## Set theory with a proper class of indiscernibles

### Volume 259 / 2022

#### Abstract

We investigate an extension of $\mathrm {ZFC}$ set theory, denoted $\mathrm {ZFI}_{ \lt }$, which is equipped with a well-ordering $ \lt $ of the universe V of set theory, and a proper class $I$ of indiscernibles over $(\mathrm {V},\in , \lt )$. Our main results are Theorems A, B, and C below. Note that the equivalence of conditions (ii) and (iii) in Theorem A was established in an earlier (2004) published work of the author. In what follows, $\mathrm{GBC}$ is the Gödel–Bernays theory of classes with global choice. In Theorem C the symbol $\rightarrow $ is the usual Erdős-arrow notation for partition calculus.

Theorem A. *The following are equivalent for a sentence $\varphi $ in the language $\{=,\in \}$ of set theory:*

(i) $\mathrm {ZFI}_{\mathrm { \lt }}\vdash \varphi .$

(ii) $\mathrm {ZFC}+{\mit\Lambda} \vdash \varphi ,$ *where* ${\mit\Lambda} =\{\lambda _{n}:n\in \omega \}$, *and* $\lambda _{n}$ *is the sentence asserting the existence of an *$n$*-Mahlo cardinal* $\kappa $ *such that* $\mathrm {V}(\kappa )$ *is a* ${\mit\Sigma} _{n}$*-elementary submodel of the universe* $\mathrm {V}$.

(iii) $\mathrm{GBC}$ $+$ “$\textrm{Ord}$ is weakly compact” $\vdash \varphi $.

Theorem B. *Every *$\omega $-*model of* $\mathrm {ZFI}_{\mathrm { \lt }}$*satisfies *$\mathrm {V}\neq \mathrm {L}$.

Theorem C. *The sentence expressing *$\forall m,n\in \omega $ $( \mathrm {Ord}\rightarrow ( \mathrm {Ord}) _{m}^{n}) $ *is not provable in the theory* $T= \textrm{GBC}$ $+$ “$\textrm {Ord}$ *is weakly compact*” , *assuming* $T$ *is consistent.*

The paper also includes results about the interpretability relationship between the theories $\mathrm {ZFC}+{\mit\Lambda} $, $\mathrm {ZFI}_{\mathrm { \lt }}$, and $\mathrm{GBC}$ $+$ “$\textrm{Ord}$ is weakly compact”.