Abstract

Let A be the adjacency matrix of a connected graph G . If z is a column vector, we say that a vertex of G is positive, nonnegative, null, etc. if the corresponding entry of z has that property. For z such that Az⩾ az, we bound the number of components in the subgraph included by positive vertices. For eigenvectors z having a null element, we bound the number of components in the graph included by nonnull vertices. Finally, bounds are established for the number of null elements in an eigenvector, for the multiplicity of an eigenvalue and for the magnitudes of the second and last eigenvalues of a general or a bipartite graph.

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