Good material Visual Group Theory - Lecture 6 on YouTube is the best Galois theroy lecture. https://www.youtube.com/watch?v=Buv4Y74_z7I&list=PLwV-9DG53NDxU337smpTwm6sef4x-SCLv&index=31 You can fined slides of the lecture at below. http://www.math.clemson.edu/~macaule/classes/m20_math4120/ Kelin four group Hasse diagrams complex number is algebraically closed. fundamental theorem of algebra Any plynomial with complex coefficients, having degree d, has at least one complex root.(Gauss) How to listen it I listened 6.1 - 6.3 at the first day.