Ian Stewart is Professor Emeritus of Mathematics at the University of Warwick and the author of the bestseller Professor Stewart’s Cabinet of Mathematical Curiosities. His recent books include Significant Figures, Incredible Numbers, Seventeen Equations that Changed the World, Professor Stewart’s Casebook of Mathematical Mysteries and Calculating the Cosomos (all published by Profile). His app, Incredible Numbers, was published jointly by Profile and Touch Press in 2014. He is a fellow society of the Royal Society and was the 2015 recipient of the American Mathematical Association Euler Book Prize.