A Characterisation of Anti-Lowner Functions

TL;DR

This paper characterizes anti-L"owner functions, which are functions on (0, +∞) with positive semidefinite anti-L"owner matrices, extending the classical operator monotone function theory and with applications in Lyapunov equations.

Contribution

It provides a characterization of anti-L"owner functions, answering a question posed by R. Bhatia, and extends the understanding of matrix positivity conditions.

Findings

Characterization of anti-L"owner functions established.

Connection to Lyapunov-type equations demonstrated.

Extension of operator monotone function theory to anti-L"owner matrices.

Abstract

According to a celebrated result by L\"owner, a real-valued function $f$ is operator monotone if and only if its L\"owner matrix, which is the matrix of divided differences $L_f=(\frac{f(x_i)-f(x_j)}{x_i-x_j})_{i,j=1}^N$, is positive semidefinite for every integer $N>0$ and any choice of $x_1,x_2,...,x_N$. In this paper we answer a question of R. Bhatia, who asked for a characterisation of real-valued functions $g$ defined on $(0,+\infty)$ for which the matrix of divided sums $K_g=(\frac{g(x_i)+g(x_j)}{x_i+x_j})_{i,j=1}^N$, which we call its anti-L\"owner matrix, is positive semidefinite for every integer $N>0$ and any choice of $x_1,x_2,...,x_N\in(0,+\infty)$. Such functions, which we call anti-L\"owner functions, have applications in the theory of Lyapunov-type equations.