全文获取类型
收费全文 | 578958篇 |
免费 | 5627篇 |
国内免费 | 1859篇 |
专业分类
化学 | 296172篇 |
晶体学 | 8466篇 |
力学 | 26244篇 |
综合类 | 9篇 |
数学 | 80639篇 |
物理学 | 174914篇 |
出版年
2020年 | 3446篇 |
2019年 | 3666篇 |
2018年 | 12016篇 |
2017年 | 12728篇 |
2016年 | 10677篇 |
2015年 | 6119篇 |
2014年 | 7689篇 |
2013年 | 22791篇 |
2012年 | 20470篇 |
2011年 | 31690篇 |
2010年 | 19690篇 |
2009年 | 19550篇 |
2008年 | 28059篇 |
2007年 | 31255篇 |
2006年 | 18342篇 |
2005年 | 23111篇 |
2004年 | 18162篇 |
2003年 | 16209篇 |
2002年 | 14196篇 |
2001年 | 14764篇 |
2000年 | 11475篇 |
1999年 | 9013篇 |
1998年 | 7414篇 |
1997年 | 7234篇 |
1996年 | 7190篇 |
1995年 | 6593篇 |
1994年 | 6257篇 |
1993年 | 6121篇 |
1992年 | 6763篇 |
1991年 | 6702篇 |
1990年 | 6288篇 |
1989年 | 6128篇 |
1988年 | 6369篇 |
1987年 | 6035篇 |
1986年 | 5816篇 |
1985年 | 8236篇 |
1984年 | 8448篇 |
1983年 | 6905篇 |
1982年 | 7526篇 |
1981年 | 7424篇 |
1980年 | 7172篇 |
1979年 | 7271篇 |
1978年 | 7523篇 |
1977年 | 7302篇 |
1976年 | 7396篇 |
1975年 | 6983篇 |
1974年 | 6940篇 |
1973年 | 7316篇 |
1972年 | 4397篇 |
1971年 | 3309篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
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
52.
A simple scheme is developed for treatment of vertical bed topography in shallow water flows. The effect of the vertical step on flows is modelled with the shallow water equations including local energy loss terms. The bed elevation is denoted with zb‐ for the left and zb+ for the right values at each grid point, hence exactly representing a discontinuity in the bed topography. The surface gradient method (SGM) is generalized to reconstruct water depths at cell interfaces involving a vertical step so that the fluxes at the cell interfaces can accurately be calculated with a Riemann solver. The scheme is verified by predicting a surge crossing a step, a tidal flow over a step and dam‐break flows on wet/dry beds. The results have shown good agreements compared with analytical solutions and available experimental data. The scheme is efficient, robust, and may be used for practical flow calculations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
53.
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
54.
55.
A method using third order moments for estimating the regression coefficients as well as the latent state scores of the reduced-rank regression model when the latent variable(s) are non-normally distributed is presented in this paper. It is shown that the factor analysis type indeterminacy of the regression coefficient matrices is eliminated. A real life example of the proposed method is presented. Differences of this solution with the reduced-rank regression eigen solution are discussed. 相似文献
56.
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
57.
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
58.
59.
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
60.
J. P. Levine 《Inventiones Mathematicae》1989,96(3):571-592