I'm not sure what to do about this issue. Here are three answers:
- Good Books On Abstract Algebra
- Good abstract algebra books for self study
- Requesting abstract algebra book recommendations
Identical text, no context, only links to PDF. Some seem legal, most are not.
Now I understand that the issue on Math.SE and SE in general is not obvious (from this and links from there, for instance). It's not even clear to me whether these links may cause a legal problem to SE Inc. (which is probably their only concern). My personal policy is to put only links to legal material, as I don't want Math.SE to become an index to illegal content. But it's only me.
However here the repetition and the lack of original content in the answer seem abusive to me.
More generally, is it possible to have a clear, definitive answer: is a link to an illegal copy of copyrighted material accepted here or not? If not, what's the proper action?
The only answer I can find is this, that states: "There might or might not be significant legal constraints on what can be linked in math.SE discussions (externally hosted downloads). This would depend on many factors including differences in laws between countries."
That's still quite vague. And beyond the purely legal question, there is also the position of the community about this and the policy of the site.