Equality sounds a straightforward idea, but there are subtle problems in theoretical computer science. Professor Thorsten Altenkirch explains how his late friend Martin Hofmann solved one of the biggest problems. More of Thorsten on Type Theory: https://bit.ly/C_Thor_playlist Thorsten's paper dedicated to Martin: https://bit.ly/C_Thor_Paper https://www.facebook.com/computerphile https://twitter.com/computer_phile This video was filmed and edited by Sean Riley. Computer Science at the University of Nottingham: https://bit.ly/nottscomputer Computerphile is a sister project to Brady Haran's Numberphile. More at http://www.bradyharan.com/