How do I reference an unpublished article in my paper?

Question

I am currently writing a paper which is partly based on the unpublished work of a known mathematician. The "unpublished work" is basically a memo that has not appeared in any journal, but has widely circulated since its release and is available on the "Publications" list of the author's homepage.

I would really like to use this article in my paper, but I am usually against referencing an unpublished "memo", for obvious reasons. Since this is a different case (a high-quality paper from a known mathematician), how do I proceed with including it in my paper?

Answer 1

Referencing such a document is a well-know proof technique, often stated as follows (cf. proof techniques):

Proof by reference to inaccessible literature: The author cites a simple corollary of a theorem to be found in a privately circulated memoir of the Slovenian Philological Society, 1883.

In your case, the quality of the work seems to be a non-issue due to the reputation of the mathematician, but what is an issue is the long-term availability of the document.

I ran into references to unpublished works a few times. For a particular example in my mind, everyone could find it, apparently, several decades ago, but now the document is gone. I must say I really hate it.

So, provide the reader with all the information you can find that would help them to get the manuscript. E.g.:

Abra K. Dabra, On the importance of being published, 1883, Slovenian Philological Society, unpublished technical report, retrieved from the author's Web page http://www.example.com on 2018-02-30.

or

Abra K. Dabra, On the importance of being published, 1883, Slovenian Philological Society, unpublished technical report, available from the library of the Slovenian Philological Society, street house-no, 1000 Ljubljana, Slovenia.

or

Abra K. Dabra, On the importance of being published, 1883, Slovenian Philological Society, unpublished, retrieved from jstor.org/... on 2018-02-30.

etc.

In addition, as @darij-grinberg says, you may ask the author to store his/her paper with an independent service for long-term archival purposes.

If you reference hard claims with their proofs, you might wish to copy the material to the appendix of your document to make your document self-contained. Of course, get the permission from the original author first, acknowledge him or her properly, and mark the corresponding part as a citation.

Answer 2

1. You can help save the preprint for posterity. For example, in the Internet Archive's Wayback Machine, if you try to access a page that has not been archived, it will automatically ask whether you want the current version to be archived. This isn't fully bulletproof (future owners of the website may make the archived versions inaccessible through robots.txt, which a few universities unfortunately do), and it's sub-optimal for preprints that undergo changes (because you can force the Wayback Machine to archive a non-archived page, but you cannot force it to update an already-archived page if the archived version is obsolete); but it's better than nothing. Another service is archive.is. Sure, these services may disappear one day, so it's best to put your eggs in as many baskets as you can be bothered to.

2. Sometimes, just mailing the author and asking them to post their preprint on the arXiv will work. If they consider it not sufficiently fleshed out for the arXiv, there is also the Zenodo repository, which from my understanding is less restrictive (although the widespread opinion that arXiv is just for submission-ready preprints isn't true: it's good for everything that is readable, interesting and correct to the authors' knowledge; this includes technical reports, lecture notes and expository work).