with parameter "a" > 0. If the system is perturbed to the value formula_2 its subsequent sequence of values is formula_3 If "a" < 1, these numbers get closer and closer to 0 regardless of the starting value formula_2 while if "a" > 1 the numbers get larger and larger without bound. But if "a" = 1, the numbers do neither of these: instead, all future values of "x" equal the value formula_5 Thus the case "a" = 1 exhibits marginal stability.