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Register a new userOn the inertia index of a mixed graph with the matching number
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A mixed graph $widetilde{G}$ is obtained by orienting some edges of $G$, where $G$ is the underlying graph of $widetilde{G}$. The positive inertia index, denoted by $p^{+}(G)$, and the negative inertia index, denoted by $n^{-}(G)$, of a mixed graph $widetilde{G}$ are the integers specifying the numbers of positive and negative eigenvalues of the Hermitian adjacent matrix of $widetilde{G}$, respectively. In this paper, we study the positive and negative inertia index of the mixed unicyclic graph. Moreover, we give the upper and lower bounds of the positive and negative inertia index of the mixed graph, and characterize the mixed graphs which attain the upper and lower bounds respectively.
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A signed graph is a pair $(G,Sigma)$, where $G=(V,E)$ is a graph (in which parallel edges and loops are permitted) with $V={1,ldots,n}$ and $Sigmasubseteq E$. The edges in $Sigma$ are called odd edges and the other edges of $E$ even. By $S(G,Sigma)$ we denote the set of all symmetric $ntimes n$ real matrices $A=[a_{i,j}]$ such that if $a_{i,j} < 0$, then there must be an even edge connecting $i$ and $j$; if $a_{i,j} > 0$, then there must be an odd edge connecting $i$ and $j$; and if $a_{i,j} = 0$, then either there must be an odd edge and an even edge connecting $i$ and $j$, or there are no edges connecting $i$ and $j$. (Here we allow $i=j$.) For a symmetric real matrix $A$, the partial inertia of $A$ is the pair $(p,q)$, where $p$ and $q$ are the number of positive and negative eigenvalues of $A$, respectively. If $(G,Sigma)$ is a signed graph, we define the emph{inertia set} of $(G,Sigma)$ as the set of the partial inertias of all matrices $A in S(G,Sigma)$. In this paper, we present a formula that allows us to obtain the minimal elements of the inertia set of $(G,Sigma)$ in case $(G,Sigma)$ has a $1$-separation using the inertia sets of certain signed graphs associated to the $1$-separation.
A signed graph is a pair $(G,Sigma)$, where $G=(V,E)$ is a graph (in which parallel edges are permitted, but loops are not) with $V={1,...,n}$ and $Sigmasubseteq E$. By $S(G,Sigma)$ we denote the set of all symmetric $Vtimes V$ matrices $A=[a_{i,j}]$ with $a_{i,j}<0$ if $i$ and $j$ are connected by only even edges, $a_{i,j}>0$ if $i$ and $j$ are connected by only odd edges, $a_{i,j}in mathbb{R}$ if $i$ and $j$ are connected by both even and odd edges, $a_{i,j}=0$ if $i ot=j$ and $i$ and $j$ are non-adjacent, and $a_{i,i} in mathbb{R}$ for all vertices $i$. The stable inertia set of a signed graph $(G,Sigma)$ is the set of all pairs $(p,q)$ for which there exists a matrix $Ain S(G,Sigma)$ with $p$ positive and $q$ negative eigenvalues which has the Strong Arnold Property. In this paper, we study the stable inertia set of (signed) graphs.
The fractional matching number of a graph G, is the maximum size of a fractional matching of G. The following sharp lower bounds for a graph G of order n are proved, and all extremal graphs are characterized in this paper. (1)The sum of the fractional matching number of a graph G and the fractional matching number of its complement is not less than n/2 , where n is not less than 2. (2) If G and its complement are non-empty, then the sum of the fractional matching number of a graph G and the fractional matching number of its complement is not less than (n+1)/2, where n is not less than 28. (3) If G and its complement have no isolated vertices, then the sum of the fractional matching number of a graph G and the fractional matching number of its complement is not less than (n+4)/2, where n is not less than 28.
A complex unit gain graph (or ${mathbb T}$-gain graph) is a triple $Phi=(G, {mathbb T}, varphi)$ (or $(G, varphi)$ for short) consisting of a simple graph $G$, as the underlying graph of $(G, varphi)$, the set of unit complex numbers $mathbb{T}= { z in C:|z|=1 }$ and a gain function $varphi: overrightarrow{E} rightarrow mathbb{T}$ with the property that $varphi(e_{i,j})=varphi(e_{j,i})^{-1}$. In this paper, we prove that $2m(G)-2c(G) leq r(G, varphi) leq 2m(G)+c(G)$, where $r(G, varphi)$, $m(G)$ and $c(G)$ are the rank of the Hermitian adjacency matrix $H(G, varphi)$, the matching number and the cyclomatic number of $G$, respectively. Furthermore, the complex unit gain graphs $(G, mathbb{T}, varphi)$ with $r(G, varphi)=2m(G)-2c(G)$ and $r(G, varphi)=2m(G)+c(G)$ are characterized. These results generalize the corresponding known results about undirected graphs, mixed graphs and signed graphs. Moreover, we show that $2m(G-V_{0}) leq r(G, varphi) leq 2m(G)+b(G)$ holds for any subset $V_0$ of $V(G)$ such that $G-V_0$ is acyclic and $b(G)$ is the minimum integer $|S|$ such that $G-S$ is bipartite for $S subset V(G)$.
Let $ Pi_q $ be the projective plane of order $ q $, let $psi(m):=psi(L(K_m))$ the pseudoachromatic number of the complete line graph of order $ m $, let $ ain { 3,4,dots,tfrac{q}{2}+1 } $ and $ m_a=(q+1)^2-a $. In this paper, we improve the upper bound of $ psi(m) $ given by Araujo-Pardo et al. [J Graph Theory 66 (2011), 89--97] and Jamison [Discrete Math. 74 (1989), 99--115] in the following values: if $ xgeq 2 $ is an integer and $min {4x^2-x,dots,4x^2+3x-3}$ then $psi(m) leq 2x(m-x-1)$. On the other hand, if $ q $ is even and there exists $ Pi_q $ we give a complete edge-colouring of $ K_{m_a} $ with $(m_a-a)q$ colours. Moreover, using this colouring we extend the previous results for $a={-1,0,1,2}$ given by Araujo-Pardo et al. in [J Graph Theory 66 (2011), 89--97] and [Bol. Soc. Mat. Mex. (2014) 20:17--28] proving that $psi(m_a)=(m_a-a)q$ for $ ain {3,4,dots,leftlceil frac{1+sqrt{4q+9}}{2}rightrceil -1 } $.
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