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Let S(Gσ) be the skew adjacency matrix of the oriented graph Gσ of order n and λ1,λ2,…,λn be all eigenvalues of S(Gσ). The skew spectral radius ρs(Gσ) of Gσ is defined as max{|λ1|,|λ2|,…,|λn|}. In this paper, we investigate oriented graphs whose skew spectral radii do not exceed 2. 相似文献
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A graph G with no isolated vertex is total domination vertex critical if for any vertex v of G that is not adjacent to a vertex of degree one, the total domination number of G-v is less than the total domination number of G . We call these graphs γt-critical. If such a graph G has total domination number k, we call it k -γt-critical. We verify an open problem of k -γt-critical graphs and obtain some results on the characterization of total domination critical graphs of order n=Δ(G)(γt(G)-1)+1. 相似文献
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We prove that if G is a finite simple group which is the unit group of a ring, then G is isomorphic to: (a) a cyclic group of order 2; or (b) a cyclic group of prime order 2k−1 for some k; or (c) a projective special linear group PSLn(F2) for some n≥3. Moreover, these groups do all occur as unit groups. We deduce this classification from a more general result, which holds for groups G with no non-trivial normal 2-subgroup. 相似文献
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Let G be a simple connected graph of order n with degree sequence d1,d2,…,dn in non-increasing order. The signless Laplacian spectral radius ρ(Q(G)) of G is the largest eigenvalue of its signless Laplacian matrix Q(G). In this paper, we give a sharp upper bound on the signless Laplacian spectral radius ρ(Q(G)) in terms of di, which improves and generalizes some known results. 相似文献
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Let F be an infinite field with characteristic not equal to two. For a graph G=(V,E) with V={1,…,n}, let S(G;F) be the set of all symmetric n×n matrices A=[ai,j] over F with ai,j≠0, i≠j if and only if ij∈E. We show that if G is the complement of a partial k -tree and m?k+2, then for all nonsingular symmetric m×m matrices K over F, there exists an m×n matrix U such that UTKU∈S(G;F). As a corollary we obtain that, if k+2?m?n and G is the complement of a partial k-tree, then for any two nonnegative integers p and q with p+q=m, there exists a matrix in S(G;R) with p positive and q negative eigenvalues. 相似文献
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Denote by D(G)=(di,j)n×n the distance matrix of a connected graph G with n vertices, where dij is equal to the distance between vertices vi and vj in G . The least eigenvalue of D(G) is called the least distance eigenvalue of G , denoted by λn. In this paper, we determine all the graphs with λn∈[−2.383,0]. 相似文献
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