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Sequences and limits
 Infinite sequences
  An infinite sequence is an ordered list of real numbers indexed by the natural numbers n Î N
                and denoted  {an},          a1,  a2,  a3, . . . ,  an, . . .
where, by an given is a rule to calculate the nth term of the sequence.
 Graphing the terms of a sequence on the number line
Thus, for example
(1)   an = n    for  n = 1, 2, 3,. . .  gives the sequence,   1,  2,  3,  4,  5,. . .     shown on the number line
(2) for  n = 1, 2, 3,. . .  gives the sequence, shown below
(3) for  n = 1, 2, 3,. . .  gives the sequence, shown below
(4) for  n = 1, 2, 3,. . .  gives the sequence, shown below
(5) for  n = 1, 2, 3,. . .  gives shown below

Observe that, the first and the third sequence shown above both increase, the second and the fourth decrease, and that the terms of the fifth sequence oscillate (alternate) from the left to the right approaching closer and closer to 1.
 ::  The limit of a sequence
  The definition of the limit of a sequence

A number L is called the limit of a sequence {an} if for every positive number e there exist a natural number  n0  such that if  n > n0,  then   | an - L | < e.
That is, a number L is the limit of a sequence if the distance between the term an and L  becomes arbitrary small by choosing n large enough (see the examples above).
The n0 denotes the value of the index n starting from which the distance  an - L  becomes smaller than the given e. Since the value of n0 depends on the size of e it is usually written as n0 (e).
 Convergence of a sequence
Therefore, if a sequence {an} has a limit L we write
        or   an ® L    as   n ® oo
and we say that that a sequence an has the limit L as n tends to infinity.
A sequence {an} is convergent if it has a limit. Otherwise, we say the sequence is divergent.
Thus, the sequences, (3), (4) and (5), from the example above, all converge or tend to the limit 1.
Example:  Let examine the limit of the sequence (3) given by
Solution:  Prove that
The sequence tends to the limit 1 as the distance
can become arbitrary small by choosing the natural number n sufficiently large.
For example, if we choose  n = 100  the distance between an and 1 is 1/n = 0.01 that is,
for all  n > 100  the distance  | an - 1 | < 0.01. 
Therefore,  n0(e) = 101 meaning, starting from the 101st term further, the distance of the remaining terms of the sequence and 1, is always less than 0.01.
Example:  Find the limit of the sequence (2) given by
Solution:  Let prove that
Using the definition of limit we must find a natural number n0(e) such that  | an - 0 | < e  for all  n > n0.
Therefore, if   n > n0   then   |1/n - 0 | = |1/n | = 1/n < 1/n0 < e.
Suppose we wish to make the difference (or the distance) between the anth term and the limit L to be less than e = 0.001 = 1/1000. 
Then, as 1/n < e or  n > 1/e  it follows  n > 1000  that is, starting from  n0 = 1001 the distance  an - L becomes smaller than the given e.
Observe that the absolute value of terms of the sequences that converge to zero become arbitrary small as n  tends to infinity that is,       | an | < e  for all  n > n0 (e).
For example such sequences are, therefore
The same way we can prove that the sequence (5) above, given by converges to 1.
As can be seen on the number line above, terms of the sequence alternate from left to right approaching closer and closer to 1 as n tends to infinity.
That is, the sequence alone alternately converges from left to right approaching closer 
and closer to 0 as n tends to infinity.
 
 
 
 
 
 
 
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