# The complex moment problem: determinacy and extendibility

## TL;DR

This paper investigates the uniqueness and extendibility of complex moment sequences, establishing conditions under which positive definite extensions are unique and exploring the structure of representing measures.

## Contribution

It proves the uniqueness of positive definite extensions for determinate complex moment sequences with atomless measures at zero and explores related open problems and special cases.

## Key findings

- Extension is unique if the sequence is determinate and has no atom at zero.
- Partial solutions are provided for measures supported on algebraic curves.
- Examples illustrate the theoretical results and special cases.

## Abstract

Complex moment sequences are exactly those which admit positive definite extensions on the integer lattice points of the upper diagonal half-plane. Here we prove that the aforesaid extension is unique provided the complex moment sequence is determinate and its only representing measure has no atom at $0$. The question of converting the relation is posed as an open problem. A partial solution to this problem is established when at least one of representing measures is supported in a plane algebraic curve whose intersection with every straight line passing through $0$ is at most one point set. Further study concerns representing measures whose supports are Zariski dense in $\mathbb C$ as well as complex moment sequences which are constant on a family of parallel "Diophantine lines". All this is supported by a bunch of illustrative examples.