Categories and General Algebraic Structures with Applications (Jan 2017)
Choice principles and lift lemmas
Abstract
We show that in ZF set theory without choice, the Ultrafilter Principle (UP) is equivalent to several compactness theorems for Alexandroff discrete spaces and to Rudin's Lemma, a basic tool in topology and the theory of quasicontinuous domains. Important consequences of Rudin's Lemma are various lift lemmas, saying that certain properties of posets are inherited by the free unital semilattices over them. Some of these principles follow not only from UP but also from DC, the Principle of Dependent Choices. On the other hand, they imply the Axiom of Choice for countable families of finite sets, which is not provable in ZF set theory.
