What does a basis cost?¶
A note by Yaashaa Golovanov · AMMOC Seminar Notes, No. 1This week, in our Research Seminars in Linear Algebra for Middle and High-schoolers, my pupils proved that every vector space has a basis. Given Zorn’s Lemma it takes three lines.
Then we asked the better question. What does that theorem cost?
Over the Zermelo–Fraenkel axioms, five statements turn out to be the same statement:
- (1)The Axiom of Choice
- (2)Zermelo’s Well-Ordering Theorem
- (3)The Hausdorff Maximal Principle
- (4)Zorn’s Lemma
- (5)Every vector space over every field has a basis.
Courses use (4) ⇒ (5) and stop there. The direction that carries the content is (5) ⇒ (1), proved by Andreas Blass in 1984: the bare existence of bases hands back the full Axiom of Choice.
And Choice is not decided by the axioms at all. Gödel showed in 1938 that it cannot be refuted. Cohen showed in 1963, by inventing forcing, that it cannot be proved either. There are models of set theory in which the real numbers, viewed as a vector space over the rationals, have no basis whatsoever.
Blass’s proof carried one hypothesis he could not remove: the Axiom of Foundation. That question stood open for over forty years.
On 17 September 2026 a preprint appeared that closes it. Fernandes, Mezabarba, and Rodrigues give a foundation-free proof—and in fact they need only vector spaces over fields of characteristic zero (arXiv:2609.20140). It is a preprint and not yet refereed, so read it as news rather than as settled literature.
Here is what my pupils took from it. The sentence on our board is one a careful fourteen-year-old can prove. Its exact axiomatic strength was being sharpened in a paper posted the week before we covered it.
They were delighted. So was I.
There is no school mathematics and then real mathematics.
There is one subject, and if you teach it honestly, the frontier is never far away.
Saarang Agarwal, Aaditya Sahu, Shaurya Patil, Vibhu Konduru, Sritha Uppaluru, Yuri Mikhyalov, Nikolai N. Mnev, Parth Sakharam Andhare, Noah Kotto and Ashwika Nukala.
A. Blass, ‘Existence of bases implies the axiom of choice’, in Axiomatic Set Theory, Contemporary Mathematics 31, American Mathematical Society, 1984, pp. 31–33.
G. Fernandes, R. M. Mezabarba and V. de O. Rodrigues, ‘Existence of bases implies the axiom of choice, a foundation-free proof’, preprint, 2026, arXiv:2609.20140.