全文获取类型
收费全文 | 1155篇 |
免费 | 26篇 |
国内免费 | 3篇 |
专业分类
化学 | 671篇 |
晶体学 | 12篇 |
力学 | 83篇 |
数学 | 122篇 |
物理学 | 211篇 |
无线电 | 85篇 |
出版年
2022年 | 15篇 |
2021年 | 21篇 |
2020年 | 19篇 |
2019年 | 21篇 |
2018年 | 13篇 |
2017年 | 19篇 |
2016年 | 30篇 |
2015年 | 14篇 |
2014年 | 33篇 |
2013年 | 92篇 |
2012年 | 72篇 |
2011年 | 65篇 |
2010年 | 40篇 |
2009年 | 30篇 |
2008年 | 45篇 |
2007年 | 46篇 |
2006年 | 53篇 |
2005年 | 47篇 |
2004年 | 35篇 |
2003年 | 44篇 |
2002年 | 41篇 |
2001年 | 9篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 11篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 14篇 |
1988年 | 8篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 14篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1974年 | 8篇 |
1973年 | 13篇 |
1945年 | 6篇 |
1941年 | 11篇 |
1936年 | 9篇 |
排序方式: 共有1184条查询结果,搜索用时 9 毫秒
1.
Human faces undergo considerable amounts of varialions with aging. While face recognition systems have been proven to be sensitive to factors such as illumination and pose, their sensitivity to facial aging effects is yet to be studied. How does age progression affect the similarity between a pair of face images of an individual? What is the confidence associated with establishing the identity between a pair of age separated face images? In this paper, we develop a Bayesian age difference classifier that classifies face images of individuals based on age differences and performs face verification across age progression. Further, we study the similarity of faces across age progression. Since age separated face images invariably differ in illumination and pose, we propose preprocessing methods for minimizing such variations. Experimental results using a database comprising of pairs of face images that were retrieved from the passports of 465 individuals are presented. The verification system for faces separated by as many as nine years, attains an equal error rate of 8.5%. 相似文献
2.
3.
On the departure process of a leaky bucket system with long-range dependent input traffic 总被引:2,自引:0,他引:2
Due to the strong experimental evidence that the traffic to be offered to future broadband networks will display long-range
dependence, it is important to study the possible implications that such traffic may have for the design and performance of
these networks. In particular, an important question is whether the offered traffic preserves its long-range dependent nature
after passing through a policing mechanism at the interface of the network. One of the proposed solutions for flow control
in the context of the emerging ATM standard is the so-called leaky bucket scheme. In this paper we consider a leaky bucket
system with long-range dependent input traffic. We adopt the following popular model for long-range dependent traffic: Time
is discrete. At each unit time a random number of sessions is initiated, having the distribution of a Poisson random variable
with mean λ. Each of these sessions has a random duration τ, where the integer random variable τ has finite mean, infinite
variance, and a regularly varying tail, i.e., P(τ >К) ~ К-Lα
L(К), where 1 < α < 2 L(·) is a slowly varying function. Once a session is initiated, it generates one cell at each unit of time until its termination.
We examine the departure process of the leaky bucket policing mechanism driven by such an arrival process, and show that it
too is long-range dependent for any token buffer size and any - finite or infinite - cell buffer size. Moreover, upper and
lower bounds for the covariance sequence of the output process are established. The above results demonstrate that long-range
dependence cannot be removed by the kinds of flow control schemes that are currently being envisioned for broadband networks.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
4.
In this paper, the problem of simultaneous structure from motion estimation for multiple independently moving objects from a monocular image sequence is addressed. Two Bayesian algorithms are presented for solving this problem using the sequential importance sampling (SIS) technique. The empirical posterior distribution of object motion and feature separation parameters is approximated by weighted samples. The first algorithm addresses the problem when only two moving objects are present. A singular value decomposition (SVD)-based sample clustering algorithm is shown to be capable of separating samples related to different objects. A pair of SIS procedures is used to track the posterior distribution of the motion parameters. In the second algorithm, a balancing step is added into the SIS procedure to preserve samples of low weights so that all objects have enough samples to propagate empirical motion distributions. By using the proposed algorithms, the relative motions of all the moving objects with respect to the camera can be simultaneously estimated. Both algorithms have been tested on synthetic and real-image sequences. Improved results have been achieved. 相似文献
5.
