Eigenvalue -1 and triangle-free graphs

Document Type : Original Paper


Department of Mathematics, K. N. Toosi University of Technology, P. O. Box 16765-3381, Tehran, Iran


Determining the maximum order of graphs whose adjacency matrices have an eigenvalue $mu$ with multiplicity $k$, is a problem which has been studied by several authors. The situation of the problem is quite different for the eigenvalues $-1,0$. In this paper, we investigate this problem for triangle-free graphs and for the eigenvalue $mu=-1$. As the main result of the paper, we prove that the order of graphs with maximum degree $d$ and the eigenvalue $-1$ with multiplicity $k>1$ is at most $k+d+1$. We also characterize the graphs attainting the lower bound.


Main Subjects

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