Let $G$ be an undirected connected graph with $n$, $n\ge 3$, vertices and $m$ edges with Laplacian eigenvalues ${\mu }_1\ge {\mu }_2\ge \cdots \ge {\mu }_{n-1}>{\mu }_n=0$. Denote by ${\mu }_I={\mu }_{r_1}+{\mu }_{r_2}+\cdots +{\mu }_{r_k}$, $1\le k\le n-2$, $1\le r_1<r_2<\cdots <r_k\le n-1$, the sum of $k$ arbitrary Laplacian eigenvalues, with ${\mu }_{I_1}={\mu }_1+{\mu }_2+\cdots +{\mu }_k$ and ${\mu }_{I_n}={\mu }_{n-k}+\cdots +{\mu }_{n-1}$. Lower bounds of graph invariants ${\mu }_{I_1}-{\mu }_{I_n}$ and ${\mu }_{I_1}/{\mu }_{I_n}$ are obtained. Some known inequalities follow as a special case.

Let F be an analytic function from an open subset Ω of the complex plane into the algebra of n×n matrices. Denoting by $s_1,...,s_n$ the decreasing sequence of singular values of a matrix, we prove that the functions $log{s}_1\left(F\left(\lambda \right)\right)+...+log{s}_k\left(F\left(\lambda \right)\right)$ and $lo{g}^{+}{s}_1\left(F\left(\lambda \right)\right)+...+lo{g}^{+}{s}_k\left(F\left(\lambda \right)\right)$ are subharmonic on Ω for 1 ≤ k ≤ n.

Starting with Dürer's magic square which appears in the well-known copper plate engraving Melencolia we consider the class of melancholic magic squares. Each member of this class exhibits the same 86 patterns of Dürer's magic square and is magic again. Special attention is paid to the eigenstructure of melancholic magic squares, their group inverse and their Moore-Penrose inverse. It is seen how the patterns of the original Dürer square to a large extent are passed down also to the inverses of the...

In this paper we studied the classical numerical range of matrices in sp(2n, C). We obtained some result on the relationship between the numerical range of a matrix in and that [...] of its diagonal block, the singular values of its off-diagonal block A2.

A matrix $𝒜$ whose entries come from the set $\{+,-,0\}$ is called a sign pattern matrix, or sign pattern. A sign pattern is said to be potentially nilpotent if it has a nilpotent realization. In this paper, the characterization problem for some potentially nilpotent double star sign patterns is discussed. A class of double star sign patterns, denoted by $𝒟SSP(m,2)$, is introduced. We determine all potentially nilpotent sign patterns in $𝒟SSP(3,2)$ and $𝒟SSP(5,2)$, and prove that one sign pattern in $𝒟SSP(3,2)$ is potentially stable.

Let A be an invertible 3 × 3 complex matrix. It is shown that there is a 3 × 3 permutation matrix P such that the product PA has at least two distinct eigenvalues. The nilpotent complex n × n matrices A for which the products PA with all symmetric matrices P have a single spectrum are determined. It is shown that for a n × n complex matrix [...] there exists a permutation matrix P such that the product PA has at least two distinct eigenvalues.