## Eleftherios Tachtsis

*On certain non-constructive properties of infinite-dimensional vector spaces*

Comment.Math.Univ.Carolin. 59,3 (2018) 285-309.**Abstract:**In set theory without the axiom of choice (${\rm AC}$), we study certain non-constructive properties of infinite-dimensional vector spaces. Among several results, we establish the following: (i) None of the principles AC$^{\rm LO}$ (AC for linearly ordered families of nonempty sets)---and hence AC$^{\rm WO}$ (AC for well-ordered families of nonempty sets)---\break${\rm DC}({<}\kappa)$ (where $\kappa$ is an uncountable regular cardinal), and ``for every infinite set~$X$, there is a bijection $f\colon X\rightarrow\{0,1\}\times X$", implies the statement ``there exists a field $F$ such that every vector space over $F$ has a basis" in ZFA set theory. The above results settle the corresponding open problems from Howard and Rubin ``Consequences of the axiom of choice", and also shed light on the question of Bleicher in ``Some theorems on vector spaces and the axiom of choice" about the set-theoretic strength of the above algebraic statement. (ii) ``For every field $F$, for every family $\mathcal{V}=\{V_{i}\colon i\in I\}$ of nontrivial vector spaces over $F$, there is a family $\mathcal{F}=\{f_{i}\colon i\in I\}$ such that $f_{i}\in F^{V_{i}}$ for all $ i\in I$, and $f_{i}$ is a nonzero linear functional" is equivalent to the full AC in ZFA set theory. (iii) ``Every infinite-dimensional vector space over $\mathbb{R}$ has a norm" is not provable in ZF set theory.

**Keywords:** choice principle; vector space; base for vector space; nonzero linear functional; norm on vector space; Fraenkel--Mostowski permutation models of ${\rm ZFA}+\neg{\rm AC}$; Jech--Sochor first embedding theorem

**DOI:** DOI 10.14712/1213-7243.2015.258

**AMS Subject Classification:** 03E25 03E35 15A03 15A04

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