On the arithmetic Hilbert depth

Abstract

Let $h:\mathbb Z \to \mathbb Z_{\geq 0}$ be a nonzero function with $h(k)=0$ for $k\ll 0$. We define the Hilbert depth of $h$ by $\operatorname{hdepth}(h)=\max\{d\;:\; \sum_{j\leq k} (-1)^{k-j}\binom{d-j}{k-j}h(j)\geq 0\text{ for all }k\leq d\}$. We show that $\operatorname{hdepth}(h)$ is a natural generalization for the Hilbert depth of a subposet $\operatorname{P}\subset 2^{[n]}$ and we prove some basic properties of it. Given $h(j)=\begin{cases} aj^n+b,& j\geq 0 \\ 0, & j<0 \end{cases}$, with $a,b,n$ positive integers, we compute $\operatorname{hdepth}(h)$ for $n=1,2$ and we give upper bounds for $\operatorname{hdepth}(h)$ for $n\geq 3$. More generally, if $h(j)=\begin{cases} P(j),& j\geq 0 \\ 0,& j<0 \end{cases}$, where $P(j)$ is a polynomial of degree $n$, with non-negative integer coefficients, and $P(0)>0$, we show that $\operatorname{hdepth}(h)\leq 2^{n+1}$.