Sequences and limits
Infinite sequences
Sequences notation - the rule for the n-th term of a sequence
The limit of a sequence
Infinite Sequences
An infinite sequence is an ordered list of real numbers indexed by the natural numbers n Î N denoted  {an}
a1a2a3, . . . ,  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 on the number line
 (3) for   n = 1, 2, 3,. . .  gives the sequence,
shown on the number line
 (4) for   n = 1, 2, 3,. . .   gives the sequence,
shown on the number line
 (5) for   n = 1, 2, 3, . . .    gives the sequence,
shown on the number line
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.
Calculus contents B