Properties of k-Descending Trees

Abstract

For any real-valued $k > 1$, we consider the tree rooted at 0, where each positive integer $n$ has parent $\lfloor\frac{n}{k}\rfloor$. The average number of children per node is $k$, thus this definition gives a natural way to extend $k$-ary trees to irrational $k$. We focus on the sequence $r_d$: the count of nodes at depth $d$. We first prove there exists some constant $\rho(k)$ such that $r_d \sim \rho(k)\cdot k^d$. We then study a family of values $k=\frac{a + \sqrt{a+4b}}{2}$, where we prove the sequence satisfies the exact recurrence $r_d = a\cdot r_{d-1} + b \cdot r_{d-2}$. This generalizes a special case when $k$ is the golden ratio and $r_d$ is the Fibonacci sequence.