[Math] Continued fraction of the golden ratio

continued-fractionsgolden ratio

It is known, that the continued fraction of $\phi = \frac{1+\sqrt{5}}{2}$ is $[\bar{1}]$. This can be shown via the equation $x^2-x-1=0$:

$$ x^2-x-1=0 \Rightarrow x = 1+\frac{1}{x} = 1+ \frac{1}{1+\frac{1}{x}} = \cdots $$

As far as I can see, the only thing that has been used here is that $\phi$ is a root of the polynomial $x^2-x-1$. My question:

This polynomial has 2 roots. Why do we get the continued fraction of $\phi$ and not those of the other root? What has to be done to get the continued fraction of the other root with this method?

Best Answer

The golden ratio $$ x : 1 = 1 + x : x $$ leads to the equation $$ x^2 - x - 1 = 0 \quad (\#) $$ It can be transformed to two different equations of the form $$ x = F(x) $$ which then can be used to substitute the $x$ on the right hand side by $F(x)$ $$ x = F(F(x)) = F(F(F(x))) = \cdots $$

Your transformed version of $(\#)$ was this equation: $$ x = 1 + \frac{1}{x} \quad (*) $$

The repeated substitution of the RHS $x$ in $(*)$ with the RHS term $$ x \mapsto 1 + \frac{1}{x} $$ results in the expansion \begin{align} x &= 1 + \frac{1}{x} \quad (\mbox{EX}1.1) \\ &= 1 + \frac{1}{1 + \frac{1}{x}} \quad (\mbox{EX}1.2) \\ &= 1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{x}}} \quad (\mbox{EX}1.3) \\ &= \cdots \end{align} and leads to the continued fraction $$ x = 1 + \frac{1\rvert}{\lvert 1} + \frac{1\rvert}{\lvert1} + \cdots \quad (\mbox{CF}1) $$

This continued fraction is positive (consisting only of additions and divisions of positive numbers) and will converge to the positive solution of $(\#)$.

Note that each of the steps $(\mbox{EX}1.n)$ is an equation equivalent to equation $(\#)$, having two solutions for $x$, while $(\mbox{CF}1)$ converges only to the positive root. This is because the continued fraction is the limit of these finite fractions: \begin{align} c_0 &= 1 \\ c_1 &= 1 + \frac{1}{1} = 2 \\ c_2 &= 1 + \frac{1}{1 + \frac{1}{1}} = 1.5 \\ c_3 &= 1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{1}}} = 1.\overline{6} \\ c_4 &= 1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{1 + \frac{1}{1}}}} = 1.6 \\ \end{align}

Note that compared to the expansions $(\mbox{EX}1.n)$ the right hand side $1/x$ is dropped, which makes the difference.

To get the other root, the golden ratio equation $(\#)$ needs to be transformed into $$ x = \frac{1}{-1 + x} \quad (**) $$ Repeated substitution of the RHS $x$ in $(**)$ with the RHS term $$ x \mapsto \frac{1}{-1 + x} $$ results in the expansion \begin{align} x &= 0 + \frac{1}{-1 + x} \quad (\mbox{EX}2.1) \\ &= 0 + \frac{1}{-1 + \frac{1}{-1 + x}} \quad (\mbox{EX}2.2) \\ &= 0 + \frac{1}{-1 + \frac{1}{-1 + \frac{1}{-1 + x}}} \quad (\mbox{EX}2.3) \\ &= \cdots \end{align} leads to the continued fraction $$ x = 0 + \frac{1\rvert}{\lvert-1} + \frac{1\rvert}{\lvert-1} + \cdots \quad (\mbox{CF}2) $$ for the negative root of $(\#)$.

Note that all equations $(\mbox{EX}2.m)$ are equivalent to equation $(\#)$ and the equations $(\mbox{EX}1.n)$, thus have two solutions for $x$.

However the derived continued fraction $(\mbox{CF}2)$ is the limit of these finite fractions: \begin{align} d_0 &= 0 \\ d_1 &= 0 + \frac{1}{-1} = -1 \\ d_2 &= 0 + \frac{1}{-1 + \frac{1}{-1}} = -0.5 \\ d_3 &= 0 + \frac{1}{-1 + \frac{1}{-1 + \frac{1}{-1}}} = -0.\overline{6} \\ d_4 &= 0 + \frac{1}{-1 + \frac{1}{-1 + \frac{1}{-1 + \frac{1}{-1}}}} = -0.6\\ \end{align} They result from dropping the right hand side $1/(-1+x)$ sub term.

Note that the negative root is nicely approached from above by the even numbered terms and from below by the odd numbered terms.