Description: What’s the hardest math I’ve ever used in real life? Me, myself, directly — not by using a cellphone or a GPS device that somebody else designed. And in “real life” - not while studying or teaching mathematics? I use addition and subtraction daily, adding up bills or calculating change. I use percentages often, though mostly it is just “add 15 percents”. I seldom use multiplication and division: when I buy in bulk, or when I need to know how many tiles I need to replace my kitchen floor. I’ve used powers twice in my life, doing calculations related to mortgages. I’ve used a tiny bit of linear algebra for a tiny bit of non-math-related computer graphics I’ve played with. And for a long time, that was all. In my talk I will tell you how recently a math topic discovered only in the 1800s made a brief and modest appearance in my non-mathematical life. There are many books devoted to that topic, yet for all I know, nobody ever needed the actual gory formulas for such a simple reason before. This lecture is co-sponsored by the Fields Institute for Research in Mathematical Sciences