On the deductive strength of the Erdős–Dushnik–Miller theorem and two order-theoretic principles

Abstract

We provide answers to open questions from Banerjee and Gopaulsingh (Bull Pol Acad Sci Math 71: 1–21, 2023) about the relationship between the Erdős–Dushnik–Miller theorem (\(\textsf{EDM}\)) and certain weaker forms of the Axiom of Choice (\(\textsf{AC}\)), and we properly strengthen some results from Banerjee and Gopaulsingh (2023). We also settle a part of an open question of Lajos Soukup (stated in Banerjee and Gopaulsingh (2023) [Question 6.1]) about the relationship between the following two order-theoretic principles, which [as shown in Banerjee and Gopaulsingh (2023)] are weaker than \(\textsf{EDM}\): (a) “Every partially ordered set such that all of its antichains are finite and all of its chains are countable is countable” (this is known as Kurepa’s theorem), and (b) “Every partially ordered set such that all of its antichains are countable and all of its chains are finite is countable”. In particular, we prove that (b) does not imply (a) in \(\textsf{ZF}\) (i.e., Zermelo–Fraenkel set theory without \(\textsf{AC}\)). Moreover, with respect to (b), we answer an open question from Banerjee and Gopaulsingh (2023) about its relationship with the following weak choice form: “Every set is either well orderable or has an amorphous subset”; in particular, we show that (b) follows from, but does not imply, the latter weak choice principle in \(\textsf{ZFA}\) (i.e., Zermelo–Fraenkel set theory with atoms).