arXiv subjects
Publications and source records attributed to Sizhong Zhou.
Let $k\geq2$ be an integer. A tree $T$ is called a $k$-tree if $d_T(v)\leq k$ for each $v\in V(T)$, that is, the maximum degree of a $k$-tree is at most $k$. Let $λ_1(D(G))$ denote the distance spectral radius in $G$, where $D(G)$ denotes the distance matrix of $G$. In this paper, we verify a upper bound for $λ_1(D(G))$ in a connected graph $G$ to guarantee the existence of a spanning $k$-tree in $G$.
Let $α\in[0,1)$, and let $G$ be a connected graph of order $n$ with $n\geq f(α)$, where $f(α)=14$ for $α\in[0,\frac{1}{2}]$, $f(α)=17$ for $α\in(\frac{1}{2},\frac{2}{3}]$, $f(α)=20$ for $α\in(\frac{2}{3},\frac{3}{4}]$ and $f(α)=\frac{5}{1-α}+1$ for $α\in(\frac{3}{4},1)$. A path factor is a spanning subgraph $F$ of $G$ such that every component of $F$ is a path with at least two vertices. Let $k\geq2$ be an integer. A $P_{\geq k}$-factor means a path-factor with each component being a path of order at least $k$. A graph $G$ is called a $P_{\geq k}$-factor covered graph if $G$ has a $P_{\geq k}$-factor containing $e$ for any $e\in E(G)$. Let $A_α(G)=αD(G)+(1-α)A(G)$, where $D(G)$ denotes the diagonal matrix of vertex degrees of $G$ and $A(G)$ denotes the adjacency matrix of $G$. The largest eigenvalue of $A_α(G)$ is called the $A_α$-spectral radius of $G$, which is denoted by $ρ_α(G)$. In this paper, it is proved that $G$ is a $P_{\geq2}$-factor covered graph if $ρ_α(G)>η(n)$, where $η(n)$ is the largest root of $x^{3}-((α+1)n+α-4)x^{2}+(αn^{2}+(α^{2}-2α-1)n-2α+1)x-α^{2}n^{2}+(5α^{2}-3α+2)n-10α^{2}+15α-8=0$. Furthermore, we provide a graph to show that the bound on $A_α$-spectral radius is optimal.
Let $\mathcal{A}$ be a set of connected graphs. Then a spanning subgraph $A$ of $G$ is called an $\mathcal{A}$-factor if each component of $A$ is isomorphic to some member of $\mathcal{A}$. Especially, when every graph in $\mathcal{A}$ is a path, $A$ is a path factor. For a positive integer $d\geq2$, we write $\mathcal{P}_{\geq d}=\{P_i|i\geq d\}$. Then a $\mathcal{P}_{\geq d}$-factor means a path factor in which every component admits at least $d$ vertices. A graph $G$ is called a $(\mathcal{P}_{\geq d},m)$-factor deleted graph if $G-E'$ admits a $\mathcal{P}_{\geq d}$-factor for any $E'\subseteq E(G)$ with $|E'|=m$. A graph $G$ is called a $(\mathcal{P}_{\geq d},k)$-factor critical graph if $G-Q$ has a $\mathcal{P}_{\geq d}$-factor for any $Q\subseteq V(G)$ with $|Q|=k$. In this paper, we present two degree conditions for graphs to be $(\mathcal{P}_{\geq3},m)$-factor deleted graphs and $(\mathcal{P}_{\geq3},k)$-factor critical graphs. Furthermore, we show that the two results are best possible in some sense.
For a nonnegative integer $k$, a graph $G$ is said to be $k$-factor-critical if $G-Q$ admits a perfect matching for any $Q\subseteq V(G)$ with $|Q|=k$. In this article, we prove spectral radius conditions for the existence of $k$-factor-critical graphs. Our result generalises one previous result on perfect matchings of graphs. Furthermore, we claim that the bounds on spectral radius in Theorem 3.1 are sharp.
Let $a$ and $b$ be two positive integers with $a\leq b$, and let $G$ be a graph with vertex set $V(G)$ and edge set $E(G)$. Let $h:E(G)\rightarrow[0,1]$ be a function. If $a\leq\sum\limits_{e\in E_G(v)}{h(e)}\leq b$ holds for every $v\in V(G)$, then the subgraph of $G$ with vertex set $V(G)$ and edge set $F_h$, denoted by $G[F_h]$, is called a fractional $[a,b]$-factor of $G$ with indicator function $h$, where $E_G(v)$ denotes the set of edges incident with $v$ in $G$ and $F_h=\{e\in E(G):h(e)>0\}$. A graph $G$ is defined as a fractional $[a,b]$-deleted graph if for any $e\in E(G)$, $G-e$ contains a fractional $[a,b]$-factor. The size, spectral radius and signless Laplacian spectral radius of $G$ are denoted by $e(G)$, $ρ(G)$ and $q(G)$, respectively. In this paper, we establish a lower bound on the size, spectral radius and signless Laplacian spectral radius of a graph $G$ to guarantee that $G$ is a fractional $[a,b]$-deleted graph.
