Limit of a SequenceLimit of a Sequence
The notion of limit of a sequence is very natural. Indeed,
consider our scientist who is collecting data everyday. Set
to be the sequence generated by our scientist
( 
is the data collected after n days). Imagine that after a
certain day the numbers are very close to each other. Therefore our
scientist will decide that the experiment settled down to a
equilibrium state, meaning that no change occured to the data. The danger here is that,
though the data collected after that date are closer to each other,
you should not, in general, believe that the system settles
down. Small changes may be responsible for weird behavior.
This is the beginning of the Chaos Theory. But, this is not the subject
treated here. We will focus more on the nice experiment where the
system settles down to an equilibrium state. To better illustrate this phenomena, let us consider the following
example.
Example: Take a calculator, set it to "radian mode" and enter the number 1. Then, hit the function Cosine over and over again. Analyze the output of this experiment.
Answer: Then, we have
.
Next, we have
.
If we proceed with this we get
Clearly, the numbers are getting closer to something that starts as
0.73.
To better appreciate the sequence, we graph the points 
on
a plane (see the Figure below).
 |
Example: Do the same as the example above with the Sine function.
Answer: We have
.
Next, we have
.
If we proceed with this we get
Clearly, the numbers are getting smaller and smaller (see Figure below). In fact, the
numbers do get closer to 0 as close as one wishes!!!
 |
Remark: It is amazing to see how slow this sequence gets to 0. There are mathematical reasons behind this which we will not discuss here. But, keep in mind that many people are interested in them (that is, speed of convergence).
After discussing the above two examples one will wonder if any sequence has the same faith (meaning, it gets closer to a number). Unfortunately, the
answer is NO. Let us consider a slightly more complicated example.
Example: As before, take your calculator and enter the number 0.3. Second, program your machine to compute y = f(x)= 4(1-x)x. Then, keep on doing the same as you did in the previous two examples. Finally, analyze the output.
Answer: In this case we have 
. Then
.
If we keep on iterating, we get
| n | xn |
| 1 | 0.3 |
| 2 | 0.84 |
| 3 | 0.5376 |
| 4 | 0.994345 |
| 5 | 0.0224922 |
| 6 | 0.0879454 |
| 7 | 0.320844 |
| 8 | 0.871612 |
| 9 | 0.447617 |
| 10 | 0.989024 |
 |
By the way, this
sequence is used as a discrete mathematical model for
population
dynamics (called the discrete logistic model).
Let us summarize what we just noticed on these examples.
Consider a sequence of numbers . Sometimes the
numbers get closer and closer to a number L (we will write 
). And sometimes the
numbers do not exhibit such behavior. If it does, we say that the
sequence is convergent and has a limit
equal to L. We will write
,
or
.
It may happen that we will say n gets larger to express that 
. If a sequence is not convergent, it is called divergent.
Let us discuss the above definition. A sequence is convergent if there exists a number L such that the numbers
get closer and closer to L as n gets larger.
We have to make sure that the claim is justified. That is, gets
really close to L. We do not want 
to get close to L and then
when you go to 
it is not!!! This
will be terrible in some serious calculations. So, when we say that gets close to L, as
n gets large, we mean that regardless of how close you want to
be to L, if you go far enough you will get there....
Meaning, if I
want my to be close to L up to 100 Digits, then I am sure if n
is big enough, I will get close to L up to 100 Digits (this will
happen if 
). In other words, let us set
to be a very small number (which measures the error like
), then
there exists 
such that for
every 
, we have
The integer N tells you how far you have to go to get closer to L up
to 
. Meaning that, N is somehow responsible for the speed
of the sequence; how fast it goes to its limit...
Definition: The sequence converges to the
number L, if and only if,
for every , there exists
such that
for
every
Some authors will use 
, instead. No
harm is done, do not worry about it.
Example: Show that
.
Answer: Let . We know that there exists an
integer such that
.
Let . Then, we have
Remark: Keep in mind that measures the error between the numbers and the limit L, while the integer N measures how fast the sequence gets closer to the limit L.
For more on Limit of Sequences, click HERE.
S.O.S MATHematics home page 