The multidimensional truncated Moment Problem: Carath\'eodory Numbers from Hilbert Functions

In this paper we improve the bounds for the Carath'eodory number, especially on algebraic varieties and with small gaps (not all monomials are present). We provide explicit lower and upper bounds on algebraic varieties, ${\mathbb{R}}^n$, and $[0,1{]}^n$. We also treat moment problems with small gaps. We find that for every $\epsilon >0$ and $d\in \mathbb{N}$ there is a $n\in \mathbb{N}$ such that we can construct a moment functional $L:$\mathbb{R}$[x_1,\dots,x_n]{\leq d}\rightarrow$\mathbb{R}$$whichneedsatleast$(1-\varepsilon)\cdot\left(\begin{smallmatrix} n+d\ n\end{smallmatrix}\right)$atoms$l{x_i}$. Consequences and results for the Hankel matrix and flat extension are gained. We find that there are moment functionals $L:\mathbb{R}[x_1,\dots ,x_n{]}_{\le 2d}\to \mathbb{R}$ which need to be extended to the worst case degree $4d$, $\tilde{L}:\mathbb{R}[x_1,\dots ,x_n{]}_{\le 4d}\to \mathbb{R}$, in order to have a flat extension.