E1(о)Ed型距离正则图的关于余弦序列的不等式

给出了E1(о)Ed型距离正则图的关于余弦序列的不等式.     

型距离正则图的关于余弦序列的不等式

给出了     

具有广义可行性余弦序列的E1(。)Ed型距离正则图

给出了具有广义可行性余弦序列的E1(?)Ed型距离正则图的特征,并计算了这类图的交叉数.     

具有广义可行性余弦序列的E1?Ed型距离正则图

给出了具有广义可行性余弦序列的E₁?E_d型距离正则图的特征,并计算了这类图的交叉数.     

关于加权距离正则图和立方图

令G=(V,E)是简单的连通k-正则图;w_1相似文献   

E1οEd型距离正则图及 Q-多项式的性质

设Г是一个直径d ≥ 3的非二部距离正则图,其特征值θ0＞θ1＞…＞θ<,d.设θ1 ∈{θ1,θd),θd,是θ1在{θ1,θd}中的余.又设Г是具有性质E1οEd=|X|-1＞(qd-11dEd-1 qd1dEd)的E1○Ed型距离正则图,σ0,σ1,…,σd,ρ0,ρ1,…,ρd和β0,β1,…,βd分别是关于θ1',θd和θd-1'的余弦序列.利用上述余弦序列,给出了Г关于θ1或θd是 Q-多项式的充要条件.     

距离正则图的推广

本文研究了直径为d(Γ) ≥ 2的距离正则图Γ的补图.利用Γ的交叉数分别证明了当d=2时,Γ的补图式强正则;当d ≥ 3时,Γ的补图是广义强正则.将文献[2]中的距离正则图Grassmann图、对偶极图、Hamming图推广到它们的补图,从而得到广义强正则图.     

E1 ο Ed型距离正则图及 Q -多项式的性质

设$\Gamma$ 是一个直径$d\geq 3$的非二部距离正则图，其特征值 $\theta_{0}>\theta_{1}>\cdots>\theta_{d}.$ 设$\theta_{1'}\in\{ \theta_{1},\theta_{d}\}, $\theta_{d'}$ 是$\theta_{1'}$ 在 $\{\theta_{1},\theta_{d}\}$中的余. 又设 $\Gamma$ 是具有性质$E_{1}\circ E_{d}=|X|^{-1}(q^{d-1}_{1d}E_{d-1}+q^{d}_{1d}E_{d})$的$E_{1}\circ E_{d}$型距离正则图,$\sigma_{0},\sigma_{1},\cdots,\sigma_{d}$,$\rho_{0},\rho_{1},\cdots,\rho_{d}$和$\beta_{0},\beta_{1},\cdots,\beta_{d}$ 分别是关于$\theta_{1'}$,$\theta_{d'}$ 和 $\theta_{d-1}$的余弦序列.利用上述余弦序列,给出了 $\Gamma$关于 $\theta_{1}$ 或$\theta_{d}$是$Q$ -多项式的充要条件.     

链状正则图的平均距离

本文构造了一类链状正则图G_k∶δ,求出了它们的平均距离D(G_k.δ),并得到关系式上式等号成立当且仅当δ=4f且k=0.这个估计式指出了施容华猜想[1]D(G)≤n/(δ 1)不成立. 文中进一步证明了这一类链状正则图有最大的直径,所以可以作出猜想: 若G是n阶连通图,则D(G)<(n 1)/(δ 1),其中δ是图G的最小度。     

一类(a+1,3)型的距离正则图

设Γ是直径为d且型为(α 1,3)的距离正则图,其中α≥2.用l(c,a,b)表示交叉阵列l(Γ)中列(c,a,b)~t的个数,记r=r(Γ)=l(c_1,a_1,b_1),s=s(Γ)=l(c_(r 1),a(r 1),b_(r 1)).那末,若c_(r 1)=3且a_(r 1)=3a,则d=r s 1,c_d=4且Γ为正则拟2d边形.     

Pseudo 1-homogeneous distance-regular graphs

Let Γ be a distance-regular graph of diameter d≥2 and a ₁≠0. Let θ be a real number. A pseudo cosine sequence for θ is a sequence of real numbers σ ₀,…,σ _d such that σ ₀=1 and c _i σ _i−1+a _i σ _i +b _i σ _i+1=θ σ _i for all i∈{0,…,d−1}. Furthermore, a pseudo primitive idempotent for θ is E _θ =s ∑ _i=0 ^d σ _i A _i , where s is any nonzero scalar. Let be the characteristic vector of a vertex v∈VΓ. For an edge xy of Γ and the characteristic vector w of the set of common neighbours of x and y, we say that the edge xy is tight with respect to θ whenever θ≠k and a nontrivial linear combination of vectors , and Ew is contained in . When an edge of Γ is tight with respect to two distinct real numbers, a parameterization with d+1 parameters of the members of the intersection array of Γ is given (using the pseudo cosines σ ₁,…,σ _d , and an auxiliary parameter ε). Let S be the set of all the vertices of Γ that are not at distance d from both vertices x and y that are adjacent. The graph Γ is pseudo 1-homogeneous with respect to xy whenever the distance partition of S corresponding to the distances from x and y is equitable in the subgraph induced on S. We show Γ is pseudo 1-homogeneous with respect to the edge xy if and only if the edge xy is tight with respect to two distinct real numbers. Finally, let us fix a vertex x of Γ. Then the graph Γ is pseudo 1-homogeneous with respect to any edge xy, and the local graph of x is connected if and only if there is the above parameterization with d+1 parameters σ ₁,…,σ _d ,ε and the local graph of x is strongly regular with nontrivial eigenvalues a ₁ σ/(1+σ) and (σ ₂−1)/(σ−σ ₂).     

Antipodal distance-regular graphs of diameter four and five

An antipodal distance-regular graph of diameter four or five is a covering graph of a connected strongly regular graph. We give existence conditions for these graphs and show for some types of strongly regular graphs that no nontrivial covers exist. © 1999 John Wiley & Sons, Inc. J Combin Designs 7: 69–77, 1999     

On the primitive idempotents of distance-regular graphs

Let Γ denote a distance-regular graph with diameter d3. Let E, F denote nontrivial primitive idempotents of Γ such that F corresponds to the second largest or the least eigenvalue. We investigate the situation that there exists a primitive idempotent H of Γ such that EF is a linear combination of F and H. Our main purpose is to obtain the inequalities involving the cosines of E, and to show that equality is closely related to Γ being Q-polynomial with respect to E. This generalizes a result of Lang on bipartite graphs and a result of Pascasio on tight graphs.     

On distance-regular Cayley graphs of generalized dicyclic groups

Let G be a generalized dicyclic group with identity 1. An inverse closed subset S of $G?\{1\}$ is called minimal if $〈S〉=G$ and there exists some $s\in S$ such that $〈S?\{s,s^{?1}\}〉\ne G$. In this paper, we characterize distance-regular Cayley graphs $\mathrm{Cay}(G,S)$ of G under the condition that S is minimal.     

A characterization of Q-polynomial distance-regular graphs

Let Γ denote a distance-regular graph with diameter D3. Let θ denote a nontrivial eigenvalue of Γ and let denote the corresponding dual eigenvalue sequence. In this paper we prove that Γ is Q-polynomial with respect to θ if and only if the following (i)–(iii) hold:

(i) There exist such that

(1)

(ii) There exist such that the intersection numbers a_i satisfy

for 0iD, where and are the scalars which satisfy Eq. (1) for i=0, i=D, respectively.

(iii) for 1iD.