20 Answers votes newest views recent 46What are your favorite instructional counterexamples? 35Describe a topic in one sentence. 3What is the first interesting theorem in (insert subject here)? 6A learning roadmap for Representation Theory 23Why do functions in complex analysis behave so well? (as opposed to functions in real analysis) 12Theorems for nothing (and the proofs for free) 3What is the field with one element? 3Good introductory references on algebraic stacks? 8equivalence of Grothendieck-style versus Cech-style sheaf cohomology 10Learning new mathematics 8What are some fundamental “sources” for the appearance of pi in mathematics? 1Various concepts of “closure” or “completion” in mathematics 5Notions of Matrix Differentiation 3Thorough Introduction to Singular Value Decomposition 4Quasi-coherent sheaves in the Functor-of-points approach 17Heuristic explanation of why we lose projectives in sheaves. 4How to Compute the coordinate ring of flag variety? 3Simplex method for SDP? 4Expository treatment of Schubert Cells Paper 4How do you rotate a matrix to maximum sparsity?