On the external branches of coalescents with multiple collisions
Abstract
A recursion for the joint moments of the external branch lengths for coalescents with multiple collisions (
Λ
-coalescents) is provided. This recursion is used to derive asymptotic results as the sample size
n
tends to infinity for the joint moments of the external branch lengths and for the moments of the total external branch length of the Bolthausen-Sznitman coalescent. These asymptotic results are based on a differential equation approach, which is as well useful to obtain exact solutions for the joint moments of the external branch lengths for the Bolthausen-Sznitman coalescent. The results for example show that the lengths of two randomly chosen external branches are positively correlated for the Bolthausen--Sznitman coalescent, whereas they are negatively correlated for the Kingman coalescent provided that
n≥4
.