I am reading this paper $ <$Interlacing Families IV: Bipartite Ramanujan Graphs of All Sizes$>$, which proved the existence of the Ramanujan Graphs of all sizes.
This paper uses the notion of Haar measure on $O(n)$, the orthogonal group on $\mathbb{R}^n$. I searched on Google and Wiki about this but it is too hard for me since I am not familiar with abstract measure theory.
I am wondering if there is an easier way to understand the specific example, which is the Haar measure on $O(n)$. What I know about measure theory is the Lebesgue meausre and to construct the integral given an meausre.
