One of the first people to stumble over imaginary numbers (namely the square root of -15) thought it was "subtle" and "useless". More links & stuff in full description below ↓↓↓ Featuring Barry Mazur. Extra footage from this interview: https://www.youtube.com/watch?v=i4GCd2xOCDg Barry's book on this topic: http://bit.ly/RootMinusFifteen Barry on scale and similarity: https://www.youtube.com/watch?v=ItiFO5y36kw Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub Numberphile is supported by the Mathematical Sciences Research Institute (MSRI): http://bit.ly/MSRINumberphile Videos by Brady Haran Brady's videos subreddit: http://www.reddit.com/r/BradyHaran/ Brady's latest videos across all channels: http://www.bradyharanblog.com/ Sign up for (occasional) emails: http://eepurl.com/YdjL9 Numberphile T-Shirts: null Other merchandise: https://store.dftba.com/collections/numberphile