NIU Ai-hong,WANG Guo-ping,QIN Zheng-xin, MU Shan-zhi

(1.School of Mathematical Sciences,Xinjiang Normal University, Urumqi 830054,China; 2.College of Mathematics and System Sciences,Xinjiang University, Urumqi 830046,China; 3.Department of Mathematics,Jiangsu University of Technology, Changzhou Jiangsu 213001,China)

Characterization of bipartite graph With maximum spectral radius

NIU Ai-hong1,WANG Guo-ping1,QIN Zheng-xin2, MU Shan-zhi3

(1.School of Mathematical Sciences,Xinjiang Normal University, Urumqi 830054,China; 2.College of Mathematics and System Sciences,Xinjiang University, Urumqi 830046,China; 3.Department of Mathematics,Jiangsu University of Technology, Changzhou Jiangsu 213001,China)

The adjacency matrix A(G)of a graph G is the n×n matrix With its(i,j)-entry equal to 1 if viand vjare adjacent,and 0 otherwise.The spectral radius of G is the largest eigenvalue of A(G).In this paper we determine the graphs With maximum spectral radius among all trees,and all bipartite unicyclic,bicyclic,tricyclic,tetracyclic,pentacyclic and quasi-tree graphs,respectively.

bipartite graph;cycle;spectral radius

0 Introduction

Let G be a connected simple graph With Vertex set V(G)={v1,v2,···,vn}.The adjacency matrix of G,denoted by A(G),is the n×n matrix With its(i,j)-entry equal to 1 if viand vjare adjacent,and 0 otherwise.The spectral radius of G is the largest eigenvalue of A(G). The spectral radius of a connected graph has been studied extensively(see［1-4］).Zhai,Liu and Shu［5］characterized the bipartite graphs With given diameter which attain the maximum and the second largest spectral radius,respectively.Nath and Paul［6］determined the graphs With minimum distance spectral radius among all connected bipartite graphs of order n With a given matching number and Vertex connectivity.Zhang and Zhang［7］obtained the graphs With maximum Laplacian spectral radius among all bipartite graphs With k cut-edges,and among all bipartite bicyclic graphs,respectively.Li,Shiu and Chan［8］gaVe the graphs With maximum Laplacian spectral radius among all bipartite graphs with(edge-)connectivity at most k.

Li,Shiu and Chan［9］determined the graphs with maximum Laplacian spectral radii among all trees,and all bipartite unicyclic,bicyclic,tricyclic and quasi-treegraphs,respectively.In this paper we determine the graphs with maximum spectral radius among all trees,and all bipartite unicyclic,bicyclic,tricyclic,tetracyclic,pentacyclic and quasi-tree graphs,respectively.

1 Extremal bipartite graphs

For each v∈V(G),let NG(v)(or N(v)for short)be the set of Vertices which are adjacent to v in G.

Lemma 1.1［10］Let u and v be two vertices o? a connected graph G,and suppose that v1,v2,···,vs∈N(v)N(u).Let G*be the graph obtained ?rom G by deleting the edges vviand adding the edges uvi(1≤i≤s).Let x be the Perron vector o?G corresponding toρ(G).I? x(u)≥x(v),thenρ(G*)＞ρ(G).

Let G be a bipartite graph With bipartition(X,Y).If X=X1∪X2∪···∪Xhand Y=Y1∪Y2∪···∪Yhsatisfy Xiand Yiare not em pty,and the neighborhood of each Vertex in Xiis Y1∪Y2∪···∪Yh+1-i(1≤i≤h),then we call G double nested graph as in［9］.If |Xi|=aiand|Yi|=bi(i=1,2,···,h),then G is denoted by D(a1,a2,···,ah;b1,b2,···,bh).

Theorem 1.2 I?G=(X,Y)attains the maximum spectral radius among all bipartite graphs,then G is a double nested graph with all pendant edges attached at a vertex.

P roo f Let X={v1,v2,···,vj}and Y={vj+1,vj+2,···,vn}such that x(v1)≥x(v2)≥···≥x(vj)＞0 and x(vj+1)≥x(vj+2)≥···≥x(vn)＞0.Then we have the following three claim s.

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Claim 1 A ll cut-edges in G are pendant edges and are attached at a common Vertex.

If it is not so,then,by Lemma 1.1,we can obtain the other bipartite graph H such that ρ(H)＞ρ(G),which contradicts maximality of G.

C laim 2 N(v1)?N(v2)?···?N(vj)and N(vj+1)?N(vj+2)?···?N(vn). MoreoVer,for some l,N(vl)=N(vl+1)if and only if x(vl)=x(vl+1).

The Claim 2 is clearly true from Lemma 1.1.

Claim 3 G is a double nested graph.

