# Unpredictability, per definitions of chaotic behavior

Apparently I've been confused about the meaning(s) of "chaotic behavior". I always thought it meant that infinitesimal perturbations of a system parameter would lead to large changes in the system's behavior, and thus that the behavior of the system is effectively unpredictable even though it might be deterministic.

More recently, though, I get the impression that sometimes "chaotic behavior" has a second definition in which it simply means "aperiodic behavior". This from the paper, Complexity in Linear Systems .... Perhaps there are additional definitions of "chaotic behavior". But: would deterministic aperiodic behavior be effectively unpredictable in the same sense as the unpredictability per the first definition?

• There are many confusing aspects to this question: 1) Sensitivity to small perturbations is about perturbations of the initial condition, not of the parameters. 2) Chaos is not only about sensitivity to small perturbations. Otherwise $\dot{y} = y$ would be chaotic. 3) What exactly do you mean by aperiodic? Does it include quasi-periodic, exploding, or fixed-point dynamics? That paper you cite does not contain this word. – Wrzlprmft Oct 28 '18 at 7:22
• Is there an important difference between perturbations of initial conditions, vs perturbations of system variables at an arbitrary time? I've always thought of "initial conditions" as being all the system conditions at the starting point of an experiment or a calculation. By aperiodic, I mean the Miriam-Webster definition: 1) of irregular occurrence : not periodic; 2) not having periodic vibrations : not oscillatory. That is, aperiodic behavior cannot be described precisely as O(t) = O(t+n delta t). – S. McGrew Oct 28 '18 at 12:36
• So, a system with two periodic components A and B whose periods are ka and kb, where ka and kb are in the ratio ka/kb = R where R is irrational would, I think, be aperiodic because there would be no delta t that makes O(t) = O(t+n delta t). (O(t) is the system state at time t). – S. McGrew Oct 28 '18 at 12:45
• @S.McGrew, Your example sounds like it could indeed be aperiodic, but then it's not chaotic, but rather quasiperiodic. – stafusa Oct 29 '18 at 2:01
• @S.McGrew, The distinction between parameters and state variables is usually very important: the state variables usually evolve as a function of, say, time and the state variables' previous values; while the parameters are almost always constant and, when not, they vary independently of the state variables. Also, in a sense, when you change a parameter, you're changing they system under study. – stafusa Oct 29 '18 at 2:04