全文获取类型
收费全文 | 82814篇 |
免费 | 1135篇 |
国内免费 | 1128篇 |
专业分类
化学 | 28026篇 |
晶体学 | 854篇 |
力学 | 6971篇 |
综合类 | 78篇 |
数学 | 32427篇 |
物理学 | 16721篇 |
出版年
2023年 | 70篇 |
2022年 | 132篇 |
2021年 | 148篇 |
2020年 | 197篇 |
2019年 | 211篇 |
2018年 | 10545篇 |
2017年 | 10377篇 |
2016年 | 6305篇 |
2015年 | 1114篇 |
2014年 | 596篇 |
2013年 | 747篇 |
2012年 | 4192篇 |
2011年 | 10854篇 |
2010年 | 5983篇 |
2009年 | 6332篇 |
2008年 | 6985篇 |
2007年 | 9076篇 |
2006年 | 526篇 |
2005年 | 1598篇 |
2004年 | 1766篇 |
2003年 | 2166篇 |
2002年 | 1178篇 |
2001年 | 413篇 |
2000年 | 411篇 |
1999年 | 244篇 |
1998年 | 257篇 |
1997年 | 198篇 |
1996年 | 239篇 |
1995年 | 153篇 |
1994年 | 105篇 |
1993年 | 135篇 |
1992年 | 93篇 |
1991年 | 97篇 |
1990年 | 69篇 |
1989年 | 72篇 |
1988年 | 74篇 |
1987年 | 70篇 |
1986年 | 72篇 |
1985年 | 59篇 |
1984年 | 50篇 |
1983年 | 40篇 |
1982年 | 46篇 |
1981年 | 40篇 |
1980年 | 48篇 |
1979年 | 48篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
21.
We consider the problem of estimating a large rank-one tensor u ⊗k ∈ (ℝn)⊗k , k ≥ 3 , in Gaussian noise. Earlier work characterized a critical signal-to-noise ratio λ Bayes = O(1) above which an ideal estimator achieves strictly positive correlation with the unknown vector of interest. Remarkably, no polynomial-time algorithm is known that achieved this goal unless λ ≥ Cn(k − 2)/4 , and even powerful semidefinite programming relaxations appear to fail for 1 ≪ λ ≪ n(k − 2)/4 . In order to elucidate this behavior, we consider the maximum likelihood estimator, which requires maximizing a degree-k homogeneous polynomial over the unit sphere in n dimensions. We compute the expected number of critical points and local maxima of this objective function and show that it is exponential in the dimensions n , and give exact formulas for the exponential growth rate. We show that (for λ larger than a constant) critical points are either very close to the unknown vector u or are confined in a band of width Θ(λ−1/(k − 1)) around the maximum circle that is orthogonal to u . For local maxima, this band shrinks to be of size Θ(λ−1/(k − 2)) . These “uninformative” local maxima are likely to cause the failure of optimization algorithms. © 2019 Wiley Periodicals, Inc. 相似文献
22.
Hejia Hu Linlin Wu Mei Li Cun Xue Guangcheng Wang Siying Chen Yong Huang Lin Zheng Aimin Wang Yueting Li Zipeng Gong 《Biomedical chromatography : BMC》2020,34(1):e4714
Eucommia ulmoides Oliv. (E. ulmoides) is a valuable and nourishing medicinal herb in China that has been used in the treatment of hypertension. Given the fact that most traditional Chinese medicine is mainly used to treat disease, investigating the pharmacokinetics of traditional Chinese medicines in the pathological state is more useful than that in the normal state. However, the differences in the absorption kinetics of active ingredients of E. ulmoides extract between pathological and physiological conditions have not been reported. Therefore, in this study, the rat intestinal in situ circulatory perfusion model was used to investigate the differences in absorption kinetics of seven active ingredients of E. ulmoides extract in normal and spontaneously hypertensive rats, namely, genipinic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, cryptochlorogenic acid, (+)-pinoresinol di-O-β-D -glucopyranoside and (+)-pinoresinol 4′-O-β-D -glucopyranoside. Our results indicate that the pathological state of spontaneous hypertension may change the absorption of active components of E. ulmoides extracts, and these findings may provide a reference for improving the rational use of E. ulmoides in the clinic. 相似文献
23.
24.
25.
26.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
27.
史美华 《纯粹数学与应用数学》2002,18(2):145-150,155
设k,r分别是自然数和非零整数,Jk(n)是Jordan函数。以E(x;k,r)表示和式sum from (n≤x) Jkr(n)的渐近公式中的误差项,本文研究了E(x;k,r)的某种加权平方积分均值。 相似文献
28.
A forecasting model is developed for the number of daily applications for loans at a financial services telephone call centre. The purpose of the forecasts and the associated prediction intervals is to provide effective staffing policies within the call centre. The model building process is constrained by the availability of only 2 years and 7 months of data. The distinctive feature of the data is that demand is driven in the main by advertising. The analysis given focuses on applications stimulated by press advertising. Unlike previous analyses of broadly similar data, where ARIMA models were used, a model with a dynamic level, multiplicative calendar effects and a multiplicative advertising response is developed and shown to be effective. 相似文献
29.
Chee Sing Yap 《The Journal of the Operational Research Society》1989,40(7):649-658
This paper describes three case studies which investigated issues relating to the implementation and management of advanced information technology. The benefits derived from information technology were found to vary from one organization to another. These studies provide further evidence to support earlier research findings that senior management has an important role to play in the effective exploitation of information technology, and that the investment approach taken by an organization has a significant effect on the development of computer-based information systems. The effectiveness of a system depends not only on applying the appropriate technology, but also on how successfully technical and behavioural issues are resolved. 相似文献
30.
M I Salema A P B Póvoa A Q Novais 《The Journal of the Operational Research Society》2006,57(6):615-629
In this paper, an MILP formulation is proposed for the design of a reverse logistics network based on a warehouse location–allocation model, which optimizes, simultaneously, the forward and reverse networks. A single product model with unlimited capacity is first defined. Subsequently, the model is extended to a multi-product capacitated recovery network model, where capacity limitations and a multi-product system can be considered. The proposed model is compared to published work in the field, where different model assumptions have been proposed. Two cases are described so as to gain a better insight into the model and allow a comparative analysis. 相似文献