502 – Tilings

Since we covered some material on tilings, I figured you may find interesting the following nice expository paper by Richard Stanley and my friend and colleague Federico Ardila.

175 – Dandelin spheres

Here is a link to the Wikipedia page on Dandelin spheres, giving us the “modern” proof of the equivalence between the definition of conic sections as regions of intersection of planes and cones, with the standard definition in terms of distance to foci. The links on the Wikipedia page provide further explanations and nice graphics illustrating the argument.