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
14) Indexcopernicus
15) EBSCO
16) CORE
17) WorldCat
18) Moraref