We give sharp conditions for global attractivity of the zero solution of the nonlinear difference equation which arises in many contexts in mathematical biology xn+1 = qxn + fn (xn, ..., x n−k ), n ∈ Z, where q ∈ (0, 1] and functions fn satisfy the generalized Yorke conditions.We study the following difference equationIt is assumed that q ∈ (0, 1] and that f n : R k+1 → R, n ∈ Z, meet the generalized Yorke condition (H):The monotone continuous functional M :Eq.(1) has the unique steady state solution x = 0. The property (H2) is satisfied by f n possessing the negative Schwarzian. If (H2) holds with b = 0, then (H1) is satisfied automatically with ϑ(z) = −aM (−z).The generalized Yorke condition was introduced in [1]. It determines a wide family of scalar functional differential equations (SFDE's) which includes various important models studied in mathematical biology (e.g. let us mention the Wright, Nicholson, and Mackey-Glass equations). In particular, this family includes (1) which can be considered as a SFDE with piecewise-constant argument. In [1] we can find a simple criterion for the global stability of the unique fixed point of SFDEs considered under the generalized Yorke condition. For the particular case of Eq. (1), it reads as: Theorem 1. Suppose that q ∈ (0, 1) and f n satisfy (H). If b = 0 and the conditionholds, then lim n→∞ x n = 0 for every solution {x n } of Eq. (1). However, Theorem 1 is not the best possible global stability result for the family of difference equations satisfying (H) (we will say that Theorem 1 is not sharp, see also Theorem 2). This makes a notable difference with the main statement of [1]. Theorem 2. Assume that b = 0 and that f n , n ∈ Z, satisfy (H). Then there exists q k ∈ (0, 1) such that for q ∈ (0, q k ] the inequality
assures the global attractivity of Eq. (1). Condition (4) is sharp within the class of Eqs. (1) determined by (H)and the assumptions b = 0 and q ∈ (0, q k ] : for every triple k ∈ Z + , a < 0, q ∈ (0, q k ], which does not meet (4) there exists a periodic sequence {f n } satisfying (H) and such that the zero solution of the corresponding equation is linearly unstable. Our paper [3] contains formulae from which q k can be found explicitly, let us list some values of q k : q 1 = 0.887; q 2 = 0.796; q 3 = 0.788; q 4 = 0.795; q 5 = 0.805; q 6 = 0.815; q 7 = 0.825; q 8 = 0.834; q 9 = 0.842; q 10 = 0.849; q 100 = 0.965; q 1000 = 0.994. Upper restrictions like q ≤ q k on q are intrinsic to (1) and cannot be omitted for k > 1 since (4) does not guarantee even the local stability for q close to 1. However, as we show below, the values of q k are not optimal.Indeed, it is natural to conjecture (see