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On Largest Offspring in a Critical Branching Process with Finite Variance

Published online by Cambridge University Press:  30 January 2018

Jean Bertoin*
Affiliation:
Universität Zürich
*
Postal address: Institut für Mathematik, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland. Email address: jean.bertoin@math.uzh.ch
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Abstract

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Continuing the work in Bertoin (2011) we study the distribution of the maximal number X*k of offspring amongst all individuals in a critical Galton‒Watson process started with k ancestors, treating the case when the reproduction law has a regularly varying tail with index −α for α > 2 (and, hence, finite variance). We show that X*k suitably normalized converges in distribution to a Fréchet law with shape parameter α/2; this contrasts sharply with the case 1< α<2 when the variance is infinite. More generally, we obtain a weak limit theorem for the offspring sequence ranked in decreasing order, in terms of atoms of a certain doubly stochastic Poisson measure.

Type
Research Article
Copyright
© Applied Probability Trust 

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