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A sequence is an infinite list of numbers, like
The numbers in the sequence are called its terms.
The general form of a sequence is
where is the -th term of the sequence.
In the example (1) above, , , , and so on.
The notations or
are abbreviations for
Occasionally the indexing of the terms will start with something other
than 1. For example,
(In this case would be the -st term.)
For some sequences, it is possible to give an explicit formula
for : this means that is expressed as a function of .
For instance, the sequence (1) above can be described by
the explicit formula