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Liviu Nicolaescu
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Spectra of certain Vandermonde related matrices

Denote by $\newcommand{\bC}{\mathbb{C}}$ $\newcommand{\bT}{\mathbb{T}}$ $\bT^N$ the real torus

$$\mathbb{T}^N :=\bigl\lbrace\vec{z}\in\bC^N;\;\;|z_1|=\cdots =|z_N|=1\bigr\rbrace$$

To each $\newcommand{\vez}{\vec{z}}$ $\vez\in\bT^N$ we associate the $N\times N$ Vandermonde matrix $V(\vec{z})$

$$V_{ij}(\vec{z})= z_j^{i-1}. $$

Now form the hermitian and positive semi-definite matrix

$$ A(\vez)= V(\vez)^\ast \cdot V(\vez). $$

As is well known $A(\vez)$ is invertible if and only if $\vez$ is nondegenerate, i.e., the components $z_j$ are pairwise distinct. In general

$$\dim \ker A(\vez) = N-\nu(\vez), $$

where $\nu(\vez)$ denotes the number of distinct elements in the list $(z_1,\dotsc,z_N)$. Deenote by $\lambda_1(\vez)$ the smallest eigenvalue of $A(\vez)$. The map

$$\bT^N\ni \vez\mapsto m\lambda_1(\vez) \in [0,\infty) $$

is continuous and semi-algebraic and vanishes exactly when $\det A(\vez)$ vanishes, where we recall that

$$ \det A(\vez)=\prod_{j > k} |z_j-z_k|^2. $$

We deduce from the Lojasewicz's inequality that there exists a positive integer $m$ and a constant $C=C_m>0$ such that

$$ \lambda_1(\vez)\geq C(\det A(\vez) )^m. \tag{1}\label{1} $$

Observe that if (\ref{1}) holds for some $m$ and $C$, it also holds for any given $m'>m$ (with a different constant $C$).

  1. Is it true that in (\ref{1}) we can choose $m=1$ ?

  2. Can one indicate another explicit and notrivial lower bound for $\lambda_1(\vez)$?

Liviu Nicolaescu
  • 34.7k
  • 2
  • 91
  • 165