From: Ann Shumelda Okerson <[log in to unmask]>
Date: Sun, 19 Aug 2018 12:30:04 -0400

Worth a detour:

"Unpaywall consists of a database that includes a list of almost 20 million
freely available scholarly articles. Most researchers access it using a
browser plug-in <https://www.nature.com/articles/d41586-017-05922-9> that
was released in 2017. The service works by searching for a queried paper’s
unique digital tag — a string of numbers and letters known as its DOI, or
digital object identifier — against those of articles gathered from 50,000
journals and repositories.

"In June 2017, Unpaywall was integrated into a popular science search
engine called Web of Science, which is operated by Clarivate Analytics.
Dimensions, a service run by Digital Science that launched this year, used
Unpaywall from the get-go. These companies, and now Elsevier, pay a
subscription fee for a feed of Unpaywall’s database that is updated weekly."
[SNIP]

https://www.nature.com/articles/d41586-018-05968-3