Definition:Real Sequence

Definition

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

Notation

The notation for a 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 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 :

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

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 :

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

are:

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

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

The first few terms of the :

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

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 :

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

are:

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

$S$ is both increasing and decreasing.

Example: $\sequence {2^n}$

The first few terms of the :

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

are:

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

$S$ is strictly increasing.

Example: $\sequence {x^n}$

The first few terms of the :

$S = \sequence {x^n}$

are:

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

Example: $\sequence {n^s}$

Let $s$ be a constant.

The first few terms of the :

$S = \sequence {n^s}$

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 :

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

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

• Definition:Integer Sequence
• Definition:Rational Sequence
• Definition:Complex Sequence

• Results about real sequences can be found here.

Sources

• 1958: J.A. Green: Sequences and Series ... (next): Chapter $1$: Sequences: $1$. Infinite Sequences
• 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $2$: Metric Spaces: $\S 5$: Limits: Definition $5.1$
• 1975: W.A. Sutherland: Introduction to Metric and Topological Spaces ... (previous) ... (next): $1$: Review of some real analysis: $\S 1.2$: Real Sequences
• 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 4$: Convergent Sequences: $\S 4.2$: Sequences

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