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The study of the properties of real and complex matrices that are more close to analysis and operator theory. For instance: the properties of positive definite matrices, matrix inequalities, perturbation analysis, matrix functions, inequalities between eigenvectors and singular values, majorization.

7 votes
Accepted

On approximate simultaneous diagonalization

The answer is no in general. For a $2\times 2$-counterexample, let $A = 0$, let $B$ be the diagonal matrix with diagonal entries $1$ and $0$ (i.e. $B$ is the projection onto the first component), cho …
Jochen Glueck's user avatar
4 votes
Accepted

A matrix monotonicity question

The answer is no, in general. Here is a counterexample: Let \begin{align*} X = \begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}, \quad \text{and} \quad A = \begin{ …
Jochen Glueck's user avatar
4 votes
1 answer
150 views

Mapping inclusion theorem for the numerical range

We denote the numerical range of a complex square matrix $A \in \mathbb{C}^{n\times n}$ by $W(A)$. Let $A \in \mathbb{C}^{n\times n}$ and let $f: \mathbb{C} \to \mathbb{C}$ be, say, an entire functio …
Jochen Glueck's user avatar
8 votes
3 answers
654 views

Representation theorem for matrices (reference request)

Motivation. If $A \in \mathbb{C}^{n \times n}$ is self-adjoint (or, more generally, normal), then we all know that $$ A = \sum_{k=1}^n \lambda_k \, h_k \otimes h_k, $$ where $\lambda_1,\dots,\lambda_n …
Jochen Glueck's user avatar
58 votes
Accepted

Is this proof of Perron's theorem correct, and if so is it original?

(1) Correctness: I read all arguments in detail and couldn't find anything wrong with them. Of course, this doesn't mean too much... (2) Orginality: I think in a topic which has such an extensive his …
Jochen Glueck's user avatar
8 votes
3 answers
642 views

Commutant of the conjugations by unitary matrices

Let $\mathcal{L}(\mathbb{C}^{n \times n})$ denote the algebra of all linear mappings from $\mathbb{C}^{n \times n}$ to $\mathbb{C}^{n \times n}$ and let $\mathcal{C} \subseteq \mathcal{L}(\mathbb{C}^{ …
Jochen Glueck's user avatar