紧凑算子的纳亚克定理

B V Rajarama Bhat, Neeru Bala
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引用次数: 0

摘要

让 $A$ 是一个 $m/times m$ 复矩阵,让 $\lambda _1, \lambda _2,\ldots , \lambda _m$ 是 $A$ 的特征值,使得 $|\lambda_1|\geq |\lambda _2|geq \cdots \geq |\lambda _m|$ 排列,并且对于 $n\geq 1、让$s^{(n)}_1/geq s^{(n)}_2\geq \cdots \geq s^{(n)}_m$ 是$A^n$的奇异值。然后山本(Yamamoto,1967 年)的一个著名定理指出 $$lim _{n\to\infty}(s^{(n)}_j )^{frac{1}{n}}= |\lambda _j|, ~~\forall \,1\leq j\leq m.$$最近 S.Nayak 通过证明矩阵$|A^n|^{frac{1}{n}}$本身的序列收敛于一个正矩阵$B$,其特征值为$|\lambda _1||,|\lambda _2||,$$\ldots ,|\lambda_m|$,极大地加强了这一结果。证明中使用了纳亚克定理、斯通-韦斯特拉斯定理、伯勒-卡拉特奥多里定理以及安塞龙和帕尔默关于集合紧凑算子的一些技术结果。简单的例子表明,该结果并不成立于一般有界算子。
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Nayak's theorem for compact operators
Let $A$ be an $m\times m$ complex matrix and let $\lambda _1, \lambda _2, \ldots , \lambda _m$ be the eigenvalues of $A$ arranged such that $|\lambda _1|\geq |\lambda _2|\geq \cdots \geq |\lambda _m|$ and for $n\geq 1,$ let $s^{(n)}_1\geq s^{(n)}_2\geq \cdots \geq s^{(n)}_m$ be the singular values of $A^n$. Then a famous theorem of Yamamoto (1967) states that $$\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 $|A^n|^{\frac{1}{n}}$ itself converges to a positive matrix $B$ whose eigenvalues are $|\lambda _1|,|\lambda _2|,$ $\ldots , |\lambda _m|.$ 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.
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