19 Answers votes newest views recent 3Are integers real? 2Does each “prod-simplicial” regular cell complex come from a unique rooted tree? 1Does pi contain 1000 consecutive zeroes (in base 10)? 11Fundamental problems whose solution seems completely out of reach 5Why is the the classifying space of the natural numbers homotopy equivalent to the circle? 1Are there examples of non-orientable manifolds in nature? 0Examples of Simplicial Groupoids in Nature 0The main theorems of category theory and their applications 9Why study simplicial homotopy groups? 0Geometric intuition for limits 8When and where did the term “module” enter commutative algebra? 20Examples where it’s useful to know that a mathematical object belongs to some family of objects 5The concept of Duality 4Expressing Galois actions on fundamental groups explicitly 3Best online mathematics videos? 3What is the best way to study Rational Homotopy Theory 3Can skeleta simplify category theory? 1Classes of metric spaces with additional structure 2Classifying space of large category?