Yeah, it's an old chestnut: https://en.wikipedia.org/wiki/Common_knowledge_(logic) https://en.wikipedia.org/wiki/Common_knowledge_(logic) Spivak has a version

Yeah, it's an old chestnut: https://en.wikipedia.org/wiki/Common_knowledge_(logic) https://en.wikipedia.org/wiki/Common_knowledge_(logic) Spivak has a version of it in his Calculus book, phrased as 17 (heh!) professors who must resign if a flaw is found in their published work (hehehe), and all have a flaw in their papers, known to each other except each author.