Bounding finite-image sequences of length $ω^k$

Abstract

Given a well-quasi-order $X$ and an ordinal $α$, the set $s^F_α(X)$ of transfinite sequences on $X$ with length less than $α$ and with finite image is also a well-quasi-order, as proven by Nash-Williams. Before Nash-Williams proved it for general $α$, however, it was proven for $α<ω^ω$ by Erdős and Rado. In this paper, we revisit Erdős and Rado's proof and improve upon it, using it to obtain upper bounds on the maximum linearization of $s^F_{ω^k}(X)$ in terms of $k$ and $o(X)$, where $o(X)$ denotes the maximum linearization of $X$. We show that, for fixed $k$, $o(s^F_{ω^k}(X))$ is bounded above by a function which can roughly be described as $(k+1)$-times exponential in $o(X)$. We also show that, for $k\le 2$, this bound is not far from tight.

Theorems & Definitions (56)

• Theorem 1.1
• Theorem 1.2
• Definition 2.1
• Definition 2.2
• Theorem 2.3: De Jongh, Parikh, Schmidt DJPSchmidt
• Definition 2.4
• Theorem 2.6: Abriola et. al. Abriola
• Definition 2.7
• Definition 3.1
• Definition 3.2