Google's search engine for scholarly research. It indexes citations, abstracts, and full-text articles, books, conference proceedings, theses, online repositories, patents, legal cases, and more. Google Scholar is particularly used as a 'covering the base' approach when starting research and canvassing the literature or as a final search on a topic.
Google Scholar Content Coverage
Google Scholar Quick Guide (by University of Alaska library)
There are bibliographic data sources by subject fields:
This is a digital library portal for researchers in astronomy and physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant.
ADS maintains three bibliographic collections with more than 15 million records covering publications in astronomy and astrophysics, physics, and general science.
It is a scientific literature digital library and search engine with a focus primarily on the literature in computer and information science.
This is the authoritative gateway to mathematics literature by AMS.