A spectral product formula for repunits via a tridiagonal Toeplitz similarity

TL;DR

This paper derives explicit spectral properties, eigenvectors, and inverse formulas for a class of tridiagonal matrices related to repunits, connecting matrix determinants with cosine product evaluations.

Contribution

It provides a novel spectral analysis and explicit formulas for a specific family of tridiagonal matrices, linking their determinants to cosine products.

Findings

Explicit eigenvalues and eigenvectors derived

Closed-form inverse matrix obtained

Finite cosine product evaluation for repunit sums

Abstract

For $b>0$ and $n\geqslant 1$, we consider the $n\times n$ tridiagonal matrix $V_n(b)$ with diagonal entries $b+1$, superdiagonal entries $1$, and subdiagonal entries $b$. A diagonal similarity reduces $V_n(b)$ to a symmetric tridiagonal Toeplitz matrix and hence makes its spectrum explicit. Since $\det\left(V_n(b)\right)$ equals the geometric sum $1+b+\cdots+b^{n}$, taking determinants yields a finite cosine product evaluation for this quantity. As further consequences, we derive sharp bounds from the extremal eigenvalues, write down explicit eigenvectors with respect to a natural weighted inner product, and obtain a closed formula for $V_n(b)^{-1}$.