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Let $G$ be a simple graph with maximum degree $Delta(G)$ and chromatic index $chi(G)$. A classic result of Vizing indicates that either $chi(G )=Delta(G)$ or $chi(G )=Delta(G)+1$. The graph $G$ is called $Delta$-critical if $G$ is connected, $chi(G )=Delta(G)+1$ and for any $ein E(G)$, $chi(G-e)=Delta(G)$. Let $G$ be an $n$-vertex $Delta$-critical graph. Vizing conjectured that $alpha(G)$, the independence number of $G$, is at most $frac{n}{2}$. The current best result on this conjecture, shown by Woodall, is that $alpha(G)<frac{3n}{5}$. We show that for any given $varepsilonin (0,1)$, there exist positive constants $d_0(varepsilon)$ and $D_0(varepsilon)$ such that if $G$ is an $n$-vertex $Delta$-critical graph with minimum degree at least $d_0$ and maximum degree at least $D_0$, then $alpha(G)<(frac{{1}}{2}+varepsilon)n$. In particular, we show that if $G$ is an $n$-vertex $Delta$-critical graph with minimum degree at least $d$ and $Delta(G)ge (d+2)^{5d+10}$, then [ alpha(G) < left. begin{cases} frac{7n}{12}, & text{if $d= 3$; } frac{4n}{7}, & text{if $d= 4$; } frac{d+2+sqrt[3]{(d-1)d}}{2d+4+sqrt[3]{(d-1)d}}n<frac{4n}{7}, & text{if $dge 19$. } end{cases} right. ]
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Given a graph $G$, denote by $Delta$, $bar{d}$ and $chi^prime$ the maximum degree, the average degree and the chromatic index of $G$, respectively. A simple graph $G$ is called {it edge-$Delta$-critical} if $chi^prime(G)=Delta+1$ and $chi^prime(H)leDelta$ for every proper subgraph $H$ of $G$. Vizing in 1968 conjectured that if $G$ is edge-$Delta$-critical, then $bar{d}geq Delta-1+ frac{3}{n}$. We show that $$ begin{displaystyle} avd ge begin{cases} 0.69241D-0.15658 quad,: mbox{ if } Deltageq 66, 0.69392D-0.20642quad;,mbox{ if } Delta=65, mbox{ and } 0.68706D+0.19815quad! quadmbox{if } 56leq Deltaleq64. end{cases} end{displaystyle} $$ This result improves the best known bound $frac{2}{3}(Delta +2)$ obtained by Woodall in 2007 for $Delta geq 56$. Additionally, Woodall constructed an infinite family of graphs showing his result cannot be improved by well-known Vizings Adjacency Lemma and other known edge-coloring techniques. To over come the barrier, we follow the recently developed recoloring technique of Tashkinov trees to expand Vizing fans technique to a larger class of trees.
Kostochka and Yancey proved that every $4$-critical graph $G$ has $e(G) geq frac{5v(G) - 2}{3}$, and that equality holds if and only if $G$ is $4$-Ore. We show that a question of Postle and Smith-Roberge implies that every $4$-critical graph with no $(7,2)$-circular-colouring has $e(G) geq frac{27v(G) -20}{15}$. We prove that every $4$-critical graph with no $(7,2)$-colouring has $e(G) geq frac{17v(G)}{10}$ unless $G$ is isomorphic to $K_{4}$ or the wheel on six vertices. We also show that if the Gallai Tree of a $4$-critical graph with no $(7,2)$-colouring has every component isomorphic to either an odd cycle, a claw, or a path. In the case that the Gallai Tree contains an odd cycle component, then $G$ is isomorphic to an odd wheel. In general, we show a $k$-critical graph with no $(2k-1,2)$-colouring that contains a clique of size $k-1$ in its Gallai Tree is isomorphic to $K_{k}$.
The edge-distinguishing chromatic number (EDCN) of a graph $G$ is the minimum positive integer $k$ such that there exists a vertex coloring $c:V(G)to{1,2,dotsc,k}$ whose induced edge labels ${c(u),c(v)}$ are distinct for all edges $uv$. Previous work has determined the EDCN of paths, cycles, and spider graphs with three legs. In this paper, we determine the EDCN of petal graphs with two petals and a loop, cycles with one chord, and spider graphs with four legs. These are achieved by graph embedding into looped complete graphs.
A signed graph is a pair $(G, sigma)$, where $G$ is a graph and $sigma: E(G) to {+, -}$ is a signature which assigns to each edge of $G$ a sign. Various notions of coloring of signed graphs have been studied. In this paper, we extend circular coloring of graphs to signed graphs. Given a signed graph $(G, sigma)$ a circular $r$-coloring of $(G, sigma)$ is an assignment $psi$ of points of a circle of circumference $r$ to the vertices of $G$ such that for every edge $e=uv$ of $G$, if $sigma(e)=+$, then $psi(u)$ and $psi(v)$ have distance at least $1$, and if $sigma(e)=-$, then $psi(v)$ and the antipodal of $psi(u)$ have distance at least $1$. The circular chromatic number $chi_c(G, sigma)$ of a signed graph $(G, sigma)$ is the infimum of those $r$ for which $(G, sigma)$ admits a circular $r$-coloring. For a graph $G$, we define the signed circular chromatic number of $G$ to be $max{chi_c(G, sigma): sigma text{ is a signature of $G$}}$. We study basic properties of circular coloring of signed graphs and develop tools for calculating $chi_c(G, sigma)$. We explore the relation between the circular chromatic number and the signed circular chromatic number of graphs, and present bounds for the signed circular chromatic number of some families of graphs. In particular, we determine the supremum of the signed circular chromatic number of $k$-chromatic graphs of large girth, of simple bipartite planar graphs, $d$-degenerate graphs, simple outerplanar graphs and series-parallel graphs. We construct a signed planar simple graph whose circular chromatic number is $4+frac{2}{3}$. This is based and improves on a signed graph built by Kardos and Narboni as a counterexample to a conjecture of M{a}v{c}ajov{a}, Raspaud, and v{S}koviera.
Let Q(n,c) denote the minimum clique size an n-vertex graph can have if its chromatic number is c. Using Ramsey graphs we give an exact, albeit implicit, formula for the case c is at least (n+3)/2.
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