Complements of coalescing sets

We consider matrices of the form $qD+A$, with $D$ being the diagonal matrix of degrees, $A$ being the adjacency matrix, and $q$ a fixed value. Given a graph $H$ and $B\subseteq V(G)$, which we call a coalescent pair $(H,B)$, we derive a formula for the characteristic polynomial where a copy of same rooted graph $G$ is attached by the root to \emph{each} vertex of $B$. Moreover, we establish if $(H_1,B_1)$ and $(H_2,B_2)$ are two coalescent pairs which are cospectral for any possible rooted graph $G$, then $(H_1,V(H_1)\setminus B_1)$ and $(H_2,V(H_2)\setminus B_2)$ will also always be cospectral for any possible rooted graph $G$.Comment: 16 page

On the Laplacian and Signless Laplacian Characteristic Polynomials of a Digraph

Let D be a digraph with n vertices and a arcs. The Laplacian and the signless Laplacian matrices of D are, respectively, defined as L(D)=Deg+(D)−A(D) and Q(D)=Deg+(D)+A(D), where A(D) represents the adjacency matrix and Deg+(D) represents the diagonal matrix whose diagonal elements are the out-degrees of the vertices in D. We derive a combinatorial representation regarding the first few coefficients of the (signless) Laplacian characteristic polynomial of D. We provide concrete directed motifs to highlight some applications and implications of our results. The paper is concluded with digraph examples demonstrating detailed calculations

On the sum of the two largest signless Laplacian eigenvalues

Let $G$ be a simple connected graph and let $S_k(G)$ be the sum of the first $k$ largest signless Laplacian eigenvalues of $G$. It was conjectured by Ashraf, Omidi and Tayfeh-Rezaibe in 2013 that $S_k(G)\leq e(G)+\binom{k+1}{2}$ holds for $1\leq k\leq n-1$. They gave a proof for the conjecture when $k = 2$, but applied an incorrect key lemma. Therefore, the conjecture is still open when $k = 2$. In this paper, we prove that $S_2(G)<e(G)+3$ is true for any graphs which also confirm the conjecture when $k = 2$.Comment: 15 pages, 5 figure