Conditioned Galton-Watson trees: The shape functional, and more on the sum of powers of subtree sizes and its mean

Abstract

For a complex number $\alpha$, we consider the sum of the $\alpha$th powers of subtree sizes in Galton--Watson trees conditioned to be of size $n$. Limiting distributions of this functional $X_n(\alpha)$ have been determined for $\Re\alpha \neq 0$, revealing a transition between a complex normal limiting distribution for $\Re\alpha < 0$ and a non-normal limiting distribution for $\Re\alpha > 0$. In this paper, we complete the picture by proving a normal limiting distribution, along with moment convergence, in the missing case $\Re\alpha = 0$. The same results are also established in the case of the so-called shape functional $X_n'(0)$, which is the sum of the logarithms of all subtree sizes; these results were obtained earlier in special cases. Additionally, we prove convergence of all moments in the case $\Re\alpha < 0$, where this result was previously missing, and establish new results about the asymptotic mean for real $\alpha < 1/2$. A novel feature for $\Re\alpha=0$ is that we find joint convergence for several $\alpha$ to independent limits, in contrast to the cases $\Re\alpha\neq0$, where the limit is known to be a continuous function of $\alpha$. Another difference from the case $\Re\alpha\neq0$ is that there is a logarithmic factor in the asymptotic variance when $\Re\alpha=0$; this holds also for the shape functional. The proofs are largely based on singularity analysis of generating functions.