On logics extended with embedding-closed quantifiers

TL;DR

This paper investigates extended first-order and infinitary logics with embedding-closed quantifiers, demonstrating eventual equivalence to quantifier-free formulas in long chains of structures and introducing a game to characterize structure equivalence.

Contribution

It introduces a new approach to analyze logics with embedding-closed quantifiers, including undefinability results and a novel Ehrenfeucht-Fra"iss"e game for these logics.

Findings

Long chains of structures lead to formulas becoming quantifier-free.

Undefinability results for logics with embedding-closed quantifiers.

A new game characterizes equivalence in extended infinitary logics.

Abstract

We study first-order as well as infinitary logics extended with quantifiers closed upwards under embeddings. In particular, we show that if a chain of quasi-homogeneous structures is sufficiently long then a given formula of such a logic is eventually equivalent to a quantifier-free formula in that chain. We use this fact to produce a number of undefinability results for logics with embedding-closed quantifiers. In the final section we introduce an Ehrenfeucht-Fra\"iss\'e game that characterizes the $L$-equivalence between structures, where $L$ is the infinitary logic $L_{\infty \omega}$ extended with all embedding-closed quantifiers. In conclusion, we provide an application of this game illustrating its use.