A dead mathematician co-authored a paper after appearing in a dream: https://boingboing.net/2021/07/20/a-person-in-a-dream-co-authored-a-math-paper.html
The paper is "Higher Algebraic \(K\)-Theory of Schemes and of Derived Categories" by Robert Wayne Thomason and Thomas Trobaugh (2007), https://doi.org/10.1007/978-0-8176-4576-2_10, https://www.ams.org/mathscinet-getitem?mr=1106918
It appears to have been quite an influential one; the MR review calls it a landmark, and it has over 1000 citations on Google Scholar.
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