EBSCO

https://research.ebsco.com/c/wtftir/search

Web of Science

https://www.webofscience.com/wos/woscc/basic-search

Scopus

https://www.scopus.com/search/form.uri?display=basic#basic

Science Direct

https://www.sciencedirect.com/

Directory of Open Access Books (DOAB)

https://www.doabooks.org/

Directory of Open Access Journals (DOAJ)

https://doaj.org/

Open Textbook Library

https://open.umn.edu/opentextbooks/

Open Course Library

https://www.openwa.org/open-course-library/

MIT Open Course Ware

https://ocw.mit.edu/

IOP Science

https://iopscience.iop.org/

OAPEN Open access publishing in Europian Networks

 https://www.oapen.org/

National Science Digital Library

https://en.wikipedia.org/wiki/National_Science_Digital_Library

All Russian Mathematical Portal

https://www.mathnet.ru/

OER Commons

https://oercommons.org/

KiberLeninka

https://cyberleninka.ru/

IMF eLibrary

https://www.elibrary.imf.org

Networked Digital Library of Theses and Dissertations

https://ndltd.org/