Positive definite collections of disks

Vladimir Tkachev

Let $Q(z,w)=-{\prod }_{k=1}^n[(z-a_k)(\bar{w}-{\bar{a}}_k)-R_k^2]$. M. Putinar and B. Gustafsson proved recently that the matrix $Q(a_i,a_j)$, $1\le i,j\le n$, is positive definite i…