1. Google Scholar

2. Crossref

3. Dimensions

4. ROAD (Directory of Open Access Scholarly Resources)

5. BASE (Bielefeld Academic Search Engine)

6. Indonesia One Search

7. GARUDA (Garba Rujukan Digital)

8. ICI (Index Copernicus International)

9. Worldcat

10. Scientific Literature (Scilit)

11. Universiteit Leiden

12. Harvard University

13. Semantic Scholar

14. Eurasian Scientific Journal Index (ESJI)

15. UCLA Library

16. EuroPub