Index Medicus: Index medicus (IM) is a comprehensive bibliographic index of scientific journal articles related to medical science.

Index Copernicus: Index Copernicus (IC) is an online database of user-contributed information, including scientist profiles as well as of scientific institutions, and publications.

Google Scholar:

Crossref: Crossref is an official Digital Object Identifier (DOI) registration agency of the international DOI foundation which enables publishers to provide a cross-publisher citation linking in the online academic journal.