# The topology and geometry of automorphism groups of free groups

• ### Karen Vogtmann

University of Warwick, Coventry, UK

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## Abstract

In the 1970s Stallings showed that one could learn a great deal about free groups and their automorphisms by viewing the free groups as fundamental groups of graphs and modeling their automorphisms as homotopy equivalences of graphs. Further impetus for using graphs to study automorphism groups of free groups came from the introduction of a space of graphs, now known as Outer space, on which the group Out$(F_n$) acts nicely. The study of Outer space and its Out$(F_n$) action continues to give new information about the structure of Out$(F_n$), but has also found surprising connections to many other groups, spaces and seemingly unrelated topics, from phylogenetic trees to cyclic operads and modular forms. In this talk I will highlight various ways these ideas are currently evolving.