A spanning subgraph $F$ of $G$ is called a path factor if every component of $F$ is a path of order at least 2. Let $k\geq2$ be an integer. A $P_{\geq k}$-factor of $G$ means a path factor in which every component has at least $k$ vertices. A graph $G$ is called a $P_{\geq k}$-factor avoidable graph if for any $e\in E(G)$, $G$ has a $P_{\geq k}$-factor avoiding $e$. A graph $G$ is called a $(P_{\geq k},n)$-factor critical avoidable graph if for any $W\subseteq V(G)$ with $|W|=n$, $G-W$ is a $P_{\geq k}$-factor avoidable graph. In other words, $G$ is $(P_{\geq k},n)$-factor critical avoidable if for any $W\subseteq V(G)$ with $|W|=n$ and any $e\in E(G-W)$, $G-W-e$ admits a $P_{\geq k}$-factor. In this article, we verify that (\romannumeral1) an $(n+r+2)$-connected graph $G$ is $(P_{\geq2},n)$-factor critical avoidable if $I(G)>\frac{n+r+3}{2(r+2)}$; (\romannumeral2) an $(n+r+2)$-connected graph $G$ is $(P_{\geq3},n)$-factor critical avoidable if $t(G)>\frac{n+r+2}{2(r+2)}$; (\romannumeral3) an $(n+r+2)$-connected graph $G$ is $(P_{\geq3},n)$-factor critical avoidable if $I(G)>\frac{n+3(r+2)}{2(r+2)}$; where $n$ and $r$ are two nonnegative integers.
Let $k$ and $n$ be two nonnegative integers with $n\equiv0$ (mod 2), and let $G$ be a graph of order $n$ with a 1-factor. Then $G$ is said to be $k$-extendable for $0\leq k\leq\frac{n-2}{2}$ if every matching in $G$ of size $k$ can be extended to a 1-factor. In this paper, we first establish a lower bound on the signless Laplacian spectral radius of $G$ to ensure that $G$ is $k$-extendable. Then we create some extremal graphs to claim that all the bounds derived in this article are sharp.
In communication networks, the binding numbers of graphs (or networks) are often used to measure the vulnerability and robustness of graphs (or networks). Furthermore, the fractional factors of graphs and the fractional ID-$[a,b]$-factor-critical covered graphs have a great deal of important applications in the data transmission networks. In this paper, we investigate the relationship between the binding numbers of graphs and the fractional ID-$[a,b]$-factor-critical covered graphs, and derive a binding number condition for a graph to be fractional ID-$[a,b]$-factor-critical covered, which is an extension of Zhou's previous result [S. Zhou, Binding numbers for fractional ID-$k$-factor-critical graphs, Acta Mathematica Sinica, English Series 30(1)(2014)181--186].
For a set $\mathcal{H}$ of connected graphs, a spanning subgraph $H$ of $G$ is called an $\mathcal{H}$-factor of $G$ if each component of $H$ is isomorphic to an element of $\mathcal{H}$. A graph $G$ is called an $\mathcal{H}$-factor uniform graph if for any two edges $e_1$ and $e_2$ of $G$, $G$ has an $\mathcal{H}$-factor covering $e_1$ and excluding $e_2$. Let each component in $\mathcal{H}$ be a path with at least $d$ vertices, where $d\geq2$ is an integer. Then an $\mathcal{H}$-factor and an $\mathcal{H}$-factor uniform graph are called a $P_{\geq d}$-factor and a $P_{\geq d}$-factor uniform graph, respectively. In this article, we verify that (\romannumeral1) a 2-edge-connected graph $G$ is a $P_{\geq3}$-factor uniform graph if $δ(G)>\frac{α(G)+4}{2}$; (\romannumeral2) a $(k+2)$-connected graph $G$ of order $n$ with $n\geq5k+3-\frac{3}{5γ-1}$ is a $P_{\geq3}$-factor uniform graph if $|N_G(A)|>γ(n-3k-2)+k+2$ for any independent set $A$ of $G$ with $|A|=\lfloorγ(2k+1)\rfloor$, where $k$ is a positive integer and $γ$ is a real number with $\frac{1}{3}\leqγ\leq1$.
