“…Let λ n (G) be the least eigenvalue of a graph G of order n. It is known that λ n (G) = −ρ(G) for a bipartite graph G (see [4]). Recently, researchers have begun to pay attention to the least eigenvalues of graphs with a given value of some well-known integer graph invariant: for instance: order and size [1,2,5,13], unicyclic graphs with a given number of pendant vertices [7], matching number and independence number [14], number of cut vertices [15], connectivity, chromatic number [16], domination number [17]. This paper also gives a spectral extremal characterization on the least eigenvalue of graphs (see Theorems 1.3 and 1.4).…”