Every vector space has a basis
If you haven’t seen a Zorn’s lemma argument before, this will probably be bewildering. It is here for the sake of completeness. Theorem: Every vector space has a basis. Proof: Consider the collection...
View ArticleEvery field has an algebraic closure
Yeah. That’s right. Every field. Yesterday we showed how we could add a root of a polynomial to a field and that would make it bigger. So we can just keep adding roots over and over again until we get...
View ArticleGlobal choice and algebraic closures
It was pointed out to me today that I glazed over a set-theoretic point in my proof that every field has an algebraic closure. We appealed to Zorn’s lemma, which says: Given a partially ordered set ,...
View ArticlePhilosophy and choice
Seriously, category theory is coming soon. I promise. In the meantime, a friend (who posts on this awesome blog) and I had a conversation about the axiom of choice from a philosophical standpoint. It...
View ArticleGuest post: Isaac Solomon
Read Isaac’s blog here. From a philosophical point of view, the Axiom of Choice (AC) is often a tough sell. On the one hand, there’s something wrong with vector spaces without bases, or fields without...
View ArticlePedantry
We’ve mentioned the category (the category of sets with functions as morphisms). Consider another category which has two objects and and a morphism between them (along with the required identity...
View ArticleMore Pedantry
Similar to last time, I want to talk about something mildly pedantic, but again, very important. There’s something in-between large categories and small categories. We don’t call it a medium category...
View ArticleCoproducts in the category of Sets
As with products. Not every category has coproducts. However, does. I claim that given two sets and , their coproduct is the disjoint union of and . Since coproducts are unique, all we need to do is...
View ArticleYoneda’s Lemma (part 1)
So I think we’re ready, at least for the statement of Yoneda’s lemma. It says that for any locally small category , if is an object in , and a functor, then Moreover, the isomorphism is natural in both...
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