# Definition:Continued Fraction

Not to be confused with Definition:Generalized Continued Fraction.

## Definition

Let $F$ be a field, such as the field of real numbers $\R$.

### Finite Continued Fraction

Let $n \geq 0$ be a natural number.

Informally, a finite continued fraction of length $n$ in $F$ is an expression of the form:

$a_0 + \cfrac 1 {a_1 + \cfrac 1 {a_2 + \cfrac 1 {\ddots \cfrac {} {a_{n-1} + \cfrac 1 {a_n}} }}}$

where $a_0, a_1, a_2, \ldots, a_n \in F$.

Formally, a finite continued fraction of length $n$ in $F$ is a finite sequence, called sequence of partial quotients, whose domain is the integer interval $\left[0 \,.\,.\, n\right]$.

A finite continued fraction should not be confused with its value, when it exists.

### Infinite Continued Fraction

Informally, an infinite continued fraction in $F$ is an expression of the form:

$a_0 + \cfrac 1 {a_1 + \cfrac 1 {a_2 + \cfrac 1 {\ddots \cfrac {} {a_{n-1} + \cfrac 1 {a_n + \cfrac 1 {\ddots}}} }}}$

where $a_0, a_1, a_2, \ldots, a_n, \ldots \in F$.

Formally, an infinite continued fraction in $F$ is a sequence, called sequence of partial quotients, whose domain is $\N_{\geq 0}$.

An infinite continued fraction should not be confused with its value, when it exists.

## Simple Continued Fraction

Let $\R$ be the field of real numbers.

### Simple Finite Continued Fraction

Let $n\geq 0$ be a natural number.

A simple finite continued fraction of length $n$ is a finite continued fraction in $\R$ of length $n$ whose partial quotients are integers that are strictly positive, except perhaps the first.

That is, it is a finite sequence $a : \left[0 \,.\,.\, n\right] \to \Z$ with $a_n > 0$ for $n >0$.

### Simple Infinite Continued Fraction

A simple infinite continued fraction is a infinite continued fraction in $\R$ whose partial quotients are integers that are strictly positive, except perhaps the first.

That is, it is a sequence $a : \N_{\geq 0} \to \Z$ with $a_n > 0$ for $n >0$.

## Continued fraction expansion of a real number

### Irrational Number

Let $x$ be an irrational number.

The continued fraction expansion of $x$ is the simple continued fraction $\paren {\floor {\alpha_n} }_{n \ge 0}$ where $\alpha_n$ is recursively defined as:

$\alpha_n = \displaystyle \begin{cases} x & : n = 0 \\ \dfrac 1 {\fractpart {\alpha_{n - 1} } } & : n \ge 1 \end{cases}$

where:

$\floor {\, \cdot \,}$ is the floor function
$\fractpart {\, \cdot \,}$ is the fractional part function.

### Rational Number

Let $x$ be a rational number.

The continued fraction expansion of $x$ is found using the Euclidean Algorithm.

## Notation

A continued fraction can be denoted using ellipsis:

$a_0 + \cfrac 1 {a_1 + \cfrac 1 {a_2 + \cfrac 1 {\ddots \cfrac {} {}}}}$

which suggests the definition of its value, but it should be noted that this is only a notation.

By definition, a continued fraction is its sequence of partial quotients and can thus be denoted:

$(a_n)_{n\geq 0}$
$[a_0; a_1, a_2, \ldots ]$
$[a_0, a_1, a_2, \ldots ]$

where the last two notations are usually reserved for its value.

## Also defined as

When one is primarily concerned with simple continued fractions of real numbers, it is common to require a continued fraction to have strictly positive partial quotients, except perhaps the first.

## Also known as

A continued fraction is also known as a:

continued fraction in canonical form
regular continued fraction
simple continued fraction

as opposed to a generalized continued fraction.

## Historical Note

The concept of the continued fraction has been around a long time, since Euclid at least, and a great deal of research has been done and terminology developed.

Much of its modern theory was established by Leonhard Paul Euler.