Just happened on this YouTube video: ----- What does genius look like in math? Where does it come from? (Dandelin spheres) ----- by Grant Sanderson, in his very good series of math videos on YouTube, called "3Blue1Brown". It's particularly notable for great illustrative graphics (this one and his others). (After going through the proof that the locus of constant sum of distances to two foci in the plane is the same curve as a conic section, he gets into a discussion of genius in mathematics which, though interesting, is not the reason I'm mentioning this video.) —Dan