The fundamental group of a locally finite graph with ends—a hyperfinite approach

Tom 232 / 2016

Isaac Goldbring, Alessandro Sisto Fundamenta Mathematicae 232 (2016), 21-39 MSC: Primary 05C63; Secondary 14F35, 03H05. DOI: 10.4064/fm232-1-2


The end compactification $|\varGamma |$ of a locally finite graph $\varGamma $ is the union of the graph and its ends, endowed with a suitable topology. We show that $\pi _1(|\varGamma |)$ embeds into a nonstandard free group with hyperfinitely many generators, i.e. an ultraproduct of finitely generated free groups, and that the embedding we construct factors through an embedding into an inverse limit of free groups. We also show how to recover the standard description of $\pi _1(|\varGamma |)$ given by Diestel and Sprüssel (2011). Finally, we give some applications of our result, including a short proof that certain loops in $|\varGamma |$ are non-nullhomologous.


  • Isaac GoldbringDepartment of Mathematics, Statistics,
    and Computer Science
    University of Illinois at Chicago
    Science and Engineering Offices (M/C 249)
    851 S. Morgan St.
    Chicago, IL 60607-7045, U.S.A.
  • Alessandro SistoDepartment of Mathematics
    ETH Zürich
    HG J 14.4
    Rämistrasse 101
    8092 Zürich, Switzerland

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