Continues from: https://www.youtube.com/watch?v=Qcv1IqHWAzg Featuring Dr Emily Riehl. Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Google Plus: http://bit.ly/numberGplus Tumblr: http://numberphile.tumblr.com/ Numberphile is supported by the Mathematical Sciences Research Institute (MSRI): http://bit.ly/MSRINumberphile Videos by Brady Haran Brown papers: http://bit.ly/brownpapers A run-down of Brady's channels: http://bit.ly/bradychannels