全文获取类型
收费全文 | 136318篇 |
免费 | 1179篇 |
国内免费 | 671篇 |
专业分类
化学 | 51339篇 |
晶体学 | 1801篇 |
力学 | 9914篇 |
综合类 | 8篇 |
数学 | 36376篇 |
物理学 | 38730篇 |
出版年
2022年 | 542篇 |
2021年 | 488篇 |
2020年 | 450篇 |
2019年 | 429篇 |
2018年 | 10940篇 |
2017年 | 10702篇 |
2016年 | 6904篇 |
2015年 | 1468篇 |
2014年 | 1272篇 |
2013年 | 2796篇 |
2012年 | 6070篇 |
2011年 | 13433篇 |
2010年 | 7719篇 |
2009年 | 8165篇 |
2008年 | 9273篇 |
2007年 | 11248篇 |
2006年 | 2658篇 |
2005年 | 3459篇 |
2004年 | 3472篇 |
2003年 | 3688篇 |
2002年 | 2645篇 |
2001年 | 3247篇 |
2000年 | 2443篇 |
1999年 | 1713篇 |
1998年 | 1275篇 |
1997年 | 1210篇 |
1996年 | 1095篇 |
1995年 | 917篇 |
1994年 | 799篇 |
1993年 | 725篇 |
1992年 | 1014篇 |
1991年 | 1014篇 |
1990年 | 909篇 |
1989年 | 815篇 |
1988年 | 799篇 |
1987年 | 856篇 |
1986年 | 724篇 |
1985年 | 935篇 |
1984年 | 876篇 |
1983年 | 607篇 |
1982年 | 608篇 |
1981年 | 575篇 |
1980年 | 545篇 |
1979年 | 669篇 |
1978年 | 690篇 |
1977年 | 682篇 |
1976年 | 595篇 |
1975年 | 502篇 |
1974年 | 536篇 |
1973年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
G. Sreenivasulu Reddy J. Hanumanthachari T. Vasanthi 《Southeast Asian Bulletin of Mathematics》2003,26(2):331-335
This paper is mainly dealt with the structure of totally ordered rings (t.o. rings) whose positive cones are finitely generated as multiplicative right ideals.AMS Subject Classification (1991): 16W80, 13J25 相似文献
992.
We consider a discretized volume V consisting of finite, congruent and attached copies of a tile t. We find a group L
V
the orbit of which, when applied to t, is just V. We show the connection between the structural matrixQ in the formal solution of a boundary value problem formulated for volume V and the so called auxiliary matrix of the graph Γ
v
associated with V. We show boundary value problems to be isomorphic if the graphs associated with the volumes are isomorphic, or, if the covering
groups are Sunada pairs. 相似文献
993.
Sam Efromovich 《Annals of the Institute of Statistical Mathematics》2004,56(2):205-223
A blockwise shrinkage is a popular procedure of adaptation that has allowed the statisticians to establish an impressive bouquet
of asymptotic mathematical results and develop softwares for solving practical problems. Traditionally risks of the estimates
are studied via upper bounds that imply sufficient conditions for a blockwise shrinkage procedure to be minimax. This article
suggests to analyze the estimates via exact (non-asymptotic) lower bounds established for a no-signal setting. The approach
complements the familiar minimax, Bayesian and numerical analysis, it allows to find necessary conditions for a procedure
to attain desired rates, and it sheds a new light on popular choices of blocks and thresholds recommended in the literature.
Mathematical results are complemented by a numerical study.
Supported in part by NSF Grants DMS-9971051 and DMS-0243606. 相似文献
994.
Ruth Kellerhals 《Israel Journal of Mathematics》2004,143(1):361-379
Forn≥2, we quantify the Margulis constant ε(n) giving rise to a thick and thin decomposition of hyperbolicn-manifolds of finite volume. As a consequence, we obtain new universal lower bounds for the volume and Gromov's invariant
as well as a geometrical inequality between injectivity radius and diameter for compact manifolds. Finally, we concretise
the upper bound for the counting function of hyperbolic manifolds of dimension >4 as described by Burger, Gelander, Lubotzky
and Mozes.
