Answers with links or references

Question

When reviewing first answers or late answers, I fairly often come across answers along the following format:

See this link for the answer: [link]

or

A proof of this can be found in book X.

A couple of examples of this is user31266's answer to this question and mFriction's answer to this question.

While these are technically an answer, I feel like they are not really relevant to this site, because

1. the material in the link or book may be extremely advanced or extremely lengthy,
2. in the case of links, the links may be dead, and the answer thus become useless, and
3. in the case of books, the OP may not have access to the book in question.

So my question is, what is the usual procedure for these kind of answers? Should they be deleted or flagged, should I leave a comment, asking them to put some of the material they reference into the actual answer, or should they just be left as is?

A related question was discussed here, but I feel it doesn't quite answer my question.

Answer 1

It depends on the kind of question asked. For example, if the question is "what is wrong with my attempted proof?" then "here's a link to a correct proof" is not an answer. It should be flagged Not An Answer.

In other cases, a reference to literature may be exactly what the asker wants: see the reference-request tag.

And there is a broad spectrum in between, where posting just a reference isn't the right kind of answer, but may be an answer nonetheless. There's also a distinction between a link to some personal webpage that may disappear next week, and a stable reference to scholarly literature (this includes ArXiv). I'm more inclined to apply the NAA flag (and delete votes) to the former than to the latter.

I also consider how likely it is that a self-contained answer would be posted. If it's a calculus exercise, pointing out that it's a solved Example 14.82.6423(f) in some book isn't adding much value, since a self-contained answer will likely be given (if it hasn't been posted already). For some specialized questions, getting even a relevant reference is already a success; deleting it would trade a reference-only answer for no answer at all.