Group theory 4: Lagrange's theorem
Channel: Richard E Borcherds
Date: 2020-06-24

This is lecture 4 of an online course on mathematical group theory. 
It introduces Lagrange's theorem that the order of a subgroup divides the order of a group, and uses it to show that all groups of prime order are cyclic, and to prove Fermat's theorem and Euler's theorem.