Moment property and positivity for some algebras of fractions

Abstract

T. M. Bisgaard proved that the $*$-algebra ${\bf C}[z,\overline{z},1/z\overline{z}]$ has the moment property, that is, each positive linear functional on this $*$-algebra is a moment functional. We generalize this result to polynomials in $d$ variables $z_1,...,z_d$. We prove that there exist $3d-2$ linear polynomials as denominators such that the corresponding $*$-algebra has the moment property, while for 3 linear polynomials in case $d=2$ the moment property always fails. Further, it is shown that for the real algebras ${\bf R}[x,y,1/(x^2+y^2)]$ (the hermitean part of ${\bf C}[z,\overline{z},1/z\overline{z}]$) and ${\bf R}[x,y,x^2/(x^2+y^2),xy/(x^2+y^2)]$, all positive semidefinite elements are sums of squares. These results are used to prove that for the semigroup $*$-algebras of ${\bf Z}^2$, ${\bf N}_0\times{\bf Z}$ and ${\mathsf N}_+:=\{(k,n)\in{\bf Z}^2:k+n\geq 0\}$, all positive semidefinite elements are sums of hermitean squares.