Inner models from extended logics and the Delta-operation

TL;DR

This paper explores how slight modifications to abstract logics, specifically the $ riangle$-extension, affect the associated inner models $C(rak{L})$, revealing that such changes can lead to strictly larger inner models under certain set-theoretic assumptions.

Contribution

It demonstrates the sensitivity of inner models $C(rak{L})$ to the $ riangle$-extension of the logic, providing examples where the extended logic yields strictly larger inner models, including cases linked to $0^{ ext{sharp}}$.

Findings

Inner models $C(rak{L})$ can be strictly smaller than $C( riangle(rak{L}))$.

The $ riangle$-extension can increase the size of the inner model.

Existence of $0^{ ext{sharp}}$ implies a strict difference between $C(rak{L})$ and $C( riangle(rak{L}))$.

Abstract

If $\mathcal{L}$ is an abstract logic (a.k.a. model theoretic logic), we can define the inner model $C(\mathcal{L})$ by replacing first order logic with $\mathcal{L}$ in G\"odel's definition of the inner model $L$ of constructible sets. Set theoretic properties of such inner models $C(\mathcal{L})$ have been investigated recently and a spectrum of new inner models is emerging between $L$ and $\mathrm{HOD}$. The topic of this paper is the effect on $C(\mathcal{L})$ of a slight modification of $\mathcal{L}$ i.e. how sensitive is $C(\mathcal{L})$ on the exact definition of $\mathcal{L}$? The $\Delta$-extension $\Delta(\mathcal{L})$ of a logic is generally considered a "mild" extension of $\mathcal{L}$. We give examples of logics $\mathcal{L}$ for which the inner model $C(\mathcal{L})$ is consistently strictly smaller than the inner model $C(\Delta(\mathcal{L}))$, and in one case we show this follows from the existence of $0^{\sharp}$.