P.Veeraraghavan Ramachandran Kamlesh J Padiya Vivek Rauniyar M.Venkat Ram Reddy Herbert C Brown 《Journal of fluorine chemistry》2004,125(4):615-620
Asymmetric “allyl”boration of pentafluorobenzaldehyde with various α-pinene based “allyl”boranes provides homoallylic alcohols in high de and ee; the alcohols have been converted into δ-lactones via acryloylation, ring-closing metathesis and hydrogenation. Pentafluorophenyl analog of key pharmacophore of statin drugs has been synthesized using diastereoselective epoxidation and regioselective reduction as key steps. 相似文献
6.
K. Vinay Kumar Reddy B. Sreenivasa Reddy M. Sreenath Reddy Ch. Gopal Reddy P. Yadagiri Reddy K. Rama Reddy 《Radiation measurements》2003,36(1-6):419-423
Studies conducted by Atomic Minerals Directorate of Exploration and Research (AMD) of Hyderabad, India had established the presence of higher concentrations of uranium in Lambapur and Peddagattu areas of Nalgonda district, AP, India and it was estimated that it could be a viable source for commercial extraction. The envisaged extraction process involves dispersion of radioactive particulate matter into atmosphere. Environmental radioactive studies in and around proposed mining areas at this point of time will be extremely useful for establishing base line data before a large scale uranium extraction process comes into existence. To this end, Solid State Nuclear Track Detectors were installed to evaluate indoor radon and thoron concentration levels in the dwellings of the area. The geometric means of radon and thoron concentration levels were found to be (7.1±0.2)×101 and (6.7±0.3)×101 Bq/m3, respectively. Simultaneously, natural background radiation measurements were also made and these levels are found to vary from 770 to 3995 μGy/y in the spatial distribution. 相似文献
7.
R. Venkat narayan Vinod Kanniah Aruna Dhathathreyan 《Journal of Chemical Sciences》2006,118(2):179-184
Cobalt oxides were prepared by three different methods: (1) by reacting cobalt nitrate with oxalic acid, (2) co-precipitating
cobalt nitrate with sodium carbonate, and (3) using sodium dodecyl sulphate as organic surfactant. All three samples were
characterized before and after calcination by solvent extraction and the resulting products examined by IR spectroscopy. In
the case of method 3, the removal of surfactant was followed by TGA studies. Products from all three methods were identified
by XRD. Peaks in low angle XRD indicate the porous nature of the oxides. The morphology of the pores was studied by transmission
electron microscopy. Some irregular pore structures were obtained for samples from methods 1 and 2, with an average size of
4–6 nm. Only the product from method 3 using SDS as template showed ordered structure and optimum size, and Brunauer-Emmet-Teller
surface areas of the as-prepared, as well as the treated samples, exhibited H3 type hysteresis. The samples from the three
methods were used as catalysts in the oxidation reaction of cyclohexane under mild conditions and the catalytic efficiency
of the cobalt oxide was comparable with mesoporous cobalt oxides. 相似文献
8.
Surendra K Krishnaveni NS Reddy MA Nageswar YV Rao KR 《The Journal of organic chemistry》2003,68(5):2058-2059
A mild and efficient oxidation of alcohols with o-iodoxybenzoic acid (IBX) catalyzed by beta-cyclodextrin in a water/acetone mixture (86:14) has been developed. A series of alcohols were oxidized at room temperature in excellent yields. 相似文献
9.
2‐Acetyl thiophenethiocynate (ATT) was synthesized and its characterization study was carried out using elemental analysis, IR and NMR techniques. The new reagent was proposed as an extractant for the development of the extractive spectrophotometric method for determination of Pt(IV) metal. The reagent complexes with the metal to produce a yellow colored complex which was then extracted into ethyl acetate at pH 11.2–11.6 having an absorption maxima at 400 nm. The thermal study showed that the extraction reactions are exothermic in nature with the reagent, and the stoichiometric ratio of Pt(IV) to 2‐acetyl thiophenethiocynate in the organic phase was 1:2. The method permits separation and determination of platinum from real and binary mixtures. The separation of Pt(IV) from bivalent metal ions has been also studied. 相似文献
10.
[reaction: see text] Chiral beta-syn-alkoxyhomoallylic alcohols derived from alkoxyallylboration of aldehydes upon oxidation provided the corresponding chiral ketones. Chelation-controlled nucleophilic addition to these ketones occurred in a highly stereoselective manner to afford anti-homoallylic tertiary alcohols. This methodology has been applied for the synthesis of the C(1)-C(11) subunit of C(8)-epi-fostriecin. 相似文献