algebraic connectivity of a graph

Let $L(G)$ be the Laplacian matrix (http://planetmath.org/LaplacianMatrixOfAGraph)of a finite connected graph $G$ with $n$ vertices. Let the eigenvaluesof $L(G)$ be denoted by${\lambda }_1\le {\lambda }_2\le \mathrm{\cdots }\le {\lambda }_n$, whichis the usual notation in spectral graph theory.The connectivity of $G$ is ${\lambda }_2$.The usual notation for the algebraic connectivity is $a(G)$.The parameter is a measure of how well the graph is connected.For example, $a(G)\ne 0$ if and only if $G$ is connected.