On the structure of graph edge designs that optimize the algebraic connectivity

We take a structural approach to the problem of designing the edge weights in an undirected graph subject to an upper bound on their total, so as to maximize the algebraic connectivity. Specifically, we first characterize the eigenvector(s) associated with the algebraic connectivity at the optimum, using optimization machinery together with eigenvalue sensitivity notions. Using these characterizations, we fully address optimal design in tree graphs that is quadratic in the number of vertices, and also obtain a suite of results concerning the topological and eigen-structure of optimal designs for bipartite and general graphs.