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Morgan Rogers (he/him) (May 06 2020 at 11:06):As I've mentioned here a few times already @Jens Hemelaer and I are currently working on toposes of monoid actions. We're currently struggling with Lemma C3.2.14 of the Elephant, which gives necessary and sufficient criteria for essential geometric morphisms between presheaf toposes (induced by a functor at the level of small categories) to be proper. The proof invokes some heavy-duty indexed category machinery that is hard to follow. We thought we had a concrete family of counterexamples, although we're no longer as sure that they're genuine counter-examples.
My question is: are there errata for Johnstone's Sketches of an Elephant? A cursory search doesn't provide any, but it would be surprising if there were no known mistakes in a reference text of that size.
Mike Shulman (May 06 2020 at 16:24):I don't know of any list of errata maintained by the author, but a few errata have been recorded at https://ncatlab.org/nlab/show/Elephant.
David Michael Roberts (May 13 2020 at 05:03):I very strongly doubt Peter Johnstone would maintain an electronic list of errata. In 2013 or 2014 he handed out printed copies of his latest preprint at a talk, the only way to get a copy without someone bootlegging it. Also he said he'd never approve ebook versions of anything he's published (Baby elephant, Stone spaces, Elephant, etc). His website is practically non-existent.
David Michael Roberts (May 13 2020 at 05:04):For that matter, Vol 3 is functionally non-existent, as much as I'd like him to do a Knuth and release pre-fascicles as he goes.
Morgan Rogers (he/him) (May 13 2020 at 09:27):You should see his Cambridge office. I reckon he has filed hard copies of every article he's ever read in there.
David Michael Roberts (May 13 2020 at 23:27):The cave of wonders!