CiteSeerX is a search engine and citation database for openly accessible scientific information online. It encompasses more than 720,000 documents, mainly in the fields of computer science and the information sciences. Many publications can be downloaded free of charge as pdfs. The database was developed at the NEC Research Institute, in Princeton, USA, and is operated by Pennsylvania State University.

MathSciNet is the online version of the Mathematical Reviews journal and the technical bibliography Current Mathematical Publications. The database contains publications from mathematics, statistics, and theoretical informatics. Around 1,900 international serials and journals have been analyzed since 1940. A list of the titles analyzed is available as a pdf. The integrated author database makes it possible to find all the publications from a single author, even if a name isn't always written identically or is the same as another author's. The online tutorials contain search tips.

PsycNet is the search platform of the American Psychological Association. This platform encompasses publications in psychology, pedagogy, sociology, behavioral research, and mental health.

RePEc (Research Papers in Economics) is a joint initiative from economists in 75 countries that documents working papers, journal articles, and economic software tools. A large portion is openly accessible or directly linked to the complete texts. Additionally, information about the authors (contact address, publication lists) and institutions (research institution, faculties, etc.) is available. The RePEc data can be searched from different entry points, including IDEAS (not the complete holdings), WoPEc (complete working papers), EDIRC (institutions), and BibEc (bibliography of the working papers that aren't openly accessible).

Google Scholar is an openly accessible search engine specifically for scholarly literature. It searches through numerous scholarly servers online and indexes not only the openly accessible offers, but also those from publishing houses. Potential advantages of this server are:

• Greater scope than Web of Science and Scopus 
• Broad consideration for different types of documents (articles, books, conference articles, etc.)

However, there are numerous disadvantages, such as:

• Search results are inconsistent and cannot be reproduced
• Ranking rules are unknown
• Match lists cannot be sorted according to one's own criteria
• Citation extraction and analysis are highly disputed due to their propensity for errors
• Search tools are limited
• "Publish Or Perish" (PoP) is a free software that was developed to further edit data from Google Scholar for bibliometric analyses. It easily calculates different indicators. However, it doesn't make sense to use the software alone, since its results are only as good as its foundation, meaning Google Scholar.

Please note that we do not recommend using this service as the data is not valid.

Microsoft Academic Search is a search engine specifically for academic literature. Similar to Google Scholar it records publications and websites free of charge. The datapool, however, is pretty restricted. It has only indexed domains on the topic "computer science" since 2009. Other disciplines have only been included since 2011.

Please note that we do not recommend using this service since the data is not valid.