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JARGON.TXT
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1993-12-29
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A Short Introduction to the Jargon of Iteration Theory
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Iteration theory (just as every other complicated field) has developed its own
jargon. This list includes some of the more common terms. It may help you
understand some of the other documentation better, and it may help you
understand iteration better as well.
And if all else fails, you can use these spiffy mathematical terms to impress
your friends with your vast stores of chaotic knowledge.
Dynamical System
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A dynamical system is simply a function together with the domain the function
is defined on. The domain can be anything--a line, a line segment, the plane,
3-space, 6 dimensional space, or any of the other weird "spaces" mathematicans
are always coming up with. (In Iterate!, the domain of the function is always
the plane.)
The only restriction is that the domain and the range of the function must be
the same. Symbolically, we would write:
f: D D
This means that 'f' is a function with domain and range D. This requirement
makes sense if you think about it. When you iterate a function, you keep
feeding points from the range back into the domain. So if the range and the
domain aren't the same, you're going to be in trouble.
The reason this is called a "dynamical system" is that "dynamics" means
"movement". What we are studying when we look at a dynamical system is how
the points move around under the influence of the function.
Iteration
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What we do when study a Dynamical System is "iterate" the points. This means
you start with a point x. Then figure out f(x). Then f(f(x)), f(f(f(x))),
f(f(f(f(x)))) and so on.
Writing all this f(f(f(f(f(x))))) stuff gets pretty tiresome, so
mathematicians abbreviate by writing fⁿ(x). This means that you apply
function 'f' to point 'x' 'n' times. So f²(x)=f(f(x)) and so on.
(It would be easy to get confused and think that f²(x) means "f(x) squared".
To distinguish between the two, mathematicians write (f(x))² if they mean
"f(x) squared." It would also be easy to get confused and think that f²(x)
means "the 2nd derivative of the function f." But if you're smart enough to
take the second derivative of the function f, then you should be smart enough
to tell the difference between f²(x) meaning "the second iteration of f
applied to x" and f²(x) meaning "the second derivative of the function f.")
Orbits
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What you are interested in looking at in a dynamical system is the path the
points take when they are iterated. This path is called the "orbit".
Another way of saying the same thing: The orbit of point x consists of these
points
x, f(x), f²(x)), . . . , fⁿ(x), . . .
The orbit of a point is what you see in Iterate! when you press <Space>.
Fixed points
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Fixed points are points that don't go anywhere when they're iterated, that is,
x=f(x)=f²(x) etc.
Another way of saying the same thing: The orbit of a fixed point consists only
of the point itself.
Periodic Points
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Periodic points are points that come back to the original point after a
certain number of iterations. For instance, a period 2 point comes back to
the original point after two iterations:
x (starting point)
f(x) (a different point)
f²(x)=x (back to the starting point)
Periodic points of every different period are possible.
Once a periodic point returns to the starting point, it just repeats the same
points again until it reaches the starting point again.
For instance, here is a possible orbit for a period 5 point:
0, ½, 1, 1½, 2, 0, ½, 1, 1½, 2, 0, ½, 1, 1½, 2, 0, ½, 1, 1½, 2, . . .
As you can see, it just keeps repeating the same 5 points over and over.
So the orbit of a period 'n' point consists of just 'n' points.
Attracting Orbits
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Attracting orbits suck nearby orbits closer and closer to them. For instance,
an attracting fixed point sucks all nearby points into itself. A period 3
attracting point sucks all points near its orbit closer and closer to the
orbit (the orbit consists of three points, of course).
Repelling Orbits
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A repelling orbit drives nearby orbits away from it.
Other Types of Orbits
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Many other types of orbits are possible. For instance, there are fixed points
that are attracting in one direction and repelling in the other.
By using techniques from elementary calculus, it is relatively easy to tell
which orbits will be attracting, repelling, or something else. Check the
literature for more details on this.
Using Iterate!, you can easily find examples of all of these different types
of orbits (fixed points, periodic points, repelling orbits, attracting
orbits, etc.). You may have to try several different functions with
different parameters, and try iterating several different points in different
areas of the plane for each of them, but eventually you will see all these
different types of orbits.
Strange Attractors
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A strange attractor is similar to an attracting orbit. The difference is that
in an attracting orbit, everything is attracted into an orbit which consists
of a finite number of points. We would say the it is a "finite attractor". A
strange attractor, however, is an "infinite attractor". That is, there is an
infinite set of points that everything else is attracted to.
Where the attracting orbit consisted of only a few attracting points, you can
think of a strange attractor as being a whole shape that is attracting.
Usually this shape is a very, very weird shape; that is why it is called a
strange attractor.
As a rule, the strange attractor is a fractal, with fractal dimension less
than dimension of the dynamical system. For instance, in Iterate!, we are
iterating functions on the plane, which has dimension 2. So any strange
attractors we find in Iterate! will have dimension less than 2--say 1.7, 1.2,
or 0.5.
Usually, the dynamical system is chaotic on the strange attractor. It isn't
chaotic on the rest of the dynamical system, though, since the rest of the
system is just sucked up into the strange attractor. (See below for the
definition of chaos.)
To see a good example of a strange attractor, select the Horseshoe Map
(Function L) with default window and parameters. The "Horseshoe" shape that
you see when you iterate a point (which actually consists of horseshoes
within horseshoes within horseshoes) is a strange attractor. You will notice
that all points are drawn into this horseshoe shape--it is an attractor. You
will notice that once a point gets close to the horseshoe shape, it seems to
just jump around randomly on it--it moves chaotically on the strange
attractor. The horseshoe shape appears to have a fractal dimension between 1
and 2--probably about 1.4 or 1.5.
Another example of a strange attractor is Function F (the inverse Julia Set
function). Again, the strange attractor is a fractal with fractal dimension
between 1 and 2.
Although strange attractors _are_ strange (hence the name), a dynamical system
with a strange attractor is often easier to understand and analyze than one
without a strange attractor.
Forward and Reverse Orbits
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To make the reverse orbit of a point, think of running the function backwards.
In other words, instead of applying the function to the point repeatedly, you
apply the inverse function of to the point repeatedly. All the points you get
by doing this are the "reverse orbit".
Another way of saying the same thing: The reverse orbit of a point 'x' is all
the points that are mapped to 'x' under iteration. In other words, if
fⁿ(y)=x, then y is in the reverse orbit of x.
If mathematicians are talking about "reverse orbits", they will often refer to
the normal