Sum of Natural Numbers (second proof and extra footage)

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...

MAIN VIDEO IS AT: http://youtu.be/w-I6XTVZXww

Ed Copeland and Tony Padilla are physicists at the University of Nottingham.