A sequence is a function whose domain is the positive integers. The value
of the function at a given integer is a term of the sequence. The range of
a sequence is the set of its terms. Sequences are not typically written as
ordered pairs, or drawn as graphs; a sequence is most often represented by a
list of its terms starting with the first term, followed by the second, and so
on. A general sequence looks something like this:
*a*
_{1}, *a*
_{2}, *a*
_{3},…, *a*
_{n}
, where
*n*
is an integer, and
*a*
_{n}
is the
*n*
th term of the sequence. A
sequence can also be written as a formula for which any integer can be the
input, and the corresponding term is the output.

Here is an example sequence:
*a*
_{n} = 2*n* = 2, 4, 6, 8, 10,…
, where
*n*
is
an integer. The formula
*a*
_{n} = 2*n*
is called an explicit formula for the
sequence. By plugging any integer
*n*
into the explicit formula for the
sequence, the
*n*
th term can be found.

Most sequences are defined for all positive integers. They are called
infinite sequences. In some cases, though, a sequence may only be defined
for the positive integers up to
*n*
, a given integer. This might be the case in
a sequence that has a real-life application for which integer values above a
certain number have no meaning, or are impossible. A sequence defined only for
positive integers up to a certain integer are called finite sequences.

When a sequence is given by an explicit formula, it is easy to find the
*n*
th
term of the sequence. The desired
*n*
need only be plugged into the equation to
find the
*n*
th term. Sometimes, however, a sequence is given by listing the
first four or five terms, and then an
*n*
th term must be found. In such a case
the only way to find the
*n*
th term is to study the first few terms and try to
find a pattern, and express it in a formula. This is a form of mathematical
induction, which is essentially the process of recognizing patterns. Keep in
mind that more than one sequence may share the first few terms, so there is not
one right answer for a problem like this. Any explicit formula for which the
first given terms are correct is an acceptable formula. In fact, in some cases,
the same sequence may be given by more than one explicit formula. Mathematical
induction requires creativity.

It is often useful to be able to calculate the sum of the terms in a sequence.
The sum of the terms in a sequence is called a series. An infinite
series is the sum of the terms in an infinite sequence. A finite series
is the sum of the terms in a finite sequence. To express series conveniently,
we use summation notation. It looks like this:
*a*
_{k} = *a*
_{1} + *a*
_{2} + *a*
_{3} + ... + *a*
_{n}
. The index of summation is
*k*
. This equation might
be read "The sum of the terms of
*a*
_{k}
as
*k*
goes from
1
to
*n*
".
*a*
_{n}
is a
sequence, and the summation of the sequence is a series.

The following facts are true of summation notation, where
*a*
_{i}
and
*b*
_{i}
are
sequences and
*c*
is a constant: