Fra\"iss\'e's conjecture, partial impredicativity and well-ordering principles, part I

TL;DR

This paper investigates the logical strength of Fra"isse9's conjecture within reverse mathematics, establishing a new bound via pseudo a -comprehension and introducing a related combinatorial principle.

Contribution

It introduces a hierarchy-based bound for Fra"isse9's conjecture's strength using pseudo a -comprehension and relates it to well-ordering principles and reflection theories.

Findings

Fra"isse9's conjecture is implied by pseudo a -comprehension.

A cofinite a -Ramsey theorem variant is introduced.

Pseudo a -comprehension is connected to reflection principles.

Abstract

Fra\"iss\'e's conjecture (proved by Laver) is implied by the $\Pi^1_1$-comprehension axiom of reverse mathematics, as shown by Montalb\'an. The implication must be strict for reasons of quantifier complexity, but it seems that no better bound has been known. We locate such a bound in a hierarchy of Suzuki and Yokoyama, which extends Towsner's framework of partial impredicativity. Specifically, we show that Fra\"iss\'e's conjecture is implied by a principle of pseudo $\Pi^1_1$-comprehension. As part of the proof, we introduce a cofinite version of the $\Delta^0_2$-Ramsey theorem, which may be of independent interest. We also relate pseudo $\Pi^1_1$-comprehension to principles of pseudo $\beta$-model reflection (due to Suzuki and Yokoyama) and reflection for $\omega$-models of transfinite induction (studied by Rathjen and Valencia-Vizca\'ino). In a forthcoming companion paper, we characterize pseudo $\Pi^1_1$-comprehension by a well-ordering principle, to get a transparent combinatorial bound for the strength of Fra\"iss\'e's conjecture.