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Continued Fractions

# Continued Fractions

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
$a_{0} + \frac{b_{0}}{ a_{1} + \frac{b_{1}}{ a_{2} + \frac{b_{2}}{ \ddots }}}$
$a_{i}, b_{i}$
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,
$\frac{17}{11} = 1 + \frac{6}{11} \\[10pt] \frac{11}{6} = 1 + \frac{5}{6} \\[10pt] \frac{6}{5} = 1 + \frac{1}{5} \\[10pt] \frac{5}{1} = 5 + 0$
the continued fractions could be written as
$\frac{5}{1} =5 \\[10pt] \frac{6}{5} = 1 + \frac{1}{5} \\[10pt] \frac{11}{6} = 1 + \frac{5}{6} = 1 + \frac{1}{\frac{6}{5}} = 1 + \frac{1}{1 + \frac{1}{5}} \\[10pt] \frac{17}{11} = 1 + \frac{6}{11} = 1 + \frac{1}{\frac{11}{6}} = 1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{5}}}$

# Notation

$a_{0} + \frac{1}{ a_{1} + \frac{1}{ a_{2} + \frac{1}{ \ddots }}}$
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
$\frac{17}{11} = [1;\ 1,\ 1,\ 5]\ \ ,\frac{11}{6} = [1;\ 1,\ 5]\ \ ,\frac{6}{5} = [1; 5]\ \ ,\frac{5}{1} = [5;]$

# Computation of simple continued fractions

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 coefficient
return [ai]
return [ai] + continued_fraction_list(1/(x - ai))

# Convergents of continued fraction

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.