TITLE: Publishing papers that became classics before they were submitted
QUESTION [20 upvotes]: Sometimes the following happens: a result is proven, but the author never submits a paper for publication. In some cases, a preprint appears. In some cases, the proof is so short that it can be presented at a conference or lecture series, and the community is convinced. Maybe it later becomes incorporated in the work of others, book or lecture notes expositions appear, other work generalizes or improves upon it etc. The result is widely accepted, and the community gives due credit.
Now assume that some years later, the author decides to submit the paper to a prestigious journal. How should the editor treat it, depending on the circumstances listed above, importance of the result, etc.? The two obvious extreme points of view are "reject, since the result is not new" and "treat the paper as proving a new result with a somewhat delayed submission", but there's a whole spectrum in between, e.g., "what was good for Annals 20 years ago is not anymore, but OK for a lesser journal".
I would be especially interested in precedents that are publicly known or can be shared publicly.

REPLY [16 votes]: This happened in my career. In 1984 I published two preprints, joint with M. Lyubich, in Russian and two short announcements based on these preprints (also in Russian). These had a substantial following. In the early 1990s we wrote a paper and submitted it to a journal. The paper was rejected on the grounds that "this is a survey of the well-known results".
(Which was essentially true: by that time the results were already well-known since many people read and used our preprints). So we submitted the paper to another journal, and in 1992 the paper was published. It is my most cited paper.
The situation is quite common in the subject of holomorphic dynamics.
In 1982 Douady and Hubbard published a large preprint (>200 pages)
"Etude dynamique des polynomes complexes" and announced the main results
in a CR note. They wanted to write a book, but this plan never materialized. This preprint established the foundation of the study of Mandelbrot set.

REPLY [11 votes]: There are countless examples, especially if one includes the publication of the Nachlass of a deceased mathematician. Let me mention just one example that I have been looking forward to.
The book Integration in Finite Terms: Fundamental Sources (probably not the best hyperlink, but I don't think it has a DOI yet) is slated to be published soon. When I requested more information from one of the editors, Michael Singer, he provided me with a sneak peek at an early draft of the Preface. After explaining that the book contains four items, the first two of which are reprints (of a paper by Rosenlicht and a book by Ritt, both entitled Integration in Finite Terms), the Preface goes on to say:

The third is a revised version of Robert Risch’s unpublished On the integration of elementary functions which are built up using algebraic
operations. This latter paper reduced the problem of finding elementary
antiderivatives of elementary functions to a problem in arithmetic algebraic geometry: the problem of determining if a point on the jacobian of a curve is of finite
order. Risch presented a solution of this latter problem in [Ris70] in the context
of deciding the elementary integrability of an algebraic function in [Ris70]. The
present paper includes the complete proof. The commentary of Clemens G. Raab
discusses the impact of this paper and further developments. The final paper is the
unpublished thesis of Barry M. Trager Integration of Algebraic Functions.
This latter paper provided practical algorithms for performing the integration in finite terms of algebraic functions after the theoretical results of Risch. It is followed
by the commentary of Barry Trager, who gives further insight into the methods of
this thesis as well as subsequent developments.

The OP also asks what an editor should do in such situations. Although MO usually frowns upon opinion-based questions, I will offer an opinion anyway.  The mathematical community should do more than it currently does to encourage the publication of, or at least the wide distribution of, such material.  As a community, we often encourage people to read the masters, and we also take some pride in the "democratic" nature of our subject, meaning that mathematics is open to all.  Well, it's not open to all if some of the writings of the masters are available only to a privileged inner circle.  Publication of important and highly influential unpublished material does not solve all such problems but it is a step in the right direction.  Currently I think we place too much emphasis on novelty. I like what Harold Edwards said in his book Riemann's Zeta Function about Siegel's monumental effort to study Riemann's Nachlass and present his findings to the mathematical public.

One wonders whether anyone else would ever have unearthed this treasure if Siegel had not. It is indeed fortunate that Siegel's concept of scholarship derived from the older tradition of respect for the past rather than the contemporary style of novelty.

REPLY [11 votes]: G.A.Margulis announced his super -rigidity and arithmeticity in an ICM talk (which he could not attend). At the next ICM where he received the Fields medal, this was one of the major cited works.  He later gave a different proof in an appendix to a Russian translation of M.S Raghunathan's book on discrete subgroups of Lie groups. Later an English translation of  this appendix was published in the Inventiones.
Here is a link to the review https://mathscinet.ams.org/mathscinet-getitem?mr=739627