Towards Better: A motivated introduction to better-quasi-orders

TL;DR

This paper provides a motivated, self-contained introduction to the theory of better-quasi-orders (BQOs), highlighting their importance and the motivation behind their study in various mathematical and computational fields.

Contribution

It offers the first comprehensive, accessible introduction to BQO theory, bridging the gap in existing literature and serving as a foundational resource.

Findings

Clarifies the concept and significance of BQOs

Explains the relationship between WQOs and BQOs

Provides foundational understanding for further research

Abstract

The well-quasi-orders (WQO) play an important role in various fields such as Computer Science, Logic or Graph Theory. Since the class of WQOs lacks closure under some important operations, the proof that a certain quasi-order is WQO consists often of proving it enjoys a stronger and more complicated property, namely that of being a better-quasi-order (BQO). Several articles contains valuable introductory material to the theory of BQOs. However, a textbook entitled "Introduction to better-quasi-order theory" is yet to be written. Here is an attempt to give a motivated and self-contained introduction to the deep concept defined by Nash-Williams that we would expect to find in such a textbook.