English

Nayak's theorem for compact operators

Functional Analysis 2024-09-10 v2

Abstract

Let AA be an m×mm\times m complex matrix and let λ1,λ2,,λm\lambda _1, \lambda _2, \ldots , \lambda _m be the eigenvalues of AA arranged such that λ1λ2λm|\lambda _1|\geq |\lambda _2|\geq \cdots \geq |\lambda _m| and for n1,n\geq 1, let s1(n)s2(n)sm(n)s^{(n)}_1\geq s^{(n)}_2\geq \cdots \geq s^{(n)}_m be the singular values of AnA^n. Then a famous theorem of Yamamoto (1967) states that limn(sj(n))1n=λj,  1jm.\lim _{n\to \infty}(s^{(n)}_j )^{\frac{1}{n}}= |\lambda _j|, ~~\forall \,1\leq j\leq m. Recently S. Nayak strengthened this result very significantly by showing that the sequence of matrices An1n|A^n|^{\frac{1}{n}} itself converges to a positive matrix BB whose eigenvalues are λ1,λ2,|\lambda _1|,|\lambda _2|, ,λm.\ldots , |\lambda _m|. Here this theorem has been extended to arbitrary compact operators on infinite dimensional complex separable Hilbert spaces. The proof makes use of Nayak's theorem, Stone-Weirstrass theorem, Borel-Caratheodory theorem and some technical results of Anselone and Palmer on collectively compact operators. Simple examples show that the result does not hold for general bounded operators.

Keywords

Cite

@article{arxiv.2408.16994,
  title  = {Nayak's theorem for compact operators},
  author = {B V Rajarama Bhat and Neeru Bala},
  journal= {arXiv preprint arXiv:2408.16994},
  year   = {2024}
}

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14 Pages

R2 v1 2026-06-28T18:28:23.163Z