Dr. Mark Humphrys

School of Computing. Dublin City University.

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Chaotic functions

Functions where "nearby" Input does not generate "nearby" Output.

Two things we might want to do with a function:

  1. Represent the whole function over its range.
  2. Just find the global maximum (or minimum) point.
If you know the equation for the function: Interesting case is where no equation known.

We use a function with a known equation as an example to illustrate what the unknown function might look like.



sin(1/x)

Example where "nearby" Input does not generate "nearby" Output.

If you are told the equation for this function is sin(1/x):

If you do not have its equation, but are learning it through exemplars:

  1. Representing the whole function over its range through exemplars is very hard / impossible.

  2. Note however that just finding the global maximum (or minimum) point (or at least a joint optimum point) is not actually hard here.
    Though with chaotic functions in general, finding the global optimum may be very hard too.

Introduction

Consider the function f(x) = sin(1/x).

First consider sin(x):

sin ( 0 ) = 0
sin ( 0.5 π ) = 1 (radians)
sin ( π ) = 0
sin ( 1.5 π ) = -1

sin ( 2 π ) = 0
sin ( 2.5 π ) = 1
sin ( 3 π ) = 0
sin ( 3.5 π ) = -1
...
sin ( 10001.5 π ) = -1
sin ( 10002.5 π ) = 1
sin ( 10003.5 π ) = -1
sin ( 10004.5 π ) = 1
...

( period = 2 π )

Hence:

f ( 1 / ( 10001.5 π )) = -1
f ( 1 / ( 10002.5 π )) = 1
f ( 1 / ( 10003.5 π )) = -1
f ( 1 / ( 10004.5 π )) = 1
...
f ( 1 / ( 100000000001.5 π )) = -1
f ( 1 / ( 100000000002.5 π )) = 1
f ( 1 / ( 100000000003.5 π )) = -1
f ( 1 / ( 100000000004.5 π )) = 1
...
f ( 1 / ( 100000000000000000000001.5 π )) = -1
f ( 1 / ( 100000000000000000000002.5 π )) = 1
f ( 1 / ( 100000000000000000000003.5 π )) = -1
f ( 1 / ( 100000000000000000000004.5 π )) = 1
...
For arbitrarily high integers n:
f ( 1 / ( (2n) π )) = 0
f ( 1 / ( (2n+0.5) π )) = 1
f ( 1 / ( (2n+1) π )) = 0
f ( 1 / ( (2n+1.5) π )) = -1
As x approaches 0, f(x) bounces back and forth between 1 and -1 more and more rapidly, such that a smaller and smaller change on the x-axis can lead to the maximum change on the y-axis.

Whereas far away from 0 nearby x clearly produce nearby y.

In fact, gnuplot, because it links samples taken at fixed discrete intervals, cannot draw this graph at all properly. Question - Why is the above a very inaccurate drawing of this function?


A more hi-resolution plot (see "set samples" in gnuplot):



An even more hi-resolution plot ("set samples 10000"):



Close-up near zero:



Close-up again:



Close-up again:






Learning sin(1/x) from exemplars (input-output pairs)

We consider constructing a machine to learn f(x) from being presented with various examples of x and f(x).

We do not see an equation for the function. We do not see a graph like above. All we see are exemplars of x and f(x). The machine discovers that f has this shape through learning.

It is told (or discovers) that f(4.3) = 0.2305,
and f(4.1) = 0.2415.
So it guesses that f(4.2) = 0.2360.
Almost spot on - it's actually 0.2358.

But then it finds out that f(0.3) = - 0.19,
and f(0.1) = - 0.54.
So it guesses that f(0.2) = - 0.365.
However, this is totally wrong. In fact, f(0.2) = - 0.95.

And what is f(0.15)? The machine might now guess f(0.15) = - 0.745.
But it is wrong again. In fact, f(0.15) is not even negative. It is 0.37.


Conclusion: In approximating f, we might dedicate 90 percent of our resources (the memory structure that defines f) to approximating f for x < 1. And for x > 1, we can effectively approximate f by a straight line.

But if so, this variable-resolution approximation is something we will have learnt. We did not know a priori that f was more complex in the area < 1.

In summary:




Chaos Theory

Where a tiny error or uncertainty in input measurements can cause a massive error in output (here the largest possible error), this is a chaotic function.

If we use a finite size data structure to approximate the curve, then no matter what data structure we use, there will exist a constant   c > 0   such that, for all   x < c   our prediction is absolutely random/useless. We can predict nothing more detailed than that the y-value is in the range -1 to 1 (which we knew anyway, i.e. no predictive value at all).

For a chaotic function you can play with online, see:




Chaos Theory demo




This shows how, unless you have perfect knowledge of current conditions, your predictive value for conditions in the future is not just poor, but zero.



Two chaotic regions

sin ( (1/x) (1/(1-x)) )



sin ( (1/(x/100)) (1/(1-x)) )





One chaotic region, but not so simple period

(2*sin(3/x))+(3*cos(5/x))+(4*sin(6/x))+(1*cos(3/x))





Two chaotic regions, no simple period

sin(3/x)*sin(5/(1-x))



Maximise this function: Find x that gives maximum point on y axis.



Two chaotic regions, no simple period

sin(1/x) + ( 2 * sin(1/(1-x)) )



Maximise this function: Find x that gives maximum point on y axis.



Example of chaotic function over the whole range:

In the Chaos Theory demo, let λ = 4 and let f(x) = the result of applying the update rule to x 100 times.
Then representing this function over the whole range is very hard.
Nearby x does not produce (even remotely) nearby f(x), for the entire range of x.
There are no locally non-chaotic regions at all.
The function looks like:


gnuplot can't really draw it. To draw it is to more or less just fill the square (x=0 to x=1, y=0 to y=1) with points.



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