The unpublished papers in reference to the published papers Sometimes it happens that a published paper refers to an unpublished paper for a result used.
In this case, if we want to check this result by ourselves, we need to access to this unpublished paper.  
Question: What is the usual process for accessing to an unpublished paper?
 Is it the duty of the author (citing it in his published paper) to send it to anyone requiring it?
Or is it the duty of the editor, or of the author of the unpublished paper?   
What to do if we can not get it through these means?
 A: I did write such a unpublished-but-cited paper. My co-author did not answer to my questions about the journal where to publish it, and then left mathematics. This happened before internet time. It is still cited, usually with the caption Available from D. S. under request. Perhaps I should post the scan on ArXiv. I must confess that I began using ArXiv only a few month ago...
Of course, every member of the PDE community knows about an unpublished paper by (P.-L.) Lions, Papanicolaou & Varadhan. We refer to it as the eternal preprint, a terminology seemingly due to PLL.
A: As the reader, you can also ask mathoverflow for help finding the paper.  There have been some very interesting MO questions along this line.  In particular, this record helps others to also track down the same paper (or discover it never existed in the first place).  Feel free to add to this list since this is CW:


*

*Scott-Solovay unpublished paper on ``Boolean valued models of set theory'' (never existed)

*Looking for a copy of Leo Harrington's unpublished notes on the first nonprojectible ordinal (now we have a version online)

*Harrington's unpublished note "The constructible reals can be anything" (now we have a version online)

A: "What to do if we can not get it through these means?"...
Ask all the people you know. Someone might have a copy.
Once you get a copy, and if you find the pre-paper useful, don't forget to advertise that you have a copy, so that others can benefit too.
A: *

*I think this is a duty of the author to refer only on those results which are available.
(Perhaps not published, but posted somewhere on the Internet). I think it is a duty of the editor and referee to make sure that the authors follow this rule. A result which uses a
reference which is not available cannot be considered as proved.

*In the case when there is an unpublished reference, which I need, I search on the internet, Google scholar, author's web page, etc. If not found I write to the author and ask her to send me a copy. In pre-computer era, I would write a letter to the author in the case I am really interested in the result.
A: In a broader context, this is the problem of the role of "grey literature" in the progress of science. Wikipedia has quite an extensive overview of this issue, and there is even an academic journal devoted entirely to it. (The Grey Journal, ironically enough itself part of the grey literature.)
I would think that the access part of this issue is the least interesting in these days of the internet (if it's a recent document that is not online somewhere, it might as well not exist); the real issue is the role of unrefereed research in the progress of science: can you rely on an unrefereed result, whether it is the proof of a theorem or the outcome of a clinical trial?
