Cubic column relations in truncated moment problems

Abstract

For the truncated moment problem associated to a complex sequence $\gamma ^{(2n)}=\{\gamma _{ij}\}_{i,j\in Z_{+},i+j \leq 2n}$ to have a representing measure $\mu $, it is necessary for the moment matrix $M(n)$ to be positive semidefinite, and for the algebraic variety $\mathcal{V}_{\gamma}$ to satisfy $\operatorname{rank}\;M(n) \leq \;$ card$\;\mathcal{V}_{\gamma}$ as well as a consistency condition: the Riesz functional vanishes on every polynomial of degree at most $2n$ that vanishes on $\mathcal{V}_{\gamma}$. In previous work with L. Fialkow and M. M\"{o}ller, the first-named author proved that for the extremal case (rank$\;M(n)=$ card$\;\mathcal{V}_{\gamma}$), positivity and consistency are sufficient for the existence of a representing measure. In this paper we solve the truncated moment problem for cubic column relations in $M(3)$ of the form $Z^{3}=itZ+u\bar{Z}$ ($u,t \in \mathbb{R}$); we do this by checking consistency. For $(u,t)$ in the open cone determined by $0 < \left|u\right| < t < 2 \left|u\right|$, we first prove that the algebraic variety has exactly $7$ points and $\operatorname{rank}\;M(3)=7$; we then apply the above mentioned result to obtain a concrete, computable, necessary and sufficient condition for the existence of a representing measure.