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   16936篇
  免费   34篇
  国内免费   41篇
系统科学   92篇
丛书文集   263篇
教育与普及   30篇
理论与方法论   72篇
现状及发展   8015篇
研究方法   710篇
综合类   7649篇
自然研究   180篇
  2013年   82篇
  2012年   199篇
  2011年   392篇
  2010年   76篇
  2008年   230篇
  2007年   249篇
  2006年   293篇
  2005年   287篇
  2004年   385篇
  2003年   265篇
  2002年   262篇
  2001年   482篇
  2000年   443篇
  1999年   327篇
  1992年   288篇
  1991年   222篇
  1990年   238篇
  1989年   235篇
  1988年   203篇
  1987年   234篇
  1986年   279篇
  1985年   340篇
  1984年   231篇
  1983年   226篇
  1982年   206篇
  1981年   217篇
  1980年   224篇
  1979年   563篇
  1978年   476篇
  1977年   437篇
  1976年   342篇
  1975年   404篇
  1974年   564篇
  1973年   452篇
  1972年   473篇
  1971年   556篇
  1970年   696篇
  1969年   538篇
  1968年   539篇
  1967年   536篇
  1966年   430篇
  1965年   371篇
  1964年   121篇
  1959年   193篇
  1958年   330篇
  1957年   247篇
  1956年   190篇
  1955年   192篇
  1954年   190篇
  1948年   127篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
Selective functionalization of mesoporous silica nanospheres (MSNs) is crucial for nanoengineering of MSNs. Herein, we have combined “surface-protected etching strategy” and “cationic surfactant assisted etching strategy” to prepare functionalized MSNs with externally attached amino groups. The externally attached NH2 groups endow the catalysts with excellent catalytic performance for nitroaldol reaction between nitromethane and benzaldehyde. In addition, those NH2-MSNs can also be used to support gold nanoparticles, which display very good catalytic performance for reduction of 4-nitrophenol. It can be envisioned that the synthesis protocol developed in this work could also be extended to nanoengineered MSNs, which provides opportunities for nanoreactors design.  相似文献   
172.
The main purpose of this paper is to exposit two very different, but very general, motivational schemes in the art of parameterization and a concrete example connecting them. We introduce a dynamic version of the DOMINATING SET problem and prove that it is fixed-parameter tractable(FPT). The problem is motivated by settings where problem instances evolve. It also arises in the quest to improve a natural greedy heuristic for the DOMINATING SET problem.  相似文献   
173.
174.
175.
176.
Quite often over the last decade, heavy equipment manufacturers are trying to improve their project management performance with the use of effective project management methods. However, the challenges encountered aren’t isolated problems and a systemic approach is required. This paper exemplifies how a systemic approach can be applied to heavy equipment manufacturing project (HEMP) management, through of a multifaceted vision. The findings present in the paper are based on a two-year case study in a metallurgic equipment manufacturing enterprise in China. Based on the analysis of HEMP management problems and the contributions on systemic approach application in project management, the paper enriches existing theory by: (1) interpreting the HEMP management system with the top-down disassembly method; (2) developing the collaborative project management approach with a systems view highlighting the project characteristics management and inter-departmental collaboration; (3) making suggestions for systemic approach application in project management. For practitioners and researchers, the findings are particularly meaningful for manufacturing project management and systemic approach application.  相似文献   
177.
The positive impacts of managing projects as a portfolio are quantified by comparing the value of the integrated risk of a project portfolio and the aggregation of single project risks implemented separately. Firstly, the integrated risk is defined by proposing risky events based on set theory. Secondly, as projects interact with each other in a project portfolio, the integrated risk is evaluated by using a Bayesian network structure learning algorithm to construct an interdependent network of risks. Finally, the integrated risk of a practical case is assessed using this method, and the results show that the proposed method is an effective tool for calculating the extent of risk reduction of implementing a project portfolio and identifying the most risky project, so as to assist companies in making comprehensive decisions in the phase of portfolio selection and portfolio controlling.  相似文献   
178.
Vascular development is a dynamic process that relies on the coordinated expression of numerous genes, but the factors that regulate gene expression during blood vessel development are not well defined. ATP-dependent chromatin-remodeling complexes are gaining attention for their specific temporal and spatial effects on gene expression during vascular development. Genetic mutations in chromatin-remodeling complex subunits are revealing roles for the complexes in vascular signaling pathways at discrete developmental time points. Phenotypic analysis of these models at various stages of vascular development will continue to expand our understanding of how chromatin remodeling impacts new blood vessel growth. Such research could also provide novel therapeutic targets for the treatment of vascular pathologies.  相似文献   
179.
Studies in several areas in Arizona and New Mexico show that dwarf mistletoe ( Arceuthobium douglasii ) is rare in young Douglas-firs growing under infected overstories. Less than 5% of the Douglas-firs under 26 years old and less than 6% of those under 1.4m tall were infected in 77 mistletoe-infested stands. Both percent infection and mean dwarf mistletoe rating of young Douglas-firs increased as tree age, height, and stand dwarf mistletoe ratings increased.  相似文献   
180.
The floristic composition of the Lee Creek valley in southwestern Alberta, Canada, is presented. The valley flora consists of 299 species of vascular plants representing 173 genera and 46 families. The flora is dominated by forbs (73 percent), followed by grasses (24 percent), shrubs (12 percent), and trees (11 percent). The valley of Lee Creek is divided into upper and lower sections on the basis of elevational, climatic, and biotic differences. Floristic richness of the Lee Creek valley is quite evenly divided between the upper and lower valleys: 105 species limited to the upper valley, 95 to the lower valley, and 99 species shared by both upper and lower valleys.  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