Thursday, May 17, 2018, from 4 to 5:30pm
East Hall, room 4096
Speaker: Harry Altman (University of Michigan)
Title: Well partial orderings and their maximum extending ordinals
A well partial order is a partial order all of whose extensions to a total order are well-orders. (These are often studied as well-quasi-orders, where the requirement of antisymmetry is dropped.) In 1976 De Jongh and Parikh showed that for a given WPO X, among the ordinals obtained this way there is always a maximum o(X). We will discuss the theory of WPOs and o(X), several equivalent formulations, and how o(X) can actually be computed for some concrete WPOs.