»BASE (Bielefeld Academia Search Engine)
https://www.base-search.net/

»SciELO – Scientific Electronic Library Online
https://worldwidescience.org/

»WorldWideScience.org
https://scielo.org/es/

»Google Scholar (Google Académico
https://scholar.google.com/

»Dialnet
https://dialnet.unirioja.es/

»Scholarpedia
http://scholarpedia.org/

»Springer Link
https://link.springer.com/

»ERIC Education Resources Information Center
https://eric.ed.gov/

»Academia.edu
https://www.academia.edu/