Lecturer:
Alberto Marcone, Gabriele Puppis (University of Udine)

Board Contact:
Alberto Marcone

Wqos (well quasi-orders) are an important class of partial orders that have been repeatedly introduced and studied in various areas of mathematics. We will show the equivalence of various characterizations of wqos and we will prove the basic results. In the 1960’s Nash-Williams introduced a strengthening of the notion of wqo and called it bqo (better quasi-order): this notion led to the proof of Fraïssé’s conjecture, stating that countable linear orders are a wqo when compared by embeddability. Wqos and bqos found applications in computer science, e.g. in formal verification, and some lectures will be devoted to these applications.