For a graph $G$, the set of vertices in $G$ is denoted by $V(G)$, and the set of edges in $G$ is denoted by $E(G)$. A fractional $[a,b]$-factor of a graph $G$ is a function $h$ from $E(G)$ to $[0,1]$ satisfying $a\leq d_G^{h}(v)\leq b$ for every vertex $v$ of $G$, where $d_G^{h}(v)=\sum\limits_{e\in E(v)}{h(e)}$ and $E(v)=\{e=uv:u\in V(G)\}$. A graph $G$ is called fractional $[a,b]$-covered if $G$ contains a fractional $[a,b]$-factor $h$ with $h(e)=1$ for any edge $e$ of $G$. A graph $G$ is called fractional $(a,b,k)$-critical covered if $G-Q$ is fractional $[a,b]$-covered for any $Q\subseteq V(G)$ with $|Q|=k$. In this article, we demonstrate a neighborhood condition for a graph to be fractional $(a,b,k)$-critical covered. Furthermore, we claim that the result is sharp.
A graph $G$ is a fractional $(a,b,k)$-critical covered graph if $G-U$ is a fractional $[a,b]$-covered graph for every $U\subseteq V(G)$ with $|U|=k$, which is first defined by Zhou, Xu and Sun (S. Zhou, Y. Xu, Z. Sun, Degree conditions for fractional $(a,b,k)$-critical covered graphs, Information Processing Letters, DOI: 10.1016/j.ipl.2019.105838). Furthermore, they derived a degree condition for a graph to be a fractional $(a,b,k)$-critical covered graph. In this paper, we gain an independence number and connectivity condition for a graph to be a fractional $(a,b,k)$-critical covered graph and verify that $G$ is a fractional $(a,b,k)$-critical covered graph if $$ κ(G)\geq\max\Big\{\frac{2b(a+1)(b+1)+4bk+5}{4b},\frac{(a+1)^{2}α(G)+4bk+5}{4b}\Big\}. $$
Let $G$ be a graph, and $g,f:V(G)\rightarrow Z^{+}$ with $g(x)\leq f(x)$ for each $x\in V(G)$. We say that $G$ admits all fractional $(g,f)$-factors if $G$ contains a fractional $r$-factor for every $r:V(G)\rightarrow Z^{+}$ with $g(x)\leq r(x)\leq f(x)$ for any $x\in V(G)$. Let $H$ be a subgraph of $G$. We say that $G$ has all fractional $(g,f)$-factors excluding $H$ if for every $r:V(G)\rightarrow Z^{+}$ with $g(x)\leq r(x)\leq f(x)$ for all $x\in V(G)$, $G$ has a fractional $r$-factor $F_h$ such that $E(H)\cap E(F_h)=\emptyset$, where $h:E(G)\rightarrow [0,1]$ is a function. In this paper, we show a characterization for the existence of all fractional $(g,f)$-factors excluding $H$ and obtain two sufficient conditions for a graph to have all fractional $(g,f)$-factors excluding $H$.
Let $G$ be a graph, and $g,f:V(G)\rightarrow N$ be two functions with $g(x)\leq f(x)$ for each vertex $x$ in $G$. We say that $G$ has all fractional $(g,f)$-factors if $G$ includes a fractional $r$-factor for every $r:V(G)\rightarrow N$ such that $g(x)\leq r(x)\leq f(x)$ for each vertex $x$ in $G$. Let $H$ be a subgraph of $G$. We say that $G$ admits all fractional $(g,f)$-factors including $H$ if for every $r:V(G)\rightarrow N$ with $g(x)\leq r(x)\leq f(x)$ for each vertex $x$ in $G$, $G$ includes a fractional $r$-factor $F_h$ with $h(e)=1$ for any $e\in E(H)$, then we say that $G$ admits all fractional $(g,f)$-factors including $H$, where $h:E(G)\rightarrow [0,1]$ is the indicator function of $F_h$. In this paper, we obtain a characterization for the existence of all fractional $(g,f)$-factors including $H$ and pose a sufficient condition for a graph to have all fractional $(g,f)$-factors including $H$.
Let $G$ be a graph of order $n$, and let $a$ and $b$ be two integers with $1\leq a\leq b$. Let $h: E(G)\rightarrow [0,1]$ be a function. If $a\leq\sum_{e\ni x}h(e)\leq b$ holds for any $x\in V(G)$, then we call $G[F_h]$ a fractional $[a,b]$-factor of $G$ with indicator function $h$ where $F_h=\{e\in E(G): h(e)>0\}$. A graph $G$ is fractional independent-set-deletable $[a,b]$-factor-critical (in short, fractional ID-$[a,b]$-factor-critical) if $G-I$ has a fractional $[a,b]$-factor for every independent set $I$ of $G$. In this paper, it is proved that if $n\geq\frac{(a+2b)(2a+2b-3)+1}{b}$, $δ(G)\geq\frac{bn}{a+2b}+a$ and $|N_G(x)\cup N_G(y)|\geq\frac{(a+b)n}{a+2b}$ for any two nonadjacent vertices $x,y\in V(G)$, then $G$ is fractional ID-$[a,b]$-factor-critical. Furthermore, it is shown that this result is best possible in some sense.