Existence and Enumeration of Polynomially Transformed Matrices under Spectral and Nilpotent Constraints

Abstract

Matrix functions extend scalar function concepts to linear operators, offering a unified framework with broad applications in mathematics, science, and engineering. Classical definitions--via power series, spectral calculus, or Jordan form--capture both diagonalizable and defective matrices, revealing insights into dynamics, stability, and modal interactions. Building on this foundation, this study examines how matrix functions behave under nilpotency and diagonalizability constraints, inspired by the Jordan decomposition into spectral and nilpotent parts. The spectral part encodes eigenstructure, while the nilpotent part reflects deviations from diagonalizability. Two main problems are addressed: (1) identifying when a polynomial-transformed matrix becomes nilpotent, and (2) determining when it becomes diagonalizable. The analysis considers two scenarios--matrices with one distinct eigenvalue and those with multiple distinct eigenvalues--each posing unique algebraic and combinatorial challenges. Depending on whether the polynomial transform is sparse or dense, distinct tools are applied: combinatorial root-counting techniques such as Descartes' rule and Sturm's theorem for sparse cases, and analytic or algebraic methods like the argument principle, Rouche's theorem, and square-free factorization for dense ones. Together, these methods establish a systematic framework for studying the existence and classification of matrices with specific structural properties under polynomial transformations. The findings deepen understanding of spectral transformations and provide new perspectives for operator theory, dynamical systems, and computational linear algebra.