I am a PhD freshman working on topological graph theory and geometric group theory. I would like to learn some Bass-Serre theory. What do you think is the best introductory textbook in this topic? Thank you in advance for your recommendations.
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Bass-Serre theory textbook
"Trees" by JP Serre.
Depending on how far you want to go, there is a very short and very well written note by Wenyuan Yang here : http://bicmr.pku.edu.cn/~wyang/ggt/BassSerre.pdf
You can find the construction of the Bass-Serre tree for Amalgamated sums and HNN extensions and some related theorems.
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