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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.

10 votes

Trace of non-commutable matrices

Not to take anything away from Suvrit's answer, but this is actually much simpler. First, we can assume $M_1$ is diagonal. Call it $diag(x_1, \dotsc, x_i).$ Then the difference between the LHS and the …
Igor Rivin's user avatar
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9 votes

The multiplicity of the max eigenvalue in matrix multiplication

I am afraid you can infer almost nothing. For example, it is a result of Frobenius (1910) that every square matrix is a product of two symmetric matrices. Since the eigenvalues of the symmetric matric …
Igor Rivin's user avatar
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5 votes
Accepted

Dimension (manifold) of matrices with exact $r$ positive and $r$ negative eigenvalues

A Hermitian matrix of rank $r$ can be represented uniquely as $U D U^\ast,$ where $U$ is an $n\times r$ matrix with orthogonal rows of unit length, and $D$ is an $r\times r$ matrix (this is the singul …
Igor Rivin's user avatar
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4 votes

Exponentiating 4 by 4 matrix analytically

There is a completely explicit formula in this paper of Bensauod and Mouline (rendicotti Palermo, 2005), which is quite compact for low dimensions.
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4 votes
Accepted

Spectral properties of the LDL^T matrix factorization

Well, for the closely related Cholesky factorization, there is the following: Fast Accurate Eigenvalue Computations Using the Cholesky Factorization (1997) (by Roy Matthias), which says that the eige …
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3 votes

complexity of computing the singular vector corresponding to the smallest singular value

The question has been studied at some length. See, for example, Hubert Schwetlick and Uwe Schnabel, MR 1997360 Iterative computation of the smallest singular value and the corresponding singular vec …
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3 votes

Triangularizing a matrix with function entries

I am pretty sure this question (actually, the Jordan canonical form question) is studied at great length in Kato's perturbation theory for linear operators book (chapter 1). Kato is also very careful …
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2 votes
Accepted

How to determine an unitary operator involved in an unitary transformation?

An algorithm for arbitrary matrices is given by Heydar Radjavi in 1962 (On unitary equivalence of arbitrary matrices, TAMS). The "arbitrary" in the title is there because the problem is trivial for no …
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2 votes

Spectrum of transition matrix for symmetric random walk

The spectrum of the matrix is computed in the beginning of my preprint: Rivin, Igor. "Growth in free groups (and other stories)." arXiv preprint math/9911076 (1999). (there is a published version, …
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2 votes
Accepted

can eigenvector be found without computing the eigenvalue

If you pick a random vector $v$ and look at $v_n=A^n v/\| A^n v\|,$ that will converge to the dominant eigenvector.
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2 votes

Finding matrices $A$ such that the entries of $A^n$ have specified signs

To follow up on Terry's comment (actually, I did the computation before seeing it, but whatever): in the parabolic case, where the matrix has the form $$A = \begin{pmatrix} d & -b \\ -c & a\end{pmatr …
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2 votes

Is $1/\max(i,j)$ a bounded matrix on Hilbert spaces?

I could be very confused (likely), but notice that your matrix the sum of $N$ and $N^t,$ where $N$ is the upper triangular matrix where $N_{ij}= 1/i,$ when $i<j$ and $0$ otherwise. It seems that by D …
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1 vote

finding a unitary submatrix inside a random matrix

(1) a.s yes (2) The probability is $0.$ Unitary submatrices are of positive codimension in $GL(r),$ and since there only a finite number of $r\times r$ submatrices, the probability is $0.$ If you want …
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1 vote

Counting Boolean Normal Matrices of size $2n \times 2n$

Since the dimension of the variety of normal matrices is the same as that of the variety of symmetric matrices (for $n\times n$ complex normal matrices, the real dimension of the variety is $n^2+n,$ s …
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1 vote
Accepted

Equivalent metrics on symmetric positive definite matrices

Check out: Reverse inequality to Golden–Thompson type inequalities: Comparison of $e^{A+B}$ and $e^Ae^B$ Jean-Christophe Bourin, Yuki Seo (2007), Linear Algebra and its Applications Volume 426, Issue …
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