Happy number

MALE SPEAKER: What are you about to show us? RIA SYMONDS: Why 7 is a happy number. So let's start with the number 7. So let's square 7, because all numbers like to be squared....

MATT PARKER: I'm going to show you two of my all-time favorite diagrams. And it's a way of linking numbers as they go from one to the next. And a lot of people will have heard of-- or...