One of the first people to stumble over imaginary numbers (namely the square root of -15) thought it was "subtle" and "useless". Featuring Barry Mazur. Extra footage from this interview: http://youtu.be/i4GCd2xOCDg Barry's book on this topic: http://bit.ly/RootMinusFifteen Barry on scale and similarity: http://youtu.be/ItiFO5y36kw NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Numberphile is supported by the Mathematical Sciences Research Institute (MSRI): http://bit.ly/MSRINumberphile Videos by Brady Haran Support us on Patreon: http://www.patreon.com/numberphile Brady's videos subreddit: http://www.reddit.com/r/BradyHaran/ A run-down of Brady's channels: http://www.bradyharan.com Sign up for (occasional) emails: http://eepurl.com/YdjL9