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JOURNALS // Zhurnal Vychislitel'noi Matematiki i Matematicheskoi Fiziki // Archive

Zh. Vychisl. Mat. Mat. Fiz., 2005 Volume 45, Number 4, Pages 587–591 (Mi zvmmf664)

On the Euclidean distance to the set of matrices with a multiple zero eigenvalue

Kh. D. Ikramov

Faculty of Computational Mathematics and Cybernetics, Moscow State University, Moscow, 119992, Russia

Abstract: Let $M_n(\mathbb C)$ be the set of $n$-by-$n$ complex matrices ($n>2$), and let $\mathcal K$ and $\mathcal L$ be the subsets of $M_n(\mathbb C)$ consisting of the matrices with a rank not greater than $n-2$ and of the matrices with a multiple zero eigenvalue, respectively. It is known that the minimal distance from a matrix $A\in M_n(\mathbb C)$ to the matrices in $\mathcal K$ is attained at the same matrix $K_A$ for both the spectral and Euclidean norm. It is shown that, for the set $\mathcal L$, similar minimal distances are attained, in the general case, at different matrices in $\mathcal L$. Moreover, the Euclidean distance from $A$ to $\mathcal L$ is, in general, strictly less than the Euclidean distance from $A$ to $\mathcal K$.

Key words: spectral norm, Euclidean norm, singular values, normal matrix, departure from normality.

UDC: 519.614

Received: 13.09.2004


 English version:
Computational Mathematics and Mathematical Physics, 2005, 45:4, 563–567

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