# A proof of Shelah's eventual categoricity conjecture and an extension to accessible categories with directed colimits

## TL;DR

This paper proves Shelah's eventual categoricity conjecture for AECs in ZFC and extends it to accessible categories with directed colimits under SCH, establishing a broad categoricity transfer principle and confirming a conjecture on eventual amalgamation.

## Contribution

It provides the first ZFC proof of Shelah's conjecture for AECs and generalizes the result to accessible categories with directed colimits assuming SCH, also confirming Grossberg's conjecture.

## Key findings

- Proves Shelah's eventual categoricity conjecture in ZFC for AECs.
- Extends categoricity transfer results to accessible categories with directed colimits under SCH.
- Establishes that high enough categoricity implies eventual amalgamation in AECs and accessible categories.

## Abstract

We provide a proof, in $ZFC$, of Shelah's eventual categoricity conjecture for abstract elementary classes (AEC's). Moreover, assuming in addition the Singular Cardinal Hypothesis ($SCH$), we prove a direct generalization to the more general context of accessible categories with directed colimits. If $\mathcal{K}$ is such a category, we show that there is a cardinal $\mu$ such that if $\mathcal{K}$ is $\lambda$-categorical for some $\lambda \geq \mu$ (i.e., it has only one object of internal size $\lambda$ up to isomorphism), then $\mathcal{K}$ is eventually categorical (i.e., it is $\lambda'$-categorical for every $\lambda' \geq \mu$). When considering cardinalities of models of infinitary theories $\mathbb{T}$ of $\mathcal{L}_{\kappa, \theta}$ that axiomatize $\mathcal{K}$, the result implies, under $SCH$, the following infinitary version of Morley's categoricity theorem: let $S$ be the class of cardinals $\lambda$ which are of cofinality at least $\theta$ but are not successors of cardinals of cofinality less than $\theta$. Then, if $\mathbb{T}$ is a $\mathcal{L}_{\kappa, \theta}$ theory whose models have directed colimits and it is $\lambda$-categorical for some $\lambda \geq \mu$ in $S$, then it is $\lambda'$-categorical for every $\lambda' \geq \mu$ in $S$; moreover, we also exhibit an example that shows that the exceptions in the class $S$ are needed. Along the way we also prove Grossberg conjecture, according to which categoricity in a high enough cardinal implies eventual amalgamation. We establish this result in AEC's and, assuming in addition $SCH$, in the more general context of accessible categories whose morphisms are monomorphisms.

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Source: https://tomesphere.com/paper/1906.09169