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

## TL;DR

This paper advances bounds on the Carathéodory number for moment problems on algebraic varieties, providing explicit bounds and constructions that highlight the complexity of representing moment functionals with minimal atoms.

## Contribution

It introduces improved bounds and explicit constructions for the Carathéodory number on algebraic varieties and with small gaps, extending understanding of moment problems and flat extensions.

## Key findings

- Constructed moment functionals requiring nearly maximal atoms
- Provided explicit bounds on Carathéodory numbers on varieties
- Identified cases needing extension to higher degrees for flatness

## Abstract

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 $\varepsilon>0$ and $d\in\mathbb{N}$ there is a $n\in\mathbb{N}$ such that we can construct a moment functional $L:\mathbb{R}[x_1,\dots,x_n]_{\leq d}\rightarrow\mathbb{R}$ which needs at least $(1-\varepsilon)\cdot\left(\begin{smallmatrix} n+d\\ n\end{smallmatrix}\right)$ atoms $l_{x_i}$. Consequences and results for the Hankel matrix and flat extension are gained. We find that there are moment functionals $L:\mathbb{R}[x_1,\dots,x_n]_{\leq 2d}\rightarrow\mathbb{R}$ which need to be extended to the worst case degree $4d$, $\tilde{L}:\mathbb{R}[x_1,\dots,x_n]_{\leq 4d}\rightarrow\mathbb{R}$, in order to have a flat extension.