Lecture
This lecture introduces the bar construction method to compute the first and second homology groups of a group G, construct the classifying space BG, and nearly prove the Hopf formula for the second homology group. The bar resolution is used to construct a free ZG-resolution of the trivial ZG-module. The lecture progresses through the construction of the classifying space BG and discusses the relationship between the homology groups and the classifying space. The presentation concludes with a detailed explanation of the action of G on the cells of the classifying space and the universal cover of BG.