Continued fractions are a way of representing a number as a sum of an integer and a fraction.

Mathematically, a continued fraction is a representation

$a_{0} + \frac{b_{0}}{
a_{1} + \frac{b_{1}}{
a_{2} + \frac{b_{2}}{
\ddots
}}}$

â€‹$a_{i}, b_{i}$are complex numbers. The continued fraction with $b_{i} = 1\ \forall i$ is called a simple continued fraction and continued fractions with finite number of $a_{i}$ are called finite continued fractions.

Consider example rational numbers,

â€‹

$\frac{17}{11} = 1 + \frac{6}{11} \\[10pt]
\frac{11}{6} = 1 + \frac{5}{6} \\[10pt]
\frac{6}{5} = 1 + \frac{1}{5} \\[10pt]
\frac{5}{1} = 5 + 0$

the continued fractions could be written as

$\frac{5}{1} =5 \\[10pt]
\frac{6}{5} = 1 + \frac{1}{5} \\[10pt]
\frac{11}{6} = 1 + \frac{5}{6} = 1 + \frac{1}{\frac{6}{5}} = 1 + \frac{1}{1 + \frac{1}{5}} \\[10pt]
\frac{17}{11} = 1 + \frac{6}{11} = 1 + \frac{1}{\frac{11}{6}} = 1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{5}}}$

$a_{0} + \frac{1}{
a_{1} + \frac{1}{
a_{2} + \frac{1}{
\ddots
}}}$

A simple continued fraction is represented as a list of coefficients($a_{i}$) i.e

$x = [a_{0};\ a_{1},\ a_{2},\ a_{3},\ a_{4},\ a_{5},\ a_{6},\ \ldots]$

for the above example

$\frac{17}{11} = [1;\ 1,\ 1,\ 5]\ \ ,\frac{11}{6} = [1;\ 1,\ 5]\ \ ,\frac{6}{5} = [1; 5]\ \ ,\frac{5}{1} = [5;]$

Given a number $x$, the coefficients($a_{i}$) in its continued fraction representation can be calculated recursively using

$x_{0} = x \\[4pt]
a_{i} = \lfloor x_{i} \rfloor \\[4pt]
x_{i+1} = \frac{1}{x_{i} - a_{i}}$

The above notation might not be obvious. Observing the structure of continued fraction with few coefficients will make them more evident:

$x_{0} = a_{0} + \frac{1}{a_{1} + \frac{1}{a_{2}}},\ \ \
x_{1} = a_{1} + \frac{1}{a_{2}}, \ \ \
x_{2} = a_{2} \\[10pt]
x_{i} = a_{i} + \frac{1}{x_{i+1}} \\[10pt]
x_{i+1} = \frac{1}{x_{i} - a_{i}}$

SageMath provides functions `continued_fraction`

and `continued_fraction_list`

to work with continued fractions. Below is presented a simple implementation of `continued_fractions`

.

def continued_fraction_list(xi):ai = floor(xi)if xi == ai: # last coefficientreturn [ai]return [ai] + continued_fraction_list(1/(x - ai))

The $k^{th}$convergent of a continued fraction$x = [a_{0}; a_{1},\ a_{2},\ a_{3},\ a_{4},\ldots]$is the numerical value or approximation calculated using the first$k - 1$coefficients of the continued fraction. The first$k$convergents are

$\frac{a_{0}}{1},\ \ \
a_{0} + \frac{1}{a_{1}}, \ \ \
a_{0} + \frac{1}{a_{1} + \frac{1}{a_{2}}}, \ \ldots,\
a_{0} + \frac{1}{a_{1} + \frac{\ddots} {a_{k-2} + \frac{1}{a_{k-1}}}}$

One of the immediate applications of the convergents is that they give rational approximations given the continued fraction of a number. This allows finding rational approximations to irrational numbers.

Convergents of continued fractions can be calculated in sage

sage: cf = continued_fraction(17/11)sage: convergents = cf.convergents()sage: cf[1; 1, 1, 5]sage: convergents[1, 2, 3/2, 17/11]

Continued fractions have many other applications. One such applicable in cryptology is based on **Legendre's theorem in diophantine approximations**.

**Theorem: **if$\mid x - \frac{a}{b} \mid < \frac{1}{b^{2}}$, then$\frac{a}{b}$is a convergent of$x$.

Wiener's attack on the RSA cryptosystem works by proving that under certain conditions, an equation of the form$\mid x - \frac{a}{b} \mid$could be derived where$x$is entirely made up of public information and$\frac{a}{b}$is made up of private information. Under assumed conditions, the inequality$\mid x - \frac{a}{b} \mid < \frac{1}{b^{2}}$is statisfied, and the value$\frac{a}{b}$(private information) is calculated from convergents of$x$(public information), consequently breaking the RSA cryptosystem.