Entrywise Positivity Preservers on Green Matrices

Abstract

We classify the entrywise functions that preserve positive semidefiniteness on discrete Green matrices \(G(p,q)=(p_{\min(i,j)}q_{\max(i,j)})\) with positive parameters, without requiring the resulting matrix to retain Green structure. For matrices of all orders, the preservers are the zero function and the functions \(f(t)=\int_{[0,\infty)}t^\alpha\,d\mu(\alpha)\), where \(\mu\) is a nonzero finite positive measure and the integral is finite for every \(t>0\). Requiring the resulting matrix to be totally nonnegative reduces the nonzero preservers to \(f(t)=ct^\alpha\), where \(c>0\) and \(\alpha\ge0\). These power functions also preserve positive semidefinite Green structure, while strict Green structure is preserved precisely when \(\alpha>0\). No regularity assumption is needed for these classifications. We also characterize continuously differentiable functions that are entrywise Loewner monotone on every fixed-\(q\) Green family: this holds precisely when \(f'\) is a positive mixture of nonnegative real powers, with the zero measure allowed.