This paper is intended as an introductory survey of a newly emerging field: a
topological approach to the study of locally finite graphs that crucially
incorporates their ends. Topological arcs and circles, which may pass through
ends, assume the role played in finite graphs by paths and cycles.
We propose a homology theory for locally compact spaces with ends in which
the ends play a special role. The approach is motivated by results for graphs
with ends, where it has been highly successful. But it was unclear how the
original graph-theoretical definition could be captured in the usual language
for homology theories, so as to make it applicable to more general spaces.
We characterize the fundamental group of a locally finite graph G with ends
combinatorially, as a group of infinite words. Our characterization gives rise
to a canonical embedding of this group in the inverse limit of the (free)
fundamental groups of the finite subgraphs of G.
We show that the topological cycle space of a locally finite graph is a
canonical quotient of the first singular homology group of its Freudenthal
compactification, and we characterize the graphs for which the two coincide. We
construct a new singular-type homology for non-compact spaces with ends, which
in dimension~1 captures precisely the topological cycle space of graphs but
works in any dimension.
We prove that every rayless graph has an unfriendly partition.
We prove that every rayless graph has an unfriendly partition.