14 Answers votes newest views recent 6Why is a topology made up of ‘open’ sets? 12real symmetric matrix has real eigenvalues - elementary proof 2Relating the angle between two vectors to max and min eigenvalues 0Intuition for mean curvature. 6Does every ellipse inside a tetrahedron inside a ball fit in a triangle inside the ball? 1Optic fibers after Joseph O’Rourke 7Why are so few operations with arity bigger than 2? 9The “Dzhanibekov effect” - an exercise in mechanics or fiction? Explain mathematically a video from a space station 1Towards a metric characterization of Euclidean spaces 2Which norms have rich isometry groups? 1Find symmetries of a tree 2Why worry about the axiom of choice? 12Examples of mathematics motivated by technological considerations 8Is every finite group a group of “symmetries”?