1. Index Copernicus
2. CAS Source Index (CASSI)
3. Central & Eastern European Academic Source (CEEAS)
4. Crossref
5. MIAR
6. WorldCat
7. Bielefeld Academic Search Engine (BASE)
8. National Library of Ukraine named after VI Vernadsky
9. Google Scholar
10. OpenAIRE
11. Dimensions
12. Directory of Open Access scholarly Resources (ROAD)
13. Open policy finder
14. CORE (COnnecting REpositories)
15. Scilit
16. Ulrich's Periodicals Directory
17. Korea Institute of Science and Technology Information (KISTI)
18. SJSU (California, United States) library
19. SLUB
20. China National Knowledge Infrastructure (CNKI)
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