全文获取类型
收费全文 | 30978篇 |
免费 | 163篇 |
国内免费 | 279篇 |
专业分类
化学 | 12046篇 |
晶体学 | 266篇 |
力学 | 1406篇 |
综合类 | 12篇 |
数学 | 9320篇 |
物理学 | 8370篇 |
出版年
2020年 | 58篇 |
2019年 | 56篇 |
2018年 | 1201篇 |
2017年 | 1456篇 |
2016年 | 752篇 |
2015年 | 600篇 |
2014年 | 526篇 |
2013年 | 709篇 |
2012年 | 3203篇 |
2011年 | 2401篇 |
2010年 | 1863篇 |
2009年 | 1572篇 |
2008年 | 622篇 |
2007年 | 697篇 |
2006年 | 702篇 |
2005年 | 4523篇 |
2004年 | 3964篇 |
2003年 | 2349篇 |
2002年 | 548篇 |
2001年 | 336篇 |
2000年 | 126篇 |
1999年 | 207篇 |
1998年 | 145篇 |
1997年 | 95篇 |
1996年 | 69篇 |
1995年 | 73篇 |
1994年 | 74篇 |
1993年 | 53篇 |
1992年 | 214篇 |
1991年 | 180篇 |
1990年 | 159篇 |
1989年 | 139篇 |
1988年 | 136篇 |
1987年 | 99篇 |
1986年 | 73篇 |
1985年 | 80篇 |
1984年 | 59篇 |
1983年 | 49篇 |
1981年 | 48篇 |
1980年 | 45篇 |
1979年 | 51篇 |
1978年 | 56篇 |
1976年 | 109篇 |
1975年 | 71篇 |
1974年 | 74篇 |
1973年 | 80篇 |
1972年 | 55篇 |
1970年 | 50篇 |
1969年 | 49篇 |
1966年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
991.
Stanislaw Barder Jüri Lember Heinrich Matzinger M?rt Toots 《Methodology and Computing in Applied Probability》2012,14(2):357-382
Let X = X
1 ... X
n
and Y = Y
1 ... Y
n
be two binary sequences with length n. A common subsequence of X and Y is any subsequence of X that at the same time is a subsequence of Y; The common subsequence with maximal length is called the longest common subsequence (LCS) of X and Y. LCS is a common tool for measuring the closeness of X and Y. In this note, we consider the case when X and Y are both i.i.d. Bernoulli sequences with the parameters ϵ and 1 − ϵ, respectively. Hence, typically the sequences consist of large and short blocks of different colors. This gives an idea to
the so-called block-by-block alignment, where the short blocks in one sequence are matched to the long blocks of the same
color in another sequence. Such and alignment is not necessarily a LCS, but it is computationally easy to obtain and, therefore,
of practical interest. We investigate the asymptotical properties of several block-by-block type of alignments. The paper
ends with the simulation study, where the of block-by-block type of alignments are compared with the LCS. 相似文献
992.
993.
The paper presents a survey of the main results of I.M. Vinogradov. 相似文献
994.
This paper analyzes the level and cyclicality of regulatory bank capital for asset portfolio securitizations in relation to the cyclicality of capital requirements for the underlying loan portfolio as under Basel II/III. We find that the cyclicality of capital requirements is higher for (i) asset portfolio securitizations relative to primary loan portfolios, (ii) Ratings Based Approach (RBA) relative to the Supervisory Formula Approach, (iii) given the RBA for a point-in-time rating methodology relative to a rate-and-forget rating methodology, and (iv) under the passive reinvestment rule relative to alternative rules. Capital requirements of the individual tranches reveal that the volatility of aggregated capital charges for the securitized portfolio is triggered by the most senior tranches. This is due to the fact that senior tranches are more sensitive to the macroeconomy. An empirical analysis provides evidence that current credit ratings are time-constant and that economic losses for securitizations have exceeded the required capital in the recent financial crisis. 相似文献
995.
996.
Oliver Brücher Uwe Bergsträßer Harald Kelm Jens Hartung Marco Greb Ingrid Svoboda Hartmut Fuess 《Tetrahedron》2012,68(34):6968-6980
A cascade, composed of (i) oxovanadium(V)-catalyzed oxidation of bromide by tert-butyl hydroperoxide and (ii) stereoselective 6-endo-bromocyclization, affords 3-bromo-2-aryl-2,6,6-trimethyltetrahydropyrans from styrene-type tertiary alkenols in synthetically useful yields. (E)-Alkenols add the bromo- and the alkoxy substituent anti-selectively across the double bond, indicating a bromonium ion-mechanism for the ring closure. 6-endo-control of the alkenol cyclization thereby arises from the polar effect of the aryl substituent. Two methyl substituents bound to the alkene terminus are not similarly able to favor 6-endo-cyclization, because strain arising from methyl group repulsion, as the bromonium-activated π-bond and the hydroxyl oxygen approach, directs bromocyclization of tertiary prenyl-type substrates toward tetrahydrofuran formation. A hexasubstituted bromotetrahydropyran prepared from the oxidation/bromocyclization cascade served as starting material for synthesis of racemic aplysiapyranoid A, in a sequence of free radical and polar functional group interconversion. 相似文献
997.
998.
Miroslav Veverka Peter ?imon Ján Lokaj Eva Veverková 《Monatshefte für Chemie / Chemical Monthly》2012,10(4):65-71
Abstract
Crystals of the α-form of imatinib mesylate with various habits (e.g., polyhedral-like and plate-like) were prepared from various organic solvents (e.g., butyl lactate, 4-methyl-2-pentanone, 2-methyl-2-butanol, 2-isopropoxyethanol, propyl ether) by several precipitation methods. The methods provide imatinib mesylate in a non-needle-shaped crystalline α-form. The crystal modification was identified by hot-stage microscopy, scanning electron microscopy, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and X-ray powder diffraction (XRPD). The analyses by DSC, IR, and XRPD indicate that imatinib mesylate crystals with various habits have the same crystal structure. The plate-like habit has been also observed in the system where the organic solvent acts as a precipitant. 相似文献999.
1000.
Dorota Majda Wac?aw Makowski Maria Mańko 《Journal of Thermal Analysis and Calorimetry》2012,109(2):663-669
Thermoporosimetry, i.e., DSC measurements of melting point depression of water and heptane confined in mesopores, has been used for determination the pore size distribution of several mesoporous silicas synthesized with the use of micelle templates. Porosity of these materials was additionally characterized by low-temperature nitrogen adsorption and quasi-equilibrated thermodesorption of nonane. The pore size distributions obtained using the water thermoporosimetry were similar to those determined using the other methods, but the pore size values found for the narrow pore materials were underestimated by ca 1?nm. Too large pore sizes obtained for the wide pore silica from heptane thermoporosimetry were attributed to nonlinear dependence of the melting point depression on the reciprocal of the pore size. 相似文献