... ρ(K2) < 1.Then(1.3) has at least one pe riodic solution.Proof. Note that Ω0× Ω0is a normal solid cone of X × X.LetA1, A2,andA be the sameoperators in the proof of Theorem 2.1.Setgn=n+ω−1s=nG(n,s)hsbs, ... that (A2(u,v))ˇn σ(A2(u,v))n for any n,ˇn ∈ Z.Thus,A : Ω × Ω →Ω × Ω. Furthermore, in view of the boundedness of G and G, and the continuity of f1 and f2, it is not difficult to show ... represent the size of a populationin the time per iod n. Since it is possible that the population may be influenced by an-other factor of the form−hnf2(n,xn−τ(n)), we are therefore interested...