Model-theoretic characterizations of large cardinals (Re){}²visited
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We characterize several large cardinal notions by model-theoretic properties of extensions of first-order logic. We show that $\Pi_n$-strong cardinals, and, as a corollary, ``Ord is Woodin" and weak Vop\v{e}nka's Principle, are characterized by compactness properties involving Henkin models for sort logic. This provides a model-theoretic analogy between Vop\v{e}nka's Principle and weak Vop\v{e}nka's Principle. We also characterize huge cardinals by compactness for type omission properties of the well-foundedness logic $\mathbb L(Q^{\text{WF}})$, and show that the compactness number of the H\"artig quantifier logic $\mathbb L(I)$ can consistently be larger than the first supercompact cardinal. Finally, we show that the upward L\"owenheim-Skolem-Tarski number of second-order logic $\mathbb L^2$ and the sort logic $\mathbb L^{s,n}$ are given by the first extendible and $C^{(n)}$-extendible cardinal, respectively.
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Model theory of class-sized logics
Restricted fragments of class-sized logics regain compactness and Löwenheim-Skolem properties that exactly characterize Weak Vopěnka's Principle, Ord is Woodin, and Shelah cardinals.
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