全文获取类型
收费全文 | 18954篇 |
免费 | 859篇 |
国内免费 | 99篇 |
专业分类
化学 | 13074篇 |
晶体学 | 120篇 |
力学 | 420篇 |
数学 | 2894篇 |
物理学 | 3404篇 |
出版年
2023年 | 122篇 |
2022年 | 172篇 |
2021年 | 268篇 |
2020年 | 377篇 |
2019年 | 397篇 |
2018年 | 234篇 |
2017年 | 236篇 |
2016年 | 600篇 |
2015年 | 556篇 |
2014年 | 644篇 |
2013年 | 1024篇 |
2012年 | 1115篇 |
2011年 | 1248篇 |
2010年 | 744篇 |
2009年 | 659篇 |
2008年 | 1035篇 |
2007年 | 1020篇 |
2006年 | 949篇 |
2005年 | 880篇 |
2004年 | 772篇 |
2003年 | 637篇 |
2002年 | 605篇 |
2001年 | 309篇 |
2000年 | 290篇 |
1999年 | 273篇 |
1998年 | 241篇 |
1997年 | 231篇 |
1996年 | 264篇 |
1995年 | 247篇 |
1994年 | 217篇 |
1993年 | 211篇 |
1992年 | 199篇 |
1991年 | 174篇 |
1990年 | 174篇 |
1989年 | 174篇 |
1988年 | 149篇 |
1987年 | 125篇 |
1986年 | 114篇 |
1985年 | 166篇 |
1984年 | 161篇 |
1983年 | 126篇 |
1982年 | 153篇 |
1981年 | 157篇 |
1980年 | 132篇 |
1979年 | 140篇 |
1978年 | 158篇 |
1977年 | 115篇 |
1976年 | 100篇 |
1975年 | 117篇 |
1974年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
51.
This paper establishes a general ABC inventory classification system as the foundation for a normative model of the maintenance cost structure and stock turnover characteristics of a large, multi-item inventory system with constant demand. For any specified number of inventory classes, the model allows expression of the overall system combined ordering and holding cost in terms of (i) the re-ordering frequencies for the items in each inventory class and (ii) the inventory class structure, that is, the proportion of the total system's items that are in each inventory class. The model yields a minimum total maintenance cost function, which reflects the effect of class structure on inventory maintenance costs and turnover. If the Pareto curve (a.k.a. Distribution-by-value function) for the inventory system can be expressed (or approximated) analytically, the model can also be used to determine an optimal class structure, as well as an appropriate number of inventory classes. A special case of the model produces a simply structured, class-based ordering policy for minimizing total inventory maintenance costs. Using real data, the cost characteristics of this policy are compared to those of a heuristic, commonly used by managers of multi-item inventory systems. This cost comparison, expressed graphically, underscores the need for normative modelling approaches to the problem of inventory cost management in large, multi-item systems. 相似文献
52.
Martin I. Karakhanyan 《Journal of Contemporary Mathematical Analysis (Armenian Academy of Sciences)》2007,42(3):146-150
The paper proves some general facts on commutators that refer to Fuglede-Putnam classical theorem in the spectral theory of not necessarily selfadjoint operators. 相似文献
53.
54.
It is shown that the law of an SDE driven by fractional Brownian motion with Hurst parameter greater than 1/2 has a smooth density with respect to Lebesgue measure, provided that the driving vector fields satisfy Hörmander’s condition. The main new ingredient of the proof is an extension of Norris’ lemma to this situation. 相似文献
55.
Martin Fuchs 《manuscripta mathematica》1989,63(4):381-419
We prove an extension theorem for Sobolev functions which plays an important role in the partial regularity theory of vector valued functions which minimize degenerate variational integrals subject to some nonlinear side conditions in the image space. 相似文献
56.
57.
We demonstrate significant enhancement of four-wave mixing in coherently driven mercury isotopes to generate vacuum-ultraviolet radiation at 125 nm. The enhancement is accomplished by preparation of the mercury atoms in a state of maximum coherence, i.e. maximum nonlinear-optical polarization, driven by Stark-chirped rapid adiabatic passage (SCRAP). In this technique, a pump laser at 313 nm excites the two-photon transition between the ground state 6s21S0 and the target state 7s 1S0 in mercury. A strong, off-resonant radiation field at 1064 nm generates dynamic Stark shifts. These Stark shifts serve to induce a rapid adiabatic passage process on the two-photon transition. During the process a coherent superposition of the two states is established, which enhances the nonlinear-optical polarization in the medium to the maximum possible value. The maximum coherence permits efficient four-wave mixing of a pump laser and an additional probe laser at 626 nm. The efficiency is further enhanced, as the SCRAP process allows to stimulate the complete set of different mercury isotopes to participate in the frequency conversion process. This enlarges the effective atomic density of the medium. Thus, we observe the generation of vacuum-ultraviolet radiation at 125 nm enhanced by more than one order of magnitude with respect to conventional frequency conversion. Parallel to the frequency conversion process, we monitored the evolution of the population in the medium by laser-induced fluorescence. These data demonstrate efficient coherent population transfer by SCRAP. 相似文献
58.
Technical cellulose-2.5-acetates (CA 2.5) were characterized regarding their carbohydrate composition in comparison to the raw material. The association of the CA 2.5 samples in acetone was studied by size exclusion chromatography (SEC) using various acetone grades and styrene divinylbenzene copolymer columns. In HPLC grade acetone with and without addition of 1% water up to three different pre-humps eluted in front of the main fraction of the polymer. The evaluation of the main peak by light scattering measurements resulted in high molar masses indicating that for these technical CA 2.5 samples even the main fraction is not dissolved without association. No pre-humps or association phenomena were observed after addition of 1 ppm LiBr to HPLC grade acetone or with p.a. grade acetone. In addition pre-hump enriched and pre-hump free fractions were isolated by fractionated precipitation. The carbohydrate composition of these fractions was determined and correlated with their association pattern in SEC investigations. 相似文献
59.
We consider a class of indecomposable modules over the Virasoro Lie algebra that we call bounded admissible modules. We get results concerning the center and the dimensions of the weight spaces. We prove that these modules always contain a submodule with one-dimensional weight spaces. From this follows the proof of a conjecture of V. Kac concerning the classification of simple admissible modules.Preprint Université de Bourgogne-mai 1990 相似文献
60.
Matthias Heuchel Martin Bhning Ole Hlck Martin R. Siegert Dieter Hofmann 《Journal of Polymer Science.Polymer Physics》2006,44(13):1874-1897
Atomistic packing models have been created, which help to better understand the experimentally observed swelling behavior of glassy polysulfone and poly (ether sulfone), under CO2 gas pressures up to 50 bar at 308 K. The experimental characterization includes the measurement of the time‐dependent volume dilation of the polymer samples after a pressure step and the determination of the corresponding gas concentrations by gravimetric gas‐sorption measurements. The models obtained by force‐field‐based molecular mechanics and molecular dynamics methods allow a detailed atomistic analysis of representative swelling states of polymer/gas systems, with respect to the dilation of the matrix. Also, changes of free volume distribution and backbone mobility are accessible. The behavior of gas molecules in unswollen and swollen polymer matrices is characterized in terms of sorption, diffusion, and plasticization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1874–1897, 2006 相似文献