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5 votes

Ordinal-indexed transitive antichain of sets with urelements

Assuming Vopenka's principle (a large cardinal axiom), we can show there is no such $\phi$. In particular, a corollary of Vopenka's principle is that every proper class of directed graphs contains som …
Adam P. Goucher's user avatar
7 votes
3 answers
229 views

Ordinal-indexed transitive antichain of sets with urelements

Operate in ZFC. Can we find a function-class $\phi$ whose domain is the class of ordinals such that the following properties hold? If $x \in \phi(\alpha)$, then either $x \in \mathbb{N}$ or there ex …
Adam P. Goucher's user avatar
36 votes

Does an existence of large cardinals have implications in number theory or combinatorics?

There's an extremely elementary theorem whose only known proof relies on the existence of a rank-into-rank cardinal (basically the strongest large cardinal axiom not known to contradict ZFC). Let $R_ …
Adam P. Goucher's user avatar