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Bestvina and Feighn showed that a morphism
between two simplicial trees that commutes with the action of a group
can
be written as a product of elementary folding operations. Here a more general
morphism between simplicial trees is considered, which allow different groups to act
on
and
.
It is shown that these morphisms can again be written as a product of elementary
operations: the Bestvina–Feighn folds plus the so-called “vertex morphisms”.
Applications of this theory are presented. Limits of infinite folding sequences are
considered. One application is that a finitely generated inaccessible group must
contain an infinite torsion subgroup.
Dedicated to David Epstein on the
occasion of his 60th birthday.