A necessary condition for certain functions to preserve positive semi-definiteness on partitioned matrices

TL;DR

This paper establishes a necessary condition for functions to preserve positive semi-definiteness on partitioned matrices, showing such functions must have a power series expansion with positive coefficients.

Contribution

It proves that functions preserving positive semi-definiteness on partitioned matrices must have a power series expansion with positive coefficients, providing a key theoretical insight.

Findings

Functions with positive semi-definite preservation have power series with positive coefficients.

Necessary condition links positive semi-definiteness preservation to power series expansion.

Results apply to symmetric functions on non-negative reals and matrix partitions.

Abstract

If $f$ is a symmetric complex-valued function on the $m$-fold Cartesian product of the set of non-negative reals and $A$ is a positive semi-definite $m\times m$ matrix with eigenvalues $\lambda_j$, we set $f(A):=f(\lambda_1,\dotsc,\lambda_m)$. It is shown that if $[f(A_{\alpha\beta})]$ is positive semi-definite whenever $[A_{\alpha\beta}]$ is a positive semi-definite matrix with positive semi-definite entries $A_{\alpha\beta}$, then $f$ has a power series expansion with positive coefficients.