收费全文 | 98篇 |
免费 | 5篇 |
管理学 | 6篇 |
民族学 | 2篇 |
人口学 | 13篇 |
丛书文集 | 1篇 |
理论方法论 | 11篇 |
社会学 | 19篇 |
统计学 | 51篇 |
2023年 | 6篇 |
2022年 | 2篇 |
2020年 | 5篇 |
2019年 | 7篇 |
2018年 | 10篇 |
2017年 | 2篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 4篇 |
2013年 | 29篇 |
2012年 | 5篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 1篇 |
2007年 | 1篇 |
2006年 | 1篇 |
2005年 | 1篇 |
2002年 | 1篇 |
2001年 | 2篇 |
1997年 | 1篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1983年 | 2篇 |
Truss design is a well-known structural optimization problem that has important practical applications in various fields. Truss design problems are typically multimodal by nature, meaning that it offers multiple optimal solutions concerning the topology (combinatorial optimization problem) and/or sizes (continuous optimization problem) of the members, but they are evaluated to have similar or equally good objective function values. From a practical standpoint, it is desirable to find as many alternative designs as possible, rather than finding a single design, as often practiced. Several techniques based on classical and metaheuristic optimization methods have been developed for simultaneous optimization of topology and size of a truss. However, all these methods unable to find multiple topologies and their corresponding size solutions in a single run. A few metaheuristics incorporating niching techniques have been developed for finding multiple topologies for the truss design problem, but these studies ignored the fact that for each known topology, multiple design solutions in terms of size can be found. To address this issue, this paper proposes a bi-level truss formulation and subsequently a speciation-based bilevel niching method (BiL-NM) using such a formulation. The BiL-NM consists of a modified SPSO niching method which is robust to find multiple topologies and a canonical PSO for their corresponding size solutions. Extensive empirical studies are carried out to analyze the accuracy, robustness, and efficiency of the BiL-NM. The results confirm that the proposed BiL-NM is superior in all these three aspects over the state-of-the-art methods on several low to high-dimensional truss design problems.
相似文献Green roof systems have been increasingly implemented to enhance vegetation cover and associated ecosystem services in urban spaces, with primary goals being the reduction of peak surface runoff, enhanced water quality, and mitigation of urban heat island effects. Recently, green roofs have also received attention as a means to enhance carbon sequestration, but direct measurements of greenhouse gas fluxes from established green roof systems are largely lacking. Here we present observations of CO2 and CH4 fluxes from substrates of experimental extensive green roof units that varied in vegetation type (Sedum spp., and a native meadow species mix), substrate depth, substrate type (high vs. low organic matter content), and irrigation. We predicted that substrate CO2 effluxes would be higher in high-organic-matter substrates and that systems with high organic matter would potentially act as CH4 sources. Substrate fluxes were low compared to natural soils, with seasonal means ranging from an efflux of 0.1–0.4 µmol CO2 m-2.s-1 and uptake of ~0.00–0.04 nmol CH4 m-2.s-1, with higher fluxes late in the growing season. CO2 fluxes showed large increases in response to irrigation and were higher from the high-organic-matter substrate and with increased substrate depth. The strength of the CH4 sink increased in response to prior irrigation treatments, and CH4 emissions were detected only on low-organic-matter substrates early in the growing season. No effects of vegetation type were detected for either CO2 or CH4 flux. Our results indicate that high levels of organic matter in green roof substrates may enhance aerobic soil respiration but are not associated with CH4 emissions, which instead were only detected in low-organic-matter substrates.
相似文献