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

### Yann Pequignot

University of Los Angeles, USA

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## 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 — notably [5, 9–11, 14, 22] — contain 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.

## Cite this article

Yann Pequignot, Towards better: A motivated introduction to better-quasi-orders. EMS Surv. Math. Sci. 4 (2017), no. 2, pp. 185–218

DOI 10.4171/EMSS/4-2-2