The Fields Medal is the highest honor a mathematician can receive. Paul Erdös never won it. But a number of the people he helped did, and that leads us to what Erdös is best known for: the “Erdös number.” No, it was not a theorem or a mathematical tool. It was simply a measure of how close you were to working with Paul Erdös.

I was fortunate enough to be able to rely on the writings of Descartes, Grothendieck, and Thurston.