Mathematicians – the really, really good ones – are not like other people. One of the (true) stories recounted by Paul Hoffman concerns the mathematician who slept with his wife only on those days of the month that were prime numbers – the second, third, fifth, seventh, and so on. Not so bad early in the month, but tougher as the primes thin out (19, 23, 29). When last heard of, he was ‘serving twenty years in the Oregon State Penitentiary for kidnapping and attempted murder’. We are not told on which day of the month the crimes were committed, but my guess is the 28th.

Paul Erdös, the subject of Hoffman’s book, would never have ended up like that. He was too engrossed in his study of numbers to take time out for anything as mundane as kidnapping and murder. He barely took time out to eat, and for the last twenty-five years of his