On Tribonacci Sequences

Let a tribonacci sequence be a sequence of integers satisfying $a_k=a_{k-1}+a_{k-2}+a_{k-3}$ for all $k\ge 4$. For any positive integers $k$ and $n$, denote by $f_k(n)$ the number of tribonacci sequences with $a_1, a_2, a_3>0$ and with $a_k=n$. For all $n$, there is a maximum $k$ such that $f_k(n)$ is non-zero. Answering a question of Spiro \cite{Spiro}, we show that there is a finite upper bound (we specifically prove 561001) on $f_k(n)$ for any positive integer $n\ge 3$ and this maximum $k$. We do this by showing that $f_k(n)$ has transitions in $n$ around constant multiples of $\phi^{3k/2}$ (where $\phi$ is the real root of $\phi^3=\phi^2+\phi+1$): there exists a constant $C$ such that $f_k(n)>0$ whenever $n>C\phi^{3k/2}$ and for any constant $T$, the values of $f_k(n)$ with $n<T\phi^{3k/2}$ have an upper bound independent of $k$.