basicity of the nitrogen atoms cause significant differences of the stability and the dynamic behavior of the resulting molecules. 相似文献
220.
Giovanni Pezzatini Maria Rosa Moncelli Rolando Guidelli 《Journal of Electroanalytical Chemistry》1980,113(2):311-425
Kinetic measurements of diethylfumarate (DEF) electroreduction to diethylsuccinate in aqueous solutions of pH ranging from 1 to 10 were carried out by the polarographic technique under different experimental conditions, and DEF adsorption was measured by the chronocoulometric technique. The rate changes produced by changes in pH, in buffer concentration, in the potential d at the outer Helmholtz plane or in the charge density σi at the inner Helmholtz plane due to specifically adsorbed iodide ions, were thoroughly examined. The effects of these changes are amenable to a straightforward interpretation if the overall electrode process is assumed to be controlled by an enol-keto transformation of the enol form of diethylsuccinate for 1
(with RэDEF) for 5