全文获取类型
收费全文 | 242300篇 |
免费 | 2745篇 |
国内免费 | 868篇 |
专业分类
化学 | 130152篇 |
晶体学 | 3688篇 |
力学 | 9889篇 |
综合类 | 2篇 |
数学 | 27049篇 |
物理学 | 75133篇 |
出版年
2020年 | 1764篇 |
2019年 | 2035篇 |
2018年 | 2470篇 |
2017年 | 2400篇 |
2016年 | 3815篇 |
2015年 | 2587篇 |
2014年 | 3889篇 |
2013年 | 11146篇 |
2012年 | 8064篇 |
2011年 | 9987篇 |
2010年 | 6562篇 |
2009年 | 6459篇 |
2008年 | 8970篇 |
2007年 | 9120篇 |
2006年 | 8363篇 |
2005年 | 7822篇 |
2004年 | 6978篇 |
2003年 | 6198篇 |
2002年 | 6129篇 |
2001年 | 7196篇 |
2000年 | 5357篇 |
1999年 | 4215篇 |
1998年 | 3547篇 |
1997年 | 3576篇 |
1996年 | 3288篇 |
1995年 | 3128篇 |
1994年 | 2944篇 |
1993年 | 2999篇 |
1992年 | 3292篇 |
1991年 | 3327篇 |
1990年 | 3143篇 |
1989年 | 3098篇 |
1988年 | 3159篇 |
1987年 | 3029篇 |
1986年 | 2923篇 |
1985年 | 3985篇 |
1984年 | 4145篇 |
1983年 | 3399篇 |
1982年 | 3749篇 |
1981年 | 3645篇 |
1980年 | 3517篇 |
1979年 | 3551篇 |
1978年 | 3777篇 |
1977年 | 3606篇 |
1976年 | 3814篇 |
1975年 | 3387篇 |
1974年 | 3516篇 |
1973年 | 3819篇 |
1972年 | 2327篇 |
1971年 | 1786篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
R. J. Vanderbei 《Mathematical Programming》1989,43(1-3):31-44
The affine-scaling modification of Karmarkar's algorithm is extended to solve problems with free variables. This extended primal algorithm is used to prove two important results. First the geometrically elegant feasibility algorithm proposed by Chandru and Kochar is the same algorithm as the one obtained by appending a single column of residuals to the constraint matrix. Second the dual algorithm as first described by Adler et al., is the same as the extended primal algorithm applied to the dual. 相似文献
102.
A new geometrical method to determine the surface damage threshold for ultrashort pulses is presented. It consists in the formation of a surface damage profile by a movement of the sample across the laser beam focus. A single measurement of the maximum transversal dimension of this damage profile, which depends solely on the laser beam power, is used to calculate the local damage threshold intensity. It is also theoretically shown that the damage in the transverse dimension can be controlled under the diffraction-limited spot. 相似文献
103.
The consequences of Ge deposition on Br-terminated Si(1 0 0) were studied with scanning tunneling microscopy at ambient temperature after annealing at 650 K. One monolayer of Br was sufficient to prevent the formation of Ge huts beyond the critical thickness of 3 ML. This is possible because Br acts as a surfactant whose presence lowered the diffusivity of Ge adatoms. Hindered mobility was manifest at low coverage through the formation of short Ge chains. Further deposition resulted in the extension and connection of the Ge chains and gave rise to the buildup of incomplete layers. The deposition of 7 ML of Ge resulted in a rough surface characterized by irregularly shaped clusters. A short 800 K anneal desorbed the Br and allowed Ge atoms to reorganize into the more energetically favorable “hut” structures produced by conventional Ge overlayer growth on Si(1 0 0). 相似文献
104.
R. J. Hinde 《Few-Body Systems》2006,38(2-4):187-191
We compute the vibrational coupling between two H2 molecules from ab initio quantum chemical calculations of the H2-H2 potential carried out at the full configuration interaction level of theory using the atom-centered aug-cc-pVTZ basis set
for hydrogen. We compare the full configuration interaction results with those obtained using two variants of coupled cluster
theory and find that a fully iterative treatment of connected triples may be required to estimate the H2-H2 vibrational coupling accurately using coupled cluster theory. 相似文献
105.
In this paper, we explore the interplay of four different conjectures on certain zero-sum problems in Zp Zp. This study of the inter-relations between these conjectures leads to the conclusion that determining the structure of minimal zero sequences (see below for the precise definition) is crucial. Also, we study the analogous situation in Zn. 相似文献
106.
A. R. Volkov B. V. Shul'gin T. I. Polupanova V. N. Lebedev A. A. Nagornyi V. L. Petrov Yu. F. Kargin 《Journal of Applied Spectroscopy》1991,54(6):585-590
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991. 相似文献
107.
A firm's logistics cost, including shipping and inventory-carrying, is a substantial percentage of its sales. Nevertheless, typical inventory-control methods ignore or insufficiently represent the shipping cost. This paper describes a recursive algorithm that determines the reorder cycle-time that minimizes total logistics cost. It allows for a realistic accounting of shipping cost, which is modelled here as a function of shipping distance and weight. The algorithm uses a relaxation procedure to identify a suitable initial approximation to the optimal order cycle-time and then, through a series of recursive steps, moves to the optimal result. We demonstrate the algorithm with a single item, with a group of items that share a common order cycle, and with multi-items when item demands are random variables. Experience with this algorithm indicates that it converges to the optimal result in a very few steps. 相似文献
108.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection. 相似文献
109.
Michael R. Caputo 《Natural Resource Modeling》1989,3(2):241-259
This paper analyzes the extent to which standard dynamic renewable resource models possess refutable implications. Both the steady state comparative static and local comparative dynamic properties of the standard model are studied. A unified framework is developed which enables one to analyze the qualitative properties of any standard renewable resource model. This is achieved by explicitly linking the local stability, steady state comparative static, and local comparative dynamic properties of the model. 相似文献
110.
R. Kaufman 《Israel Journal of Mathematics》1989,67(1):117-122
A closed setE is constructed so thatA∼(E) is an inseparable Banach space but its maximal ideal space isE.
Supported in part by the National Science Foundation. 相似文献