13 Answers votes newest views recent 20Elementary applications of Krein-Milman 13Existence of prime ideals and Axiom of Choice. 11Category of categories as a foundation of mathematics 4Is there an infinite-dimensional Banach space with a compact unit ball? 3Can the Riemann hypothesis be undecidable? 3Countable Unions And The Axiom Of Countable Choice 2Is there an integration free proof (or heuristic) that once differentiable implies twice differentiable for complex functions? 2Interesting and Accessible Topics in Graph Theory 1Awfully sophisticated proof for simple facts 1Is the non-triviality of the algebraic dual of an infinite-dimensional vector space equivalent to the axiom of choice? 1Problems where we can’t make a canonical choice, solved by looking at all choices at once 1Simple Question about Category theory. 0Under what conditions a bounded linear map can be extended ?