When reviewing first answers or late answers, I fairly often come across answers along the following format:
See this link for the answer: [link]
A proof of this can be found in book X.
While these are technically an answer, I feel like they are not really relevant to this site, because
- the material in the link or book may be extremely advanced or extremely lengthy,
- in the case of links, the links may be dead, and the answer thus become useless, and
- 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.