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BASE (Bielefeld Academic Search Engine) is one of the world's most voluminous search sengines especially for academic web resources, e.g. journal articles, preprints, digital collections, images / videos or research data. Other than search engines like Google or Bing BASE searches the deep web as well. The sources which are included are intellectually selected and reviewed. BASE is operated by Bielefeld University Library.
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CiteSeerx is a scientific literature digital library and search engine that focuses primarily on the literature in computer and information science. CiteSeerx aims to improve the dissemination of scientific literature and to provide improvements in functionality, usability, availability, cost, comprehensiveness, efficiency, and timeliness in the access of scientific and scholarly knowledge.
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Scopus is an abstract and citation database for research literature published since 1996, with discovery and analytics tools. The multidisciplinary content covers the fields of Natural Sciences, Engineering, Biomedical Research as well as Arts and Humanities and the Social Sciences. Scopus generates citation search results and automatically updated researcher and institution profiles. Thus, connections between people, published ideas, and organizations can be identified.
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Web of Science is a curated collection of over 20,000 peer-reviewed, high-quality scholarly journals published worldwide (including Open Access journals) in over 250 science, social sciences, and humanities disciplines. Conference proceedings and book data are also available. Web of Science offers cited reference searching which means that you can search for records that cite a particular document.
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