Ordering Unicyclic Graphs in Terms of Their Smaller Least Eigenvalues

Let G be a simple graph with n vertices, and let A be the 0, 1 -adjacency matrix of G. We call det λI −A the characteristic polynomial of G, denoted by P G; λ , or abbreviated P G . Since A is symmetric, its eigenvalues λ1 G , λ2 G , . . . , λn G are real, and we assume that λ1 G ≥ λ2 G ≥ · · · ≥ λn G . We call λn G the least eigenvalue of G. Up to now, some good results on the least eigenvalues of simple graphs have been obtained.