全文获取类型
收费全文 | 4628篇 |
免费 | 215篇 |
出版年
2023年 | 22篇 |
2020年 | 68篇 |
2019年 | 102篇 |
2018年 | 141篇 |
2017年 | 174篇 |
2016年 | 169篇 |
2015年 | 131篇 |
2014年 | 115篇 |
2013年 | 1527篇 |
2012年 | 126篇 |
2011年 | 119篇 |
2010年 | 168篇 |
2009年 | 144篇 |
2008年 | 105篇 |
2007年 | 128篇 |
2006年 | 125篇 |
2005年 | 67篇 |
2004年 | 80篇 |
2003年 | 76篇 |
2002年 | 73篇 |
2001年 | 65篇 |
2000年 | 59篇 |
1999年 | 47篇 |
1998年 | 53篇 |
1997年 | 49篇 |
1996年 | 28篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 32篇 |
1992年 | 41篇 |
1991年 | 37篇 |
1990年 | 30篇 |
1989年 | 23篇 |
1988年 | 38篇 |
1987年 | 35篇 |
1986年 | 28篇 |
1985年 | 46篇 |
1984年 | 37篇 |
1983年 | 46篇 |
1982年 | 41篇 |
1981年 | 40篇 |
1980年 | 33篇 |
1979年 | 34篇 |
1978年 | 33篇 |
1977年 | 25篇 |
1976年 | 30篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 14篇 |
1972年 | 17篇 |
排序方式: 共有4843条查询结果,搜索用时 0 毫秒
31.
32.
The p-median problem is a powerful tool in analyzing facility location options when the goal of the location scheme is to minimize the average distance that demand must traverse to reach its nearest facility. It may be used to determine the number of facilities to site, as well as the actual facility locations. Demand data are frequently aggregated in p-median location problems to reduce the computational complexity of the problem. Demand data aggregation, however, results in the loss of locational information. This loss may lead to suboptimal facility location configurations (optimality errors) and inaccurate measures of the resulting travel distances (cost errors). Hillsman and Rhoda (1978) have identified three error components: Source A, B, and C errors, which may result from demand data aggregation. In this article, a method to measure weighted travel distances in p-median problems which eliminates Source A and B errors is proposed. Test problem results indicate that the proposed measurement scheme yields solutions with lower optimality and cost errors than does the traditional distance measurement scheme. 相似文献
33.
David M. Mark 《Geographical analysis》1987,19(3):264-272
A new and recursive algorithm for determining proximal polygons is based on quadtree concepts. The algorithm can use any distance metric, and produces a quadtree of the image of the proximal polygons. This can be converted to vector form, or be incorporated directly into a quadtree-based geographic information system, in order to solve a number of closest-point problems. The algorithm can also produce the Delaunay triangulation, which is the dual of the proximal polygons. Empirically, the running time of the algorithm is proportional to the number of centers raised to the 1.6 power. 相似文献
34.
35.
Igor V. Volvenko Andrey V. Gebruk Oleg N. Katugin Georgy M. Vinogradov Alexei M. Orlov 《Geographical Research》2023,61(4):503-511
Biogeographers, ecologists, palaeontologists, and conservation managers often deal with checklists in which not all individuals have been identified to a species level, or the accuracy of species identification is questionable. Is it possible and credible to investigate species richness based on such checklists? Studies on macrofauna in the Far Eastern seas, eastern Arctic seas, and adjacent waters of the Pacific and Arctic Oceans suggest that in different habitats and for diverse taxa, species, and higher taxa richness strongly correlate with each other and increase with an expansion in the study area and sample size according to the species–area law. Such an increase is higher in the bottom zone than in the pelagic. Species and higher taxa richness also show a decrease from lower to higher latitudes, which is in line with the Humboldt–Wallace’s law. According to Willis’ law and self-similarity in the organisation of taxonomic levels, species richness can be assessed based on the genus, family, and order richness. In other words, supraspecies richness itself can tell us the same as species richness and therefore certain global patterns revealed at the species level may also be revealed at the supraspecies level. Such a concordance in general trends among richness parameters at different taxonomic levels in practice implies that species richness can be studied based on lists that lack species identifications or lists with doubtful species identification. We suggest bolder use of supraspecies richness in science and practice, discussing the disadvantages and advantages of this approach. 相似文献
36.
37.
V. MEJDAHL 《Archaeometry》1972,14(2):245-256
This article outlines the thermoluminescent dating technique employed at Risö. The intensity of beta and gamma radiation is determined by means of the TL phosphors CaSO4:Mn and CaSO4:Dy, and the determination of accumulated exposure is based on quartz and feldspar inclusions with a size of 0.3–0.5 mm. Results are presented of a dating programme comprising sherds, bricks, burned stones and burned clay (from kilns) from seventeen excavation sites. The age of the sites ranged from a.d. 1600 to 4000 b.c. 相似文献
38.
39.
40.