Partially supported by Schweizerischer Nationalfonds No. 20-61379.00 and 20-67619.02. 相似文献
995.
We present here an improved version of the method introduced by the first author to derive
pointwise gradient estimates for the solutions of one-dimensional parabolic problems. After considering
a general qualinear equation in divergence form we apply the method to the case of a nonlinear
diffusion-convection equation. The conclusions are stated first for classical solutions and then for
generalized and mild solutions. In the case of unbounded initial datum we obtain several regularizing
effects for t > 0. Some unilateral pointwise gradient
estimates are also obtained. The case of
the Dirichlet problem is also considered. Finally, we collect, in the last section, several comments
showing the connections among these estimates and the study of the free boundaries
associated to the solutions of the diffusion-convection equation. 相似文献
996.
An exfoliated poly(4,4′-oxybis(benzene)disulfide) (POBDS)/vermiculite (VMT) nanocomposite was prepared by using cyclo(4,4′-oxybis(benzene)disulfide) (COBDS) oligomers and cetyltrimethyl ammonium bromide exchanged VMT. The POBDS/VMT nanocomposites were fabricated in two steps. First, the COBDS oligomers were used to swell and exfoliate organo VMT to afford COBDS-VMT nanocomposite precursor. Subsequently, the exfoliated POBDS-VMT nanocomposite can be made via in situ and instant melt ring-opening polymerization of the COBDS-VMT nanocomposite precursor. High molecular weight POBDS polymer can be formed in a few minutes. The nano scale dispersion of VMT layers within POBDS polymer was confirmed by both the X-ray diffraction patterns and TEM examinations. This methodology provides a potential approach to synthesize high performance polymer nanocomposite. 相似文献
997.
998.
Y. Kanai 《Journal of Radioanalytical and Nuclear Chemistry》2003,255(2):319-323
Geological and geochemical events were elucidated in by using U series nuclides in combination with chemical leaching technique. Two examples were examined: (1) in the apatite-bearing sediment column samples U(VI) is moving, while U(IV) in the samples is almost retained, and (2) the redistribution of U occurred along the column under the monsoon climate. On the other hand, U in the conglomerate sample is divided into 5 categories by the leaching technique, which are related with exchangeable, carbonate, iron oxide, organic/sulfide and silicate components, respectively. Uranium is shown to be abundant in the carbonate and iron rich fractions of conglomerate sediment. Appropriate models are proposed in each case study. 相似文献
999.
Su-Yun Huang Chuhsing Kate Hsiao Ching-Wei Chang 《Annals of the Institute of Statistical Mathematics》2003,55(3):655-670
The article provides a refinement for the volume-corrected Laplace-Metropolis estimator of the marginal likelihood of DiCiccioet al. The correction volume of probability α in DiCiccioet al. is fixed and suggested to take the value α=0.05. In this article α is selected based on an asymptotic analysis to minimize
the mean square relative error (MSRE). This optimal choice of α is shown to be invariant under linear transformations. The
invariance property leads to easy implementation for multivariate problems. An implementation procedure is provided for practical
use. A simulation study and a real data example are presented. 相似文献
1000.
T W Sloan 《The Journal of the Operational Research Society》2004,55(6):647-656
In many environments, product yield is heavily influenced by equipment condition. Despite this fact, previous research has either focused on the issue of maintenance, ignoring the effect of equipment condition on yield, or has focused on the issue of production, omitting the possibility of actively changing the machine state. We formulate a Markov decision process model of a single-stage production system in which demand is random. The product yield has a binomial distribution that depends on the equipment condition, which deteriorates over time. The objective is to choose simultaneously the equipment maintenance schedule as well as the quantity to produce in a way that minimizes the sum of expected production, backorder, and holding costs. After proving some results about the structural properties of the optimal policy, numerical problems are used to compare this method to the typical approach of solving the maintenance and production problems sequentially. The results show that the simultaneous solution provides substantial gains over the sequential approach. In the cases studied, the proposed method resulted in an average cost savings of approximately 18%. 相似文献