In Don Page’s paper Return of the Boltzmann Brains:
For example, in  I made the estimate that the probability per 4-volume for a brief brain is , where the upper exponent had already been predicted 33 years ago .
The relevant citation:
 D. Adams, The Hitchhiker’s Guide to the Galaxy (Pan Books, London, 1979).
Having skimmed the paper, I missed the whole “33 years ago” joke, and only went back to look for it when I skimmed through the citations and saw HHGG in it as #42. Notably, the paper has exactly 42 citations.