What’s your favourite number? I was ambivalent on this issue until a few months ago until I came across the following quirky result: if you start with the prime number 41 and then add 2 you get 43, which is also prime and then if you add 4 to 43, you get 47: also prime. And this continues to produce prime numbers if you add successive multiples of two to your running total, UNTIL… you get to the 41st number in this sequence, which is 41 squared.