首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5108篇
  免费   236篇
  2023年   41篇
  2020年   101篇
  2019年   117篇
  2018年   223篇
  2017年   207篇
  2016年   198篇
  2015年   160篇
  2014年   154篇
  2013年   1246篇
  2012年   181篇
  2011年   161篇
  2010年   131篇
  2009年   165篇
  2008年   190篇
  2007年   135篇
  2006年   91篇
  2005年   60篇
  2004年   65篇
  2003年   62篇
  2002年   60篇
  2001年   102篇
  2000年   67篇
  1999年   61篇
  1998年   41篇
  1997年   31篇
  1996年   53篇
  1995年   54篇
  1994年   40篇
  1993年   51篇
  1992年   31篇
  1991年   34篇
  1990年   29篇
  1989年   41篇
  1988年   39篇
  1987年   42篇
  1986年   38篇
  1985年   42篇
  1984年   50篇
  1983年   58篇
  1982年   46篇
  1981年   42篇
  1980年   45篇
  1979年   37篇
  1978年   47篇
  1977年   42篇
  1976年   52篇
  1974年   27篇
  1973年   29篇
  1972年   30篇
  1971年   30篇
排序方式: 共有5344条查询结果,搜索用时 15 毫秒
151.
152.
The sophistication of regional economic models has been demonstrated in numerous ways, most recently in the form of linking several modeling systems or in the expansion in the number of equations that can be manipulated successfully to produce impact analyses or forecasts. In this paper, an alternative perspective is employed. What do regional macro-level forecasts indicate about the process of structural change? A new methodology is illustrated that enables analysts to make forecasts of detailed structural change in the interindustry relations in an economy. Using a regional econometric input-output model developed for the Chicago metropolitan region, derived input-output tables are extracted for the period 1975–2016. These tables are then analyzed to determine the forecasted direction of structural changes for the region. The innovation illustrated here is based on a model that exploits the general equilibrium spirit of computable general equilibrium models through the adjustment of input coefficients to clear markets.  相似文献   
153.
154.
National parks can no longer be considered as pristine ecosystems protected by impenetrable boundaries. It is now realised that many forces outside national park boundaries create significant changes within those boundaries. To date, most attention has focussed upon penetration by biophysical factors. This paper addresses another aspect, external influence over decision making in the parks. It suggests that three main groups - entrepreneurs, environmentalists, and aboriginal peoples - have been dominant in different time periods, and uses case studies to illustrate the rise to current prominence of aboriginal peoples. Reasons are suggested for this ascendancy.
Les pares nationaux ne peuvent plus être considérés comme des écosystèmes primitifs protégés par des limites impénétrables. On remarque à présent que beaucoup de forces en dehors des limites des pares nationaux peuvent créer des changements significatifs à &lcaron intérieur de ces limites. Jusquà maintenant la plupart des études ont traité de la pénétration par des facteurs biophysiques. Cet article traite &dcaron un autre facteur, une influence externe qui agit sur les décisions prises au sujet des pares. Nous suggérons que trois groupes principaux - les entrepreneurs, les écologistes et les populations autochtones - ont été dominants à des époques différentes, et soumettrons des études de cas pour illustrer comment les populations autochtones sont arriveées à la dominance. Nous offrirons également des raisons pour cette ascension.  相似文献   
155.
This paper describes the use of electron spin resonance spectroscopy to estimate the degree of heating of quartzite cobbles from hearths on the floor of the archaeological remains of an eighteenth-century Dutch colonial slave lodge. A novel technique based on the comparison of line intensities for the E’and O-2 centres in quartz distinguished successfully between cobbles which had been heated to estimated temperatures ranging from 300 °C to 450° C and controls from an adjacent stream bed. This inexpensive and simple technique could be applied to a wide range of archaeological problems involving the thermal history of objects consisting of or containing quartz.  相似文献   
156.
The Greek Neolithic, here divided into five phases, is the oldest in Europe. Geographically, it is the closest to the Near East and has thus long been seen as an impoverished derivative of the latter. However, recent research has tended to emphasize the autochthonous nature of Neolithic development in Europe, including Greece. The Greek Neolithic economy, which was based almost entirely on domesticates, and its densely packed, long-lived villages strongly recall the Near East, as do also the early emphasis on fine, decorated, nonculinary pottery and the wealth of figurines. On the other hand, the evolution of stylistic patterns is specifically Greek, although generally related to trends in the Balkans. Originality in the development of the Greek Neolithic is also seen in its latest phase, with apparent decreases, rather than increases, in site density, social differentiation, and, to some extent, long-distance trade. At the same time, however, the dichotomy became much sharper between the rich agricultural plains of northern Greece and the more pastoral(?) regions of the Peloponnese and Cycladic Islands; this presages similar contrasts during the Bronze Age.  相似文献   
157.
158.
Several samples of pyramids from Giza were obtained. The samples were analysed by scanning electron microscopy, electron diffraction by X-rays, powder X-ray diffraction, inductively coupled plasmography, Fourier-transform infrared spectroscopy and other techniques. Some scientists who have recently studied the pyramids have suggested that the pyramids were poured into place, stone by stone. The prevailing theory among Egyptologists is, of course, that the pyramids were built from blocks of natural stone. All of the results obtained during this study directly support the concept that the pyramids are made of limestone and are not cementitious in nature.  相似文献   
159.
160.
A multi-regional framework is developed in order to analyze net migration over time to all 10 Canadian provinces within an integrated system of equations. "An extended gravity model is the basis for the equation specification and the use of constrained econometric estimation techniques allows for the provincial interdependence of the migration decision while at the same time ensuring that an important system-wide requirement is respected." The model is estimated using official Canadian data for the 1960s and 1970s. "The results suggest the predominance of the push factor for interprovincial migration for most provinces, although net migration to the Atlantic provinces is also shown to be subject to pull forces from the rest of the country." The effects of wage rate variables, unemployment, and political disturbances in Quebec on inter-provincial migration are noted.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号