Without loss of generality we assume that x(v1)≥x(vj+1).Suppose that a1＞a2＞···＞ahare all distinct real numbers among{x(v1),x(v2),···,x(vj)}.Let Xk={vi∈X|x(vi)=ak} (1≤k≤h).Then X=X1∪X2∪···∪Xh.The Claim 2 shows that Y=Y1∪Y2∪···∪Yh.

Up to now the proof is completed.

2 Extremal bipartite graphs With at most five cycles

If a graph on n Vertices is of n+k-1 size then it is called k cycles graph.In this section we assume that G=(X,Y)is an k cycles bipartite graph on n Vertices With the maximum spectral radius.Next we exactly characterize G when k∈{0,1,2,3,4,5}.

Theorem 2.1［11］W hen k=0,GSn,where Snis a star with order n.

It is clear that n≥k+3 when k≥1.

P roo f From Theorem 1.2 we can see that there is a partition,say X,such that for each v∈X,d(v)≥2,which im p lies that|X|=2.This shows that the result is true.

Lemma 2.3［12］,whereΔ(H)is the maximum degree o?H.

Now we giVe the characterization of G when k=2,3,4,5.

We only Verify Theorem 2.6 and other theorem s can be similarly proVed.

P roof of Theorem 2.6 From Theorem 1.2 we can see that there is a partition,say X,such that for each v∈X,d(v)≥2,which im p lies that|X|=2,3,4 or 5.In this case, we can easily know that G is isomorphic to some one of D(2;5),D(1,2;3,1)and D(2,1;2,2)if n=7,and that G is isomorphic to some one of D(1,1;5,n-7),D(1,2;3,n-6),D(1,1,1;2,2,n-7),D(1,1,2;2,1,n-7)and D(1,4;2,n-7)if n≥8.By calculation we obtain thatThus,when n=7,GD(2;5).Next we assume n≥8.

Let H0=D(1,1;5,n-7),H1=D(1,2;3,n-6),H2=D(1,1,1;2,2,n-7),H3= D(1,1,2;2,1,n-7)and H4=D(1,4;2,n-7)be shown in Fig.1.

Fig.1 H i(i=0,1,2,3,4)

Set x to be the Perron Vector of H0.By symmetry,we can set x(v3)=···=x(v7)=x3and x(v8)=···=x(vn)=x4.Let x(v1)=x1and x(v2)=x2.Then

3 Extremal bipartite quasi-tree graphs

If there exists u0∈V(G)such that G-u0is a tree,then we call G quasi-tree graph as in［9］.LetΨ(n,d0)={G|G is a bipartite graph of order n With G-u0is a tree and d(u0)=d0}. Clearly,Ψ(n,1)is the set of trees of order n,and Theorem 2.1 shows that Snattains the maximum spectral radius am ong all trees.

Theorem 3.1.Let 2≤d0≤n-2 and suppose that H∈Ψ(n,d0)attains the maximum spectral radius.Then HD(1,1;d0,n-d0-2).

P roof Let H=(X,Y).From Theorem 1.2 we can see that there is a partition,say X, such that for each v∈X,d(v)≥2.Thus,|X|=2 if u0∈X,and|X|=d0if u0∈Y.In this case,we can easily know that HD(1,1;d0,n-d0-2)or HD(1,d0-1;2,n-d0-2).

W hen d0=2,HD(1,1;2,n-4),and when d0=n-2,HD(2;n-2).

Next we assume that 3≤d0≤n-3.

Let H*=D(1,1;d0,n-d0-2)and e H=D(1,d0-1;2,n-d0-2)be shown in Fig.2.

Fig.2 H*and

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(責任編輯 李 藝)

具有最大譜半徑的二部圖的刻畫

牛愛紅1,王國平1,秦正新2,牟善志3
(1.新疆師范大學(xué)數(shù)學(xué)科學(xué)學(xué)院,烏魯木齊830054; 2.新疆大學(xué)數(shù)學(xué)與系統(tǒng)科學(xué)學(xué)院,烏魯木齊830046; 3.江蘇理工學(xué)院數(shù)學(xué)系,江蘇常州213001)

一個圖G的鄰接矩陣A(G)是n×n矩陣,如果vi和vj相鄰,那么它的(i,j)位置為1,否則為0.圖G的譜半徑是鄰接矩陣A(G)的最大特征值.本文確定了在所有的樹和所有的二部單圈圖、二部雙圈圖、二部三圈圖、二部四圈圖、二部五圈圖以及二部擬樹圖中所對應(yīng)的具有最大譜半徑的圖.

二部圖;圈;譜半徑

2015-01

國家自然科學(xué)基金(11461071)

牛愛紅,女,碩士研究生,研究方向為圖論.E-mail:949751277@qq.com.

王國平,男,教授,研究方向為圖論.E-mail:x j.wgp@163.com.

O 157.5 Document code:A

10.3969/j.issn.1000-5641.2016.01.012

1000-5641(2016)01-0096-06