Free groups and the axiom of choice

The Nielsen–Schreier theorem states that subgroups of free groups are free. As all of its proofs use the Axiom of Choice, it is natural to ask whether the theorem is equivalent to the Axiom of Choice. Other questions arise in this context, such as whether the same is true for free abelian groups, and whether free groups have a notion of dimension in the absence of Choice. In chapters 1 and 2 we define basic concepts and introduce Fraenkel–Mostowski models. In chapter 3 the notion of dimension in free groups is investigated. We prove, without using the full Axiom of Choice, that all bases of a free group have the same cardinality. In contrast, a closely related statement is shown to be equivalent to the Axiom of Choice. Schreier graphs are used to prove the Nielsen–Schreier theorem in chapter 4. For later reference, we also classify Schreier graphs of (normal) subgroups of free groups. Chapter 5 starts with an analysis of the use of the Axiom of Choice in the proof of the Nielsen–Schreier theorem. Then we introduce representative functions – a tool for constructing choice functions from bases. They are used to deduce the finite Axiom of Choice from Nielsen–Schreier, and to prove the equivalence of a strong form of Nielsen–Schreier and the Axiom of Choice. Using Fraenkel–Mostowski models, we show that Nielsen–Schreier cannot be deduced from the Boolean Prime Ideal Theorem. Chapter 6 explores properties of free abelian groups that are similar to those considered in chapter 5. However, the commutative setting requires new ideas and different proofs. Using representative functions, we deduce the Axiom of Choice for pairs from the abelian version of the Nielsen–Schreier theorem. This implication is shown to be strict by proving that it doesn’t follow from the Boolean Prime Ideal Theorem. We end with a section on potential applications to vector spaces

The weak choice principle WISC may fail in the category of sets

The set-theoretic axiom WISC states that for every set there is a set of surjections to it cofinal in all such surjections. By constructing an unbounded topos over the category of sets and using an extension of the internal logic of a topos due to Shulman, we show that WISC is independent of the rest of the axioms of the set theory given by a well-pointed topos. This also gives an example of a topos that is not a predicative topos as defined by van den Berg.Comment: v2 Change of title and abstract; v3 Almost completely rewritten after referee pointed out critical mistake. v4 Final version. Will be published in Studia Logica. License is CC-B

An axiomatic approach to the measurement of envy

We characterize a class of envy-as-inequity measures. There are three key axioms. Decomposability requires that overall envy is the sum of the envy within and between subgroups. The other two axioms deal with the two-individual setting and specify how the envy measure should react to simple changes in the individuals’ commodity bundles. The characterized class measures how much one individual envies another individual by the relative utility difference (using the envious’ utility function) between the bundle of the envied and the bundle of the envious, where the utility function that must be used to represent the ordinal preferences is the ‘ray’ utility function. The class measures overall envy by the sum of these (transformed) relative utility differences. We discuss our results in the light of previous contributions to envy measurement and multidimensional inequality measurement