These are the show notes for episode 21 of the Travels in a Mathematical World podcast. 21 is the number of squares in the unique smallest simple squared square. You can see the square with some more information on the page about its use as the logo of the The Trinity Mathematical Society at Cambridge. There is more information on squaring problems at squaring.net. More about the number 21 from Number Gossip.
In the regular Maths History series, Noel-Ann Bradshaw of the University of Greenwich and also Meetings Co-ordinator of the British Society for the History of Mathematics talks about the life of Alan Turing. You can read a biography of Turing at the MacTutor History of Maths Archive and there are a large number of links to further reading on the Alan Turing Wikipedia page.