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Questions where the notion matrix has an important or crucial role (for the latter, note the tag matrix-theory for potential use). Matrices appear in various parts of mathematics, and this tag is typically combined with other tags to make the general subject clear, such as an appropriate top-level tag ra.rings-and-algebras, co.combinatorics, etc. and other tags that might be applicable. There are also several more specialized tags concerning matrices.

20 votes
Accepted

Eigenvalue pattern

The explanation is pretty simple with a suitable change of basis. Letting $$B = \begin{pmatrix} 1 & 0 & 1 & 0 \\ i & 0 & -i & 0 \\ 0 & 1 & 0 & 1 \\ 0 & i & 0 & -i \end{pmatrix}$$ we have $$B^{-1}M …
Antoine Labelle's user avatar
11 votes
1 answer
324 views

$2$-adic valuation of Schur $P$-functions in the power-sum basis

For a partition $\lambda$, let $P_\lambda$ be the Schur $P$-functions (case $t=-1$ of Hall-Littlewood symmetric functions) and let $p_\lambda=p_{\lambda_1}p_{\lambda_1}\cdots p_{\lambda_k}$ be the pow …
Antoine Labelle's user avatar
8 votes
Accepted

On the coefficients that appear in finite groups of matrices with integer entries

We consider the conjugate by $B$ of the group of diagonal $\pm 1$ matrices, of size $2^n$. Let $X$ be a diagonal matrix with $X_{i,i}=\varepsilon_i \in \{\pm 1\}$ and suppose $\varepsilon_n=1$. … }^{n-1} \varepsilon_i x_i$$ If we take, for example, $x_i=2^i$, then these sums are all different for the $2^{n-1}$ possible choices of $X$, so at least $2^{n-1}$ different coefficients appear in the matrices
Antoine Labelle's user avatar
5 votes
Accepted

Eigenvalues invariant under 90° rotation

Assume $N$ is even (this is false when N is odd). Let $X=2B, Y=A+A^T$. Let $$P = \begin{bmatrix} 1 & 1 & 1 & \cdots & 1 \\ 1 & \zeta & \zeta^2 & \cdots & \zeta^{N-1} \\ 1 & \zeta^2 & \ze …
Antoine Labelle's user avatar
4 votes
Accepted

Similarity of two matrices

I will show that it is not possible for $\phi=\pi/2$, so it is certainly not for general $\phi$. (actually, I don't think that it is possible for any single $\phi$ except $0$ and $\pi$, by an analogou …
Antoine Labelle's user avatar
2 votes
Accepted

Upper bound of rank of a matrix

Both matrices are invertible since $B$ is a permutation matrix and $C$ is upper-triangular (with respect to any linear extension of the inclusion ordering on $2^X$) with $1$'s on the diagonal. …
Antoine Labelle's user avatar