Abstract
We study fragmentation trees of Gibbs type. In the binary case, we identify
the most general Gibbs type fragmentation tree with Aldous's beta-splitting
model, which has an extended parameter range $\beta>-2$ with respect to the
$Beta(\beta+1,\beta+1)$ probability distributions on which it is based.
In the multifurcating case, we show that Gibbs fragmentation trees are
associated with the two-parameter Poisson-Dirichlet models for exchangeable
random partitions of $\bN$, with an extended parameter range $0łe\alpha1$,
$þeta-2\alpha$ and $\alpha<0$, $þeta=-m\alpha$, $mın\bN$.
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