Let G be a connected graph. We denote by σ(G,x) and δ(G) respectively the σ-polynomial and the edge-density of G,where δ(G)=|E(G)||V(G)|2. If σ(G,x) has at least an unreal root,then G is said to be a σ-unreal gr...Let G be a connected graph. We denote by σ(G,x) and δ(G) respectively the σ-polynomial and the edge-density of G,where δ(G)=|E(G)||V(G)|2. If σ(G,x) has at least an unreal root,then G is said to be a σ-unreal graph.Let δ(n) be the minimum edge-density over all n vertices graphs with σ-unreal roots. In this paper,by using the theory of adjoint polynomials, a negative answer to a problem posed by Brenti et al. is given and the following results are obtained:For any positive integer a and rational number 0≤c≤1,there exists at least a graph sequence {G i} 1≤i≤a such that G i is σ-unreal and δ(G i)→c as n→∞ for all 1≤i≤a,and moreover, δ(n)→0 as n→∞.展开更多
For a graph G,P(G,λ)denotes the chromatic polynomial of G.Two graphs G and H are said to be chromatically equivalent,denoted by G～H,if P(G,λ)=p(H,λ).Let [G]={H|H～G}.If [G]={G},then G is said to be chromaticall...For a graph G,P(G,λ)denotes the chromatic polynomial of G.Two graphs G and H are said to be chromatically equivalent,denoted by G～H,if P(G,λ)=p(H,λ).Let [G]={H|H～G}.If [G]={G},then G is said to be chromatically unique.For a complete 5 partite graph G with 5n vertices, define θ(G)=(α(G,6)-2 n+1 -2 n-1 + 5)/2 n-2 ,where α(G,6) denotes the number of 6 independent partition s of G.In this paper, the authors show that θ(G)≥0 and determine all g raphs with θ(G)=0,1,2,5/2,7/2,4,17/4.By using these results the chromaticity of 5 partite graphs of the form G-S with θ(G)=0,1,2,5/2,7/2,4,17/4 is inve stigated,where S is a set of edges of G.Many new chromatically unique 5 partite graphs are obtained.展开更多
基金Supported by the National Natural Science Foundation of China(1 0 0 6 1 0 0 3 ) and the Science Founda-tion of the State Education Ministry of China
文摘Let G be a connected graph. We denote by σ(G,x) and δ(G) respectively the σ-polynomial and the edge-density of G,where δ(G)=|E(G)||V(G)|2. If σ(G,x) has at least an unreal root,then G is said to be a σ-unreal graph.Let δ(n) be the minimum edge-density over all n vertices graphs with σ-unreal roots. In this paper,by using the theory of adjoint polynomials, a negative answer to a problem posed by Brenti et al. is given and the following results are obtained:For any positive integer a and rational number 0≤c≤1,there exists at least a graph sequence {G i} 1≤i≤a such that G i is σ-unreal and δ(G i)→c as n→∞ for all 1≤i≤a,and moreover, δ(n)→0 as n→∞.
基金Supported by the National Natural Science Foundation of China (1 0 0 61 0 0 3) and the ScienceFoundation of the State Education Ministry of China
文摘For a graph G,P(G,λ)denotes the chromatic polynomial of G.Two graphs G and H are said to be chromatically equivalent,denoted by G～H,if P(G,λ)=p(H,λ).Let [G]={H|H～G}.If [G]={G},then G is said to be chromatically unique.For a complete 5 partite graph G with 5n vertices, define θ(G)=(α(G,6)-2 n+1 -2 n-1 + 5)/2 n-2 ,where α(G,6) denotes the number of 6 independent partition s of G.In this paper, the authors show that θ(G)≥0 and determine all g raphs with θ(G)=0,1,2,5/2,7/2,4,17/4.By using these results the chromaticity of 5 partite graphs of the form G-S with θ(G)=0,1,2,5/2,7/2,4,17/4 is inve stigated,where S is a set of edges of G.Many new chromatically unique 5 partite graphs are obtained.
