1) Public Knowledge Project (PKP)
2) Digital Object Identifier System (DOI)
3) Google Scholar
4) Crossref Event Data
5) Onesearch
6) Base-Search
7) Orcid
8) Directory of Research Journals Indexing (DRJI)
9) Directory Of Open Access Scholarly Resources (ROAD)
10) Academic Resource Index (ReasearchBib)
11) Portal Garuda
12) Publons
13) DOAJ