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Written in an informal style, the material is divided into short sections each of which deals with an important result or a new idea. A novel feature at this level is a proof of the Nielsen-Schreier theorem, using group actions on trees.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.