Bielefeld Academic Search Engine

https://www.base-search.net/

CORE

https://core.ac.uk/

Semantic Scholar

https://www.semanticscholar.org/

Baidu Scholar

Science.gov

Science.gov

Google Scholar

Microsoft Academic

WorldWideScience

https://worldwidescience.org/

Wolfram Alpha

http://www.wolframalpha.com/
“computational knowledge engine”

Refseek

http://www.refseek.com/

Educational Resources Information Center

Educational Resources Information Center (ERIC) https://eric.ed.gov/

Virtual Learning Resources Center (VLRC)

NDLTD

http://www.ndltd.org/

theses.fr

https://www.theses.fr/en/

DART-Europe E-theses Portal

DART-Europe http://www.dart-europe.eu/basic-search.php