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Every group is the outer automorphism group of an HNN-extension of a fixed triangle group
Fix an equilateral triangle group
with arbitrary. Our main result is: for every presentation
of every countable group there exists an HNN-extension
of such that . We construct the HNN-extensions explicitly, and examples are given. The
class of groups constructed have nice categorical and residual properties. In
order to prove our main result we give a method for recognising malnormal
subgroups of small cancellation groups, and we introduce the concept of
"malcharacteristic" subgroups.Comment: 39 pages. Final version, to appear in Advances in Mathematic
Linear and projective boundaries in HNN-extensions and distortion phenomena
Linear and projective boundaries of Cayley graphs were introduced
in~\cite{kst} as quasi-isometry invariant boundaries of finitely generated
groups. They consist of forward orbits , or
orbits , respectively, of non-torsion
elements~ of the group , where `sufficiently close' (forward) orbits
become identified, together with a metric bounded by 1.
We show that for all finitely generated groups, the distance between the
antipodal points and in the linear boundary is bounded
from below by , and we give an example of a group which has two
antipodal elements of distance at most . Our example is a
derivation of the Baumslag-Gersten group. \newline We also exhibit a group with
elements and such that , but . Furthermore, we introduce a notion of
average-case-distortion---called growth---and compute explicit positive lower
bounds for distances between points and which are limits
of group elements and with different growth
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