全文获取类型
收费全文 | 729篇 |
免费 | 36篇 |
国内免费 | 5篇 |
专业分类
化学 | 441篇 |
晶体学 | 2篇 |
力学 | 32篇 |
数学 | 169篇 |
物理学 | 126篇 |
出版年
2022年 | 4篇 |
2021年 | 11篇 |
2020年 | 28篇 |
2019年 | 16篇 |
2018年 | 21篇 |
2017年 | 7篇 |
2016年 | 20篇 |
2015年 | 13篇 |
2014年 | 25篇 |
2013年 | 42篇 |
2012年 | 57篇 |
2011年 | 50篇 |
2010年 | 28篇 |
2009年 | 21篇 |
2008年 | 42篇 |
2007年 | 33篇 |
2006年 | 35篇 |
2005年 | 37篇 |
2004年 | 29篇 |
2003年 | 26篇 |
2002年 | 23篇 |
2001年 | 12篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 5篇 |
1996年 | 5篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1934年 | 3篇 |
1931年 | 4篇 |
1914年 | 4篇 |
1908年 | 3篇 |
1906年 | 4篇 |
1905年 | 3篇 |
1904年 | 11篇 |
1901年 | 5篇 |
1880年 | 3篇 |
1873年 | 3篇 |
排序方式: 共有770条查询结果,搜索用时 15 毫秒
101.
Hodgkiss JM Damrauer NH Pressé S Rosenthal J Nocera DG 《The journal of physical chemistry. B》2006,110(38):18853-18858
The temperature-isotope dependence of proton-coupled electron transfer (PCET) for a noncovalent molecular dyad is reported. The system consists of an excited-state Zn(II) porphyrin that transfers an electron to a naphthalene diimide acceptor through an amidinium-carboxylate interface. Two different isotope effects are observed for variant temperature regimes. A reverse isotope effect (i.e., kH/kD < 1) is observed as T approaches 120 K (kH/kD = 0.9, 120 K), whereas a normal isotope effect (i.e., kH/kD > 1) is recovered as the temperature is increased (kH/kD = 1.2, 300 K). The transition between these limits is smooth, with a crossover temperature of T approximately 160 K. These observations are in accordance with charge-transfer dynamics that are susceptible to bath-induced fluctuations in the proton coordinate. 相似文献
102.
103.
Jin-Long Chen Yuo-Hsien Shiau Hung-Wen Chiu Niels Wessel Jürgen Kurths Woei-Chyn Chu 《Physica A》2010,389(9):1861-1868
Despite many previous studies on the association between hyperthyroidism and the hyperadrenergic state, controversies still exist. Detrended fluctuation analysis (DFA) is a well recognized method in the nonlinear analysis of heart rate variability (HRV), and it has physiological significance related to the autonomic nervous system. In particular, an increased short-term scaling exponent α1 calculated from DFA is associated with both increased sympathetic activity and decreased vagal activity. No study has investigated the DFA of HRV in hyperthyroidism. This study was designed to assess the sympathovagal balance in hyperthyroidism. We performed the DFA along with the linear analysis of HRV in 36 hyperthyroid Graves’ disease patients (32 females and 4 males; age 30 ± 1 years, means ± SE) and 36 normal controls matched by sex, age and body mass index. Compared with the normal controls, the hyperthyroid patients revealed a significant increase (P<0.001) in α1 (hyperthyroid 1.28±0.04 versus control 0.91±0.02), long-term scaling exponent , overall scaling exponent , low frequency power in normalized units (LF%) and the ratio of low frequency power to high frequency power (LF/HF); and a significant decrease (P<0.001) in the standard deviation of the R-R intervals (SDNN) and high frequency power (HF). In conclusion, hyperthyroidism is characterized by concurrent sympathetic activation and vagal withdrawal. This sympathovagal imbalance state in hyperthyroidism helps to explain the higher prevalence of atrial fibrillation and exercise intolerance among hyperthyroid patients. 相似文献
104.
Niels van der Laan Ward Romeijnders Maarten H. van der Vlerk 《Computational Management Science》2018,15(3-4):325-349
We derive bounds on the expectation of a class of periodic functions using the total variations of higher-order derivatives of the underlying probability density function. These bounds are a strict improvement over those of Romeijnders et al. (Math Program 157:3–46, 2016b), and we use them to derive error bounds for convex approximations of simple integer recourse models. In fact, we obtain a hierarchy of error bounds that become tighter if the total variations of additional higher-order derivatives are taken into account. Moreover, each error bound decreases if these total variations become smaller. The improved bounds may be used to derive tighter error bounds for convex approximations of more general recourse models involving integer decision variables. 相似文献
105.
Niels Schwartz 《Mathematische Nachrichten》2010,283(5):758-774
The relationship between a poring and a convex subring has been studied most successfully for porings with bounded inversion. The best conceivable results are known for real closed rings. The present paper focuses on the connections between the prime spectra and between the real spectra of a poring and a convex subring. Examples show that for arbitrary porings one may not expect a very close relationship. But with the assumption of bounded inversion or spectral compatibility the results are similar to those for real closed rings (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
106.
In this paper, we study the initial value problem for a class of non-linear stochastic equations of Burgers type of the following form for u:(t,x)∈(0,∞)×R?u(t,x)∈R, where q(x,D) is a pseudo-differential operator with negative definite symbol of variable order which generates a stable-like process with transition density, f,h1,h2:[0,∞)×R×R→R are measurable functions, and Ft,x stands for a Lévy space-time white noise. We investigate the stochastic equation on the whole space R in the mild formulation and show the existence of a unique local mild solution to the initial value problem by utilising a fixed point argument. 相似文献
∂tu+q(x,D)u+∂xf(t,x,u)=h1(t,x,u)+h2(t,x,u)Ft,x
107.
We present a revision to the well known Störmer–Verlet algorithm for simulating second order differential equations. The revision addresses the inclusion of linear friction with associated stochastic noise, and we analytically demonstrate that the new algorithm correctly reproduces diffusive behaviour of a particle in a flat potential. For a harmonic oscillator, our algorithm provides the exact Boltzmann distribution for any value of damping, frequency and time step for both underdamped and overdamped behaviour within the usual stability limit of the Verlet algorithm. Given the structure and simplicity of the method, we conclude that this approach can trivially be adapted for contemporary applications, including molecular dynamics with extensions such as molecular constraints. 相似文献
108.
Equivalent dose distribution analysis of Holocene eolian and fluvial quartz sands from Central Oklahoma 总被引:5,自引:0,他引:5
Kenneth Lepper Niels Agersnap Larsen Stephen W. S. McKeever 《Radiation measurements》2000,32(5-6):603-608
Holocene quartz sands were collected from fluvial terrace deposits and eolian dune deposits adjacent to the North Canadian and Cimarron Rivers and their tributaries in Central Oklahoma. Single aliquot regenerative dose optically stimulated luminescence techniques were employed to generate equivalent dose (ED) distribution histograms for each sample. We hypothesize that the ED distributions are convolutions of the distribution arising from natural sedimentary processes, influenced primarily by the degree of solar resetting experienced by individual grains, and a distribution due to experimental error. An estimation of the experimental error distribution for each sample was made and this was then deconvolved from the experimentally measured ED distribution to reveal the ‘sedimentary process’ ED distribution. Objective methods for determining EDs, uncertainties, and confidence parameters from the deconvolved distributions are presented. 相似文献
109.
In this paper, we establish the invariance of cyclic (co)homology of left Hopf algebroids under the change of Morita equivalent base algebras. The classical result on Morita invariance for cyclic homology of associative algebras appears as a special example of this theory. In our main application we consider the Morita equivalence between the algebra of complex-valued smooth functions on the classical 2-torus and the coordinate algebra of the noncommutative 2-torus with rational parameter. We then construct a Morita base change left Hopf algebroid over this noncommutative 2-torus and show that its cyclic (co)homology can be computed by means of the homology of the Lie algebroid of vector fields on the classical 2-torus. 相似文献
110.
Franco Doro Niels Akeroyd Franc Schiet Anubhav Narula 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(22):7248-7253
The Prins reaction, an acid‐catalyzed condensation of alkenes with aldehydes, is used extensively in the fragrance industry. This year we celebrate the 100th anniversary of the discovery of this reaction. In honor of this occasion we present an overview of the diverse applications of the Prins reaction in the synthesis of flavor and fragrance ingredients. To pay tribute to the inventor of the Prins reaction, Hendrik Jacobus Prins, we also provide some insight into his life, scientific, and entrepreneurial accomplishments. 相似文献