A tree-indexed autoregressive(AR) process is a time series defined on a tree which is generated by a branching process and/or a deterministic splitting mechanism. This short article is concerned with conditional heteroscedastic structure of the tree-indexed AR models. It has been usual in the literature to analyze conditional mean structure (rather than conditional variance) of tree-indexed AR models. This article pursues to identify quadratic conditional heteroscedasticity inherent in various tree-indexed AR models in a unified way, and thus providing some perspectives to the future works in this area. The identical conditional variance of sisters sharing the same mother will be referred to as the branching heteroscedasticity(BH, for short). A quasilikelihood but preliminary estimation of the quadratic BH is discussed and relevant limit distributions are derived.Keywords: Branching heteroscedasticity, quasilikelihood, tree-indexed AR.
Motivation of the StudyWe first construct a tree-index on which an AR time series (X) of interest is defined. Following the lines as in Hwang and Basawa (2011), consider the successive generation sizes {Z t } with the initial size Z 0 = 1. In particular, the super critical G-W(Galton-Watson) branching process {Z t } is defined bywhere {η t j , t = 1, 2, . . . , j = 1, 2, . . .} is an array of iid non-negative integer-valued random variables with common (offspring) mean m > 1 and variance σ 2 η ≥ 0. Let X t ( j) denote the observation on the j th individual in t th generation. In addition, let X t−1 (t( j)) denote the observation on immediate mother of the j th individual in t th generation. It is noticed that X t−1 (t( j)) is an observation made in the (t − 1) th generation. In Figure 1, note that x 2 (3(1)) = x 2 (1); x 2 (3(10)) = x 2 (6). As with e.g., Hwang (2011), one can consider two cases separately according as σ Figure 1 illustrates a tree consisting of three generations. It is noted in Figure 1 that there are random number of individuals in each generation. On the other hand, the case of σ 2 η = 0 (see Figure 2) is referred to as a multi-casting tree where each individual (mother) gives rise to exactly m-offspring (daughters) in the next generation. When m = 2, the multi-casting tree reduces to a bifurcating case, i.e., a binary-splitting tree studied by several authors including Cowan and Staudte (1986) and Basawa and Zhou (2004) among others. Most of the research on the multi-casting case of σ 2 η = 0 has been directed to identification of the conditional mean function of the models. For traditional issues on the 1 Corresponding author: Professor,