What do 5, 13 and 563 have in common?

Alex Bellos discusses how the Incans used knots in string (Quipu) to record numbers.Check out Brilliant (get 20% off their premium service): https://brillian...

We're talking Prime Numbers again... Featuring Dr James Grime.

Extra footage: http://youtu.be/AiplrfFB6h0
Our Prime Numbers extravaganza: http://bit.ly/primevids
Brown paper: http://bit.ly/brownpapers
Liar Numbers: https://www.youtube.com/watch?v=jbiaz_aHHUQ