Daniel Lakeland (@dlakelan.mastodon.sdf.org.ap.brid.gy)

Any #mathematics professors want to take a shot at explaining to someone with say a typical math major undergrad background what the deal is with the recent introduction of exacting and ultraexacting cardinals? #math #maths #settheory #zfc https://arxiv.org/abs/2411.11568

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Large cardinals, structural reflection, and the HOD Conjecture We introduce exacting cardinals and a strengthening of these, ultraexacting cardinals. These are natural large cardinals defined equivalently as weak forms of rank-Berkeley cardinals, strong forms of Jónsson cardinals, or in terms of principles of structural reflection. However, they challenge commonly held intuition on strong axioms of infinity. We prove that ultraexacting cardinals are consistent with Zermelo-Fraenkel Set Theory with the Axiom of Choice (ZFC) relative to the existence of an I0 embedding. However, the existence of an ultraexacting cardinal below a measurable cardinal implies the consistency of ZFC with a proper class of I0 embeddings, thus challenging the linear--incremental picture of the large cardinal hierarchy. We show that the existence of an exacting cardinal implies that V is not equal to HOD (Gödel's universe of Hereditarily Ordinal Definable sets), showing that these cardinals surpass the current hierarchy of large cardinals consistent with ZFC. Moreover, we prove that the existence of an exacting cardinal above an extendible cardinal implies the "V is far from HOD" alternative of Woodin's HOD Dichotomy. In particular, it follows that the consistency of ZFC with an exacting cardinal above an extendible cardinal would refute Woodin's HOD Conjecture and Ultimate-L Conjecture. Finally, we show that the consistency of ZF with certain large cardinals beyond choice implies the consistency of ZFC with the existence of an exacting cardinal above an extendible cardinal. https://arxiv.org/abs/2411.11568