Definition:Real Sequence

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Definition

A real sequence is a sequence (usually infinite) whose codomain is the set of real numbers $\R$.


Notation

The notation for a real sequence is conventionally of the form $\sequence {a_n}$, where it is understood that:

the domain of $\sequence {a_n}$ is the natural numbers: either $\set {0, 1, 2, 3, \ldots}$ or $\set {1, 2, 3, \ldots}$
the range of $\sequence {a_n}$ is the set of real numbers $\R$.

However, some sources use the notation of mappings to explicitly interpret a real sequence as a real-valued function:

$f: \N \to \R$

Hence the $n$th term can be seen denoted in two ways:

$\sequence {a_n}$
$\map f n$


Examples

Example: $\sequence {\paren {-1}^n}$

The first few terms of the real sequence:

$S = \sequence {\paren {-1}^n}_{n \mathop \ge 1}$

are:

$-1, +1, -1, +1, \dotsc$

This is an example of the real sequence:

$S = \sequence {x^n}$

where $x = -1$.

$S$ is not monotone, either increasing or decreasing.


Example: $\sequence {\dfrac {\paren {-1}^{n + 1} } n}$

The first few terms of the real sequence:

$S = \sequence {\dfrac {\paren {-1}^{n + 1} } n}$

are:

$1, -\dfrac 1 2, \dfrac 1 3, -\dfrac 1 4, \dotsc$


Example: $\sequence {n^{-1} }$

The first few terms of the real sequence:

$S = \sequence {n^{-1} }_{n \mathop \ge 1}$

are:

$1, \dfrac 1 2, \dfrac 1 3, \dfrac 1 4, \dotsc$


$S$ is strictly decreasing.


Example: $\sequence 1$

The first few terms of the real sequence:

$S = \sequence 1_{n \mathop \ge 1}$

are:

$1, 1, 1, 1, \dotsc$


$S$ is both increasing and decreasing.


Example: $\sequence {2^n}$

The first few terms of the real sequence:

$S = \sequence {2^n}_{n \mathop \ge 1}$

are:

$2, 4, 8, 16, \dotsc$


$S$ is strictly increasing.


Example: $\sequence {x^n}$

The first few terms of the real sequence:

$S = \sequence {x^n}$

are:

$x, x^2, x^3, \ldots$


Example: $\sequence {n^s}$

Let $s$ be a constant.

The first few terms of the real sequence:

$S = \sequence {n^s}$

are:

$1^s, 2^s, 3^s, \ldots$

When $s = 1$, $S$ is the sequence of natural numbers.


Example: $\sequence {\dfrac 1 2 \paren {x_{n - 1} + \dfrac 2 {x_{n - 1} } } }_{n \mathop \ge 2}$

The first few terms of the real sequence:

$S = \sequence {a_n}_{n \mathop \ge 1}$

defined as:

$a_n = \begin {cases} 2 & : n = 1 \\ \dfrac 1 2 \paren {x_{n - 1} + \dfrac 2 {x_{n - 1} } } & : n > 1 \end {cases}$

are:

$2, \dfrac 3 2, \dfrac {17} {12}, \dfrac {577} {408}, \dotsc$


Also see

  • Results about real sequences can be found here.


Sources

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