“…Let the vector functions ( , , ), ( , , ), ( , , , ) and ( , , , ) defined on the domain ( , , ) ∈ 1 × 1 × = (−∞, ∞) × (−∞, ∞) × ( , , ) ∈ 1 × 1 × 1 = (−∞, ∞) × (−∞, ∞) × 1} (1) and periodic in of period . Assume that the vector functions ( , , ), ( , , ), ( , , , ) and ( , , , ) are satisfying the following inequalities: ‖ ( , , )‖ ≤ 1 ,‖ ( , , )‖ ≤ 2…”