Old and new results on algebraic connectivity of graphs

This paper is a survey of the second smallest eigenvalue of the Laplacian of a graph G, best-known as the algebraic connectivity of G, denoted a (G). Emphasis is given on classifications of bounds to algebraic connectivity as a function of other graph invariants, as well as the applications of Fiedler vectors (eigenvectors realated to a(G)) on trees, oil hard problems in graphs and also oil the combinatorial optimization problems. Besides, limit points to a(G) and characterizations of extremal graphs to a(G) are described, especially those for which the algebraic connectivity is equal to the vertex connectivity. (C) 2006 Elsevier Inc. All rights reserved.