We called him the Professor. And he called my son Root, because he said, the flat top of his head reminded him of the square root sign. "There's a fine brain in there," the Professor said, mussing my son's hair. Root, who wore a cap to avoid being teased by his friends, gave a wary shurg. "With this one little sign, we can come to know an infinite range of numbers, even those we can't see." He …