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