What is...the Seifert-van Kampen theorem?

Goal. Explaining basic concepts of algebraic topology in an intuitive way. This time. What is...the Seifert-van Kampen theorem? Or: Cut and compute Disclaimer. Nobody is perfect, and I might have said something silly. If there is any doubt, then please check the references. Disclaimer. These videos are concerned with algebraic topology, and not general topology. (These two are not to be confused.) I assume that you know bits and pieces about general topology, but not too much, I hope. Slides. http://www.dtubbenhauer.com/youtube.html Website with exercises. http://www.dtubbenhauer.com/lecture-a... Note. The Seifert-van Kampen theorem is also called van Kampen theorem. Material used. Hatcher, Chapter 1 https://en.wikipedia.org/wiki/Seifert... Free groups and free products. https://en.wikipedia.org/wiki/Free_pr... https://en.wikipedia.org/wiki/Free_group https://en.wikipedia.org/wiki/Categor... Spanning trees. https://en.wikipedia.org/wiki/Spannin... Awesome fundamental group animations (and other great stuff). https://math.unice.fr/~indira/Mygifs.... Hatcher’s book (I sometimes steal some pictures from there). https://pi.math.cornell.edu/~hatcher/... Always useful. https://en.wikipedia.org/wiki/Counter... Mathematica. https://reference.wolfram.com/languag... #algebraictopology #topology